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Major Studies of Drugs and Drug Policy
The Report of the Canadian Government Commission of Inquiry into the Non-Medical Use of Drugs - 1972


The Report of the Canadian Government Commission of Inquiry into the Non-Medical Use of Drugs - 1972

2. Cannabis and Its Effects



Cannabis temporarily alters a variety of intellectual and cognitive functions, and these changes are related to dose, past experience, and other psychological and social variables. Of the mental abilities so far tested, acute impairment is found more consistently with complex tasks than with simple ones, and with high doses rather than medium or low quantities. Although many profound changes in thought and perception are reported by users, few of these have been documented in the laboratory. The intellectual effects of acute cannabis use are rarely detectable by the casual observer, and experienced users generally have little trouble functioning socially without detection, even after relatively high doses. Users often report considerable improvement or expansion of certain psychological capacities from the drug. but such effects have generally not been adequately explored or documented scientifically.232,598 Cannabis users often report that when very 'high', they experience some trouble following a train of thought and may easily lose track of the direction of a conversation. Changes in speech patterns have been reported.660 Some users have also noted that when 'high' they tend to get lost while walking or driving more often than when not under the influence of the drug . 271,438

In spite of the rather profound short-term subjective psychological changes reported to occur with cannabis, the experimental literature is surprisingly inconsistent in reporting objectively documented changes in intellectual capacity or performance. Even a number of studies employing what would appear to be high doses have been inconsistent in detecting cannabis effects on standard tests of cognitive and intellectual functioning. Many of the tests employed are clearly not sensitive to, or appropriate for, the rather amorphous psychological effects of cannabis. Consequently, both the positive and negative findings in the literature must be interpreted with caution.

Performance on complex tasks apparently requiring sustained attention and short-term memory are most often impaired in laboratory studies, although significant effects have also been found on simple tests at very high doses. Least effect is found with simple and familiar tasks, and those which the subject has had experience performing while 'high'. In a number of experiments. regular or chronic users performed better on standard doses than did novices or light users. 305,407,431,660

Tasks studied in the laboratory which have, in some situations, shown a cannabis-related impairment include: goal-directed arithmetic tasks, learning of a digit code and digit-symbol substitution, short-term or immediate memory tests, reading comprehension, and performance on cognitive and psychomotor tasks in the presence of distracting stimuli. Other studies have not found these effects. Simple memory tasks have given especially inconsistent results.4,111,133,161209,282,308,397,407,431,612,660   Several recent aboratory  studies suggest a dose-related impairment of attention and vigilance.111,133,431,455 Subjective reports from users confirm the notion that cannabis, in sufficient doses, often causes lapses in short-term memory and directed attention.

Mayor La Guardia's Committee investigated the effects of rather large doses of smoked and orally administered cannabis on intellectual functioning.117 Their data indicate that high doses impair a variety of mental abilities, while lower doses have little effect or may improve some of them. The conclusions that can be drawn are limited, however, by lack of statistical analyses of the data.

The research group of Melges, Tinklenberg, Hollister and others have studied certain aspects of THC-induced changes in short-term memory and temporal perception. 426,427,611,612 Significant impairment was found on a goal directed serial arithmetic task under some conditions, but not under others. This test is felt to assess short-term memory, attention and goal-directed organizing abilities, and an inability to perform properly was considered to reflect "temporal disintegration". With cannabis, subjects were shown to concentrate more on present thoughts and experiences than on aspects of the past or future. The researchers suggest that "temporal disintegration", seen at higher doses, can account for this phenomenon and may play a significant underlying role in a number of other cannabis effects, including depersonalization, a sense of timelessness, and perhaps acute psychotic or panic reactions. They also report that impairment in immediate memory with marijuana may be episodic and brief in duration, rather than following a smooth and continuous time course.

Waskow and associates found no decrement on the Wechsler Memory Scale at moderate doses of oral THC. Serial addition was affected only when a secondary distracting condition was presented concomitantly.655 Rafaelsen and co-workers reported that orally administered hashish produced a closedependent impairment on several psychological tests, which included the reproduction of sentences, learning finger labyrinths (mazes) and certain arithmetic abilities. Other tasks did not reveal significant effects.514 Another research group found that orally administered cannabis concentrate impaired reading comprehension and the learning of a digit code, and affected choice reaction time in a way which suggested a basic impairment in focussed attention and vigilance. 133

Recently an impairment in short-term memory was found in monkeys trained to smoke marijuana. The same effect was seen with orally administered THC. The monkeys exhibited some evidence of a "loss of ability or motivation to perform complex tasks." 542,680 

Abel has studied various aspects of marijuana effects on human memory processes and has described several mechanisms by which performance could be impaired.2,3,4 The drug might interfere with either acquisition of new material, storage of the information, or retrieval of stored material, or a combination of these. Abel gives evidence that retrieval mechanisms are not affected, but that acute intoxication with cannabis interferes primarily with acquisition processes involved in the storage of information perhaps by reducing the focussing of attention required for adequate rehearsal of information for efficient memory storage. By contrast, Moskowitz's recent work suggests that marijuana affects only the very initial acquistion of information, rather than its rehearsal or transmission to storage.455 Again, attention factors are suspected of playing a central role in this deficit.

The Commission has investigated the effects of marijuana and THC on several intellectual and cognitive abilities with drug doses commonly used in North America today.440 Short-term serial position memory was significantly impaired with the higher THC and marijuana doses, but not with lesser quantities. No significant drug effects were seen on the digit symbol substitution test, or on finger painting (rated on a variety of psychiatric, graphic and aesthetic dimensions) 1-1/2 to 2 hours after smoking. Subjects rated themselves significantly lower on the Clyde Mood Scale "clearthinking" factor after cannabis use than with placebo. None of these effects was significant when the test battery was repeated 4-1/2 hours after smoking.

In a separate Commission experiment, sustained attention was assessed in a 40-minute visual signal detection task which was subjectively boring and tedious, and made precise demands on the subject. Cannabis consistently reduced accuracy, and the decrement in performance (d) correlated positively with the individual's subjective rating of the intensity of the drug-induced 'high'.603 Similar effects have recently been found on an auditory signal detection task. 455 In another Commission experiment, however, no cannabiseffects were seen on a secondary choice-reaction task designed to assess  the effects of cannabis on attention and  response speed. The effects of cannabis on vigilance and response speed should be further explored in a broader stimulus-and-reward context.

Cannabis users often claim enhanced creativity, and the ability to see new and significant relationships among sensory and cognitive experiences with the drug.  The evidence in this regard mixed. 93,111,232,243,244,247,416,481,598,670  Such functions are extremely difficult to study scientifically. Creativity has not been adequately explored experimentally, even under non-drug conditions, and significant advances are not expected in the near future. While several studies have found that cannabis users tend to be more imaginative, flexible and creative than average, the association between the use of the drug and these personality characteristics has not been shown to be a drug effect.

Certain combinations of dose, set and setting can induce severe acute panic or psychotic reactions. Clinical reports indicate that such temporary states are often characterized by extreme confusion, grossly impaired judgement and immediate memory, and a significant reduction in "reality testing". Adverse reactions of this type are examined extensively in a later section.

In the past few years a considerable number of surveys have examined the relationship between the use of cannabis and reported academic performance. In most of these studies, including the Commission's national survey of high school students,350 cannabis use tended to be associated with slightly poorer grades, with heavy users doing less well than others.20,21,54,103,247,269,299,464,569,572,586,591,666 On the other hand, several studies have discovered no relationship between cannabis use and grades.60,275,494,504,650 In the Commission's national survey of university students there was no significant difference in the overall grade distributions of cannabis users and nonusers.351' Two college surveys in the United States have found that light or infrequent users had better academic records than either non-users or heavy users. 233,277 A negative correlation between the use of cannabis and school performance seems to be more frequent in high schools than in universities. Canadian surveys have been more consistent than United States studies in linking poorer performance with drug use (and have generally shown less prevalent drug consumption). It is interesting to note that in most studies. the use of tobacco and alcohol reflects a similar, but sometimes weaker, negative relationship with academic performance.

No causal inferences can be drawn from these correlational studies. Campbell, however, has suggested that the prolonged and heavy use of cannabis can lead to a temporary deterioration of the capacity to abstract and synthesize, and such a deficiency would likely be reflected in academic performance.103 This hypothesis should be systematically explored.

A number of other recent clinical and interview reports also suggest that some cognitive impairment may be associated with chronic cannabis use by some persons in North America.83, 177,334,345,383,448,511,682 Problems of thinking, attention, concentration and memory were noted in several instances. A tendency for some "excessive" users "to favour regressive and magical to rational thinking" has been reported.171 Schwarz has drawn attention to a parallel between the short-term effects of cannabis on cognitive functioning and some alleged chronic effects. He is of the opinion that, "Despite the persistence of acute toxic features in some people, in a majority of cases the effects wear off in a comparatively short period of time after drug intake is stopped."550 [P. 12]

Very little controlled research has been done on the effects of chronic or sub-chronic cannabis use on cognitive functioning. The 1946 study by Williams and associates suggests a slight reduction in performance on an I.Q. test and no change in rote-memory over a period of more than one month of heavy daily use,670 but the interpretation of the experiment is hampered by the lack of both a control group and statistical analysis of the data. Miles and associates, at the Addiction Research Foundation of Ontario, are investigating the effects of daily marijuana smoking (at various doses) in the laboratory for periods of up to two months.436 Matched control subjects were examined under identical conditions. No significant chronic changes in intellectual functioning due to marijuana have been detected in these studies. (A preliminary summary of this continuing experimental program is presented in Annex B at the end of this chapter.)

Recently in a preliminary study, Dornbush has found tentative evidence of some recovery from initial drug-induced cognitive impairment when subjects were administered a fixed marijuana dose daily for a few weeks.211 By contrast. Hollister has found no change in drug effects over a five-day period of daily oral THC administration. Several other experimental studies which are presently in progress should add significantly to the information in this area.428,626

In Jamaica, Bowman examined a total of 30 males with a 10-year or longer history of steady use of cannabis, and 24 matched non-users. These groups were not consistently different on a wide variety of measures of cognitive function and memory.74 The United States NIMH has sponsored a large-scale study of a broad range of psychological, social and physiological measures in cannabis users in Jamaica, but the results of the investigation have not yet been made public.534 A continuing study of chronic hashish smokers in Greece will also be interesting in this respect.189 So far, no gross abnormalities have been found. In Egypt, Soueif has compared 850 persons imprisoned for cannabis use with prisoners who had never taken cannabis, opium or other illegal drugs. The drug users performed less well on most of the cognitive tests.584,414

After a general inquiry, in 1893 the Indian Hemp Drugs Commission came to the conclusion that "...the moderate use of hemp drugs produces no injurious effects on the mind...... But, "It is otherwise with excessive use." The Commissioners stressed, however, that moderate use is the rule "...and that the excessive use is comparatively exceptional.295 [P. 264]

Mayor La Guardia's Committee examined 48 users in New York who had been smoking regularly for two to seventeen years. The Committee concluded that:

There is definite evidence in this study that the marijuana users were not inferior in intelligence to the general population and that they had suffered no mental or physical deterioration as a result of their use of the drug.407 [P. 141]

There have been reports from the East that certain long-term heavy cannabis users. who apparently constitute a group not unlike North American skid-row alcoholics, show a progressive deterioration in intellectual ability. We have been unable to find evidence of such long-term effects in the Western literature. Eastern and other literature concerning acute and long term effects on cognitive functioning of this character is considered in greater detail in the section on adverse reactions.


Psychomotor Performance

The effects of cannabis on psychomotor skills depends upon dose, the subject's past history of use (and probably his experience with the task while 'high') and the nature of the sensory, cognitive and muscular components involved in the task studied. The "set and setting" are also undoubtedly important, as well, but have not been adequately studied in this context.

Mayor La Guardia's Committee found that apparently large doses of oral or smoked cannabis impaired hand and body steadiness and choice reaction time. Simple reaction time and maximum tapping speed were little affected. The decrements were generally dose-related, and persons with past cannabis experience showed less impairment in performance than did those who had not taken cannabis before.407' The findings of decreased hand and body steadiness and impaired complex reaction time have been recently confirmed, as well as the observation that experience with the drug may reduce or eliminate the impairment.133,305,337 There is some suggestion that cannabis may slightly alter ocular-motor (eye movement) function,455 although the general significance of such an effect is unclear.

More complicated psychomotor skills, requiring an integration of continuously changing visual and muscular feedback and motor control have recently been studied. Weil and co-workers tested cannabis-naive subjects with two different doses of smoked marijuana and an inactive placebo. Performance on a task requiring muscular co-ordination and attention (Pursuit Rotor Test) declined as dose increased. No effect was seen with the high dose in a small group of experienced cannabis users.661' Carlin and associates found no significant cannabis effects in experienced cannabis smokers on pursuit rotor performance, either while the task was being learned, or after the subjects' tracking abilities had improved and stabilized with practice.111 Manno and associates found a dose-dependent impairment of pursuit tracking performance in a group which included both experienced and cannabis-naive individuals. A tracking impairment due to cannabis was demonstrated in three separate studies, although a dose-dependent relationship was evident only when the drug was administered on a body weight basis. Marijuana and alcohol together produced significantly greater impairment of tracking performance on one of the four stimulus patterns tested than did either drug alone. There is some suggestion that boredom and attentional factors played a significant role in these deficits.337,397,398,399,400

Crancer's group found no cannabis effects on a laboratory-simulated driving task.145 This study will be discussed in more detail below. Similarly, a recent study in Alberta found no marijuana-related impairment on a "pilot simulator" laboratory apparatus designed to assess "muscular complex coordination" skills."' Rafaelsen and associates, in a preliminary report of their cannabis and alcohol driving simulator research, noted no drug effects on "start time". but both drugs increased "brake time".515 Again, attention problems were implicated in the brake time delays. Binder reported that marijuana increased response latency in a laboratory tracking task.55

The Commission has investigated the effects of various doses of high purity THC, marijuana and alcohol on several psychomotor abilities in experienced users of these drugs.251,440,518 Both the upper cannabis and alcohol (.07% blood alcohol level - b.a.l.) doses produced subjective effects reported to be generally comparable to those typically experienced when 'high' or 'stoned'. (Although most subjects felt that the dose produced effects that were at least as intensive as their normal experience with the drug, others reported that they felt less 'high' than usual.) Simple tracking and complex tracking (involving the addition of a foot choice-reaction task and occassional polarity reversals if, the control device), tapping speed, and some automobile driving tasks were studied. The driving data will be discussed separately below.

Maximum tapping speed was unaffected by cannabis.440 In the first hour after drug administration, alcohol and, less consistently, the upper marijuana dose resulted in a decrease in simple and complex compensatory tracking performance. 518 The interaction of low marijuana and low alcohol (.03% b.a.l.) in combination typically resulted in more error in complex tracking than either low dose treatment alone. An additive drug effect was not as evident in simple tracking. A more detailed analysis of the various components of tracking error (discussed in Annex A of this chapter) suggests that the mechanisms of the effects of marijuana and alcohol on this task are somewhat distinct. Speed of reaction to tracking control polarity change tended to be slower with the higher dose of alcohol, but not with the other drug conditions. During complex tracking, whenever a number appeared on an electronic tube positioned above the tracking screen, the subject was required to push either a left or right pedal, or continue pushing a middle pedal with his foot, depending on which of three numbers was presented. No warning signal was given. This task was designed to assess both secondary attention and response time. Marijuana did not significantly change choice reaction time, but alcohol reliably decreased response speed. The drugs did not cause subjects to miss signals or respond at the wrong time any more often than with placebo, suggesting no drug effects on the level of attention required to perform this task. The effects of low alcohol and marijuana in combination on choice-reaction were not significantly different from the effects of alcohol alone. No reliable drug effects were seen on psychomotor performance when the subjects were retested four hours after smoking. The additive clecremental effects of alcohol and cannabis on psychomotor performance suggested in this study and by Manno and associates399 may be of social importance because of the increased use of these drugs in combination.

The effects of long-term chronic use of cannabis on psychomotor functioning has not been adequately explored. In William's study, subjects smoked large quantities of marijuana daily for more than one month. No gross effects were noted on muscular co-ordination, although on one test of mechanical ability there was a tendency for increased speed but less accuracy as time went on.670 The lack of a control group and statistical analysis limits the conclusions that can be drawn from this study, however. In a report of a prison study in Egypt, Souief found a tendency for inmates arrested for hashish use to perform less well on psychomotor tests than did prisoners who had never taken any illicit drugs (1,689 subjects were tested ).584,414 In two studies of chronic heavy marijuana users and matched controls in Jamaica, Bowman found no evidence of significant psychomotor impairment.74 Other studies of chronic and sub-chronic use are underway which ma be of value here. but the relevant data have not yet been presented.160,189,281,428,534

Automobile Driving

Although there are clearly a multitude of valid reasons for interest in the effects of cannabis on psychomotor skills, concern has perhaps been greatest for possible adverse effects on automobile driving. While laboratory studies of psychomotor skills and attention are of considerable interest in this regard, the predictive validity of such tests with respect to actual driving has not been demonstrated, and generalizations must be made with great caution.

Compared to most behavioural tasks studied in the laboratory, automobile driving is an extremely complex phenomenon. Certain isolated elements of the overall behavioural patterns involved have been explored, and some are reasonably well understood in abstract experimental conditions. However, the relative importance of various perceptual, cognitive and psychomotor responses in general driving and (of greater significance ultimately) in traffic accidents and fatalities is not clear. Although it would seem obvious that gross impairment in any of a number of essential functions would affect driving ability, little is known as to the actual causes of traffic accidents, and small defects in one or more components might not result in significantly increased accidents or injuries.

Many popular assumptions as to essential driving skills, which may on the surface seem valid, have not been supported by studies of driving behaviour. It has been shown that bad drivers (i.e., those with a large number of moving violations) actually had faster reaction times, performed better on a number of visual perception tasks, and were more knowledgeable about traffic laws than individuals with better driving records.144,654   Fergenson found that drivers with high accident rates were slower in a choice-reaction time test than non-accident drivers, but those with many traffic violations and no accidents performed best.184 Currie has argued that elementary psychomotor functions, like simple reaction time, are generally not significant factors in accidents, but that judgemental faculties such as the perception of danger are more important. 147  It would seem that such factors as driver attitudes, risk taking traits, general judgement, attention and susceptibility to distraction, and a variety of other variables which are very difficult to measure under natural conditions. may be more significant in contributing to automobile accidents. Waller stresses that both laboratory and epidemiological data are needed in this area. 648 Non-experimental information of value can be gained from studies of users' attitudes, experiences and driving records, and by systematic investigation of accidents. Data in these various areas will be discussed below.

Hypothetically, there are many ways in which a drug might change some component of the driver's car handling or traffic negotiation abilities. A drug might, for instance, damage his ability to assess risks, or to integrate the complex feedback involved in car handling. It could subtly impair the driver's psychomotor co-ordination or simply make him fall asleep. The drug could alter his perception of the roadway or change the way he evaluates the intentions of other drivers, or it might perhaps interact with a particular situation in the environment, regardless of the driver's skill and motivation. A drug could very well have drastic, disabling effects on driving ability and yet have little effect on accident rates if people tended not to drive after using the drug. A drug could have an effect on driving and accidents, but cause few injuries, perhaps because it would induce its users to drive more slowly and thus be involved mostly in minor accidents. (Driving slowly, however, may cause difficulties for others in fast-moving traffic.) A drug might have no effect on psychomotor functions, yet increase risk-taking or aggressiveness in a way which leads to a greater incidence of accidents and injuries.

As noted above. cannabis has a variety of perceptual, cognitive and psychomotor effects which are dose-dependent. General psychomotor skills, such as tracking ability, complex reaction time, etc., may be only slightly or not at all impaired at low doses, while higher doses often produce a more definite effect. It has been shown that cannabis can affect attention and judgement, and, if the dose is sufficiently large, can produce considerable perceptual distortion. On the other hand, cannabis does not normally seem to increase aggression and apparently often reduces it. In all, it would seem likely that under some conditions, cannabis might have adverse effects on driving, and that any such effects would vary as a function of dose and a variety of other factors.

Attitudes of Users Towards Driving

A number of surveys of regular cannabis users indicate that at least half have driven while under the influence of cannabis.239,248,251,275,415,481 Significant proportion of these persons feel that their driving is impaired by the drug, and many generally refuse to drive after using cannabis. Others deny any impairment. Many indicate that they compensate for initial difficulties and, in fact, are more careful and, consequently, safer drivers when 'high'. It has even been suggested that because of the tranquilizing effects attributed to cannabis. low doses might improve driving performance in some nervous individuals under certain conditions of tension or stress. Other observers have expressed concern over the possible effects on driving of "flashbacks" or recurrences of intoxication effects in the absence of recent drug use.

Klein and associates studied college students' attitudes to cannabis and driving, and found that infrequent and former users tended to downgrade their driving-related perceptual and psychomotor abilities with marijuana more frequently than did chronic users. In addition, non-users and infrequent users tended to disapprove of the use of marijuana by airplane pilots and commercial and private automobile drivers more consistently than did chronic users.340 Whether these attitudes reflect a differential sensitivity to marijuana effects. possibly including tolerance, or perhaps some attempt at rationalization by chronic users. or other biasing factors (either positive or negative) is not clear. The authors also presented a series of case historic examples where marijuana use was allegedly involved in crashes or poor driving.

Experimental Studies of Driving Skills

Experiments of drug effects on certain driving tasks, or on simulated or putative components of actual driving behaviour, may answer certain specific questions regarding cause and effect in the experimental context, but they generally lack documented predictive validity with respect to performance under normal conditions of driving. In any experimental research on driving, of either the laboratory driving-simulator or driving-course type, some components of 'real' driving will be absent. In most laboratory simulator tests there are simulated demands on both the car handling and traffic negotiation functions, but actual vehicle control is lacking and many of the normal lines of feedback to the driver do not exist. In a simulator, the driver generally only uses vision (and perhaps hearing), while in real driving he receives feedback through his bodily senses (kinesthetic and vestibular) as to the consequences of his control movements. When automobiles are used in a controlled driving course, the driver receives normal feedback on performance, since he is handling a real car, but he does not have to cope with traffic or dangerous situations. Drug effects on real traffic negotiation are rarely studied for safety reasons. In either kind of test, the risks and payoffs involved are quite different than in normal driving, and in most experimental tests there is no risk in poor performance at all, except perhaps to the subject's self-esteem. On the other hand, subjects are always aware that they are "performing" and under observation. In most testing of driver behaviour there is a limited and arbitrary choice of roadway and vehicle characteristics, so that interactions of driver factors with different aspects of the roadway and vehicle, which may be of the most interest, are not seen. The problem of validating any experimental tests against driving performance under normal conditions has not yet been solved, and the relative value of laboratory simulator and testtrack driving measures is a matter of dispute.

In 1969 Crancer and co-workers from the Washington State Department of Motor Vehicles published a study of marijuana effects on a laboratory driving simulator. 145 Scores on a very similar task had previously been shown to correlate slightly with subjects' driving records. The overall performance of 36 regular cannabis users tested with a single dose of marijuana was not different from control, although there was less careful monitoring of the speedometer under the drug, and "driving" tended to be slower.144 The main study was followed by two cursory investigations. Four subjects were retested with approximately three times the original dose and none showed a significant change in performance. Furthermore, four marijuana-naive subjects were tested after smoking enough marijuana to become 'high' (equal to or greater than the amount used in the main experiment). No significant change in scores occurred with the drug in these subjects either.

The investigators cautioned that the study does not necessarily indicate that marijuana will not impair driving:

However. we feel that. because the simulator task is a less complex but related task, deterioration in simulator performance implies deterioration in actual driving performance. We are less willing to assume that non-deterioration in simulator performance implies non-deterioration in actual driving. 145 [P. 854]

In order to obtain some standard reference point for the study, and to ascertain the sensitivity of the performance task to known drug-induced impairment, the subjects were also tested under a single dose of alcohol, designed to produce a blood alcohol level (b.a.l.) corresponding to the legal standard of presumed driving impairment in Washington (0.10% b.a.l.). The actual b.a.l. achieved was 0.11%, a relatively high dose.144 The average number of errors under alcohol was significantly greater than that acquired under either the no-drug or marijuana conditions. The Crancer study has been widely quoted and distorted in the literature and is often referenced to demonstrate that marijuana does not affect driving-a conclusion not drawn by the original investigators.

H. Kalant has pointed out that comparisons between the drugs must be made with caution due to the single doses used in the main study.310 He also suggested that, even though it would not have been easy for the subjects to fake good driving performance under marijuana, an anti-alcohol bias, sometimes seen in marijuana users, could have resulted in poorer performance in the alcohol condition. The study has also been criticized for comparing a heavy alcohol dose with a mild cannabis dose. However, Crancer apparently gave rather large quantities of marijuana, although the actual potency of the material used is in question. In another laboratory, marijuana from the same supply was found to be much weaker than originally estimated. 527

As noted earlier. Rafaelsen's group found that "socially relevant" doses of cannabis taken orally increased "brake time" (and apparently the number of missed signals) but had no effect on "start time" in response to the appropriate driving signals in their laboratory driving simulators.515 The U.C.L.A. group has released very little information about their driving simulator research. but has suggested impairment due to marijuana in some other apparently driving-related tasks.455,631 Other general studies of psychomotor skills of possible relevance to driving have been discussed above.

The Commission has conducted an initial investigation of the effects of two levels of smoked marijuana and a single dose of alcohol on a limited range of automobile driving tasks in 16 regular users of both drugs.251 Subjects' ratings indicated that the upper cannabis dose and the alcohol (.07% b.a.l.) generally produced effects comparable to their typical levels of "highness" or intoxication achieved with these drugs. Subjects were tested under all conditions and received first a drink and then a smoke in each session, one of which was a placebo on experimental drug days. In the control session, the subjects were given a disguised non-alcoholic drink and a "joint" of thoroughly extracted marijuana placebo. Subjects were tested for three quarters of an hour on a driving course (marked out with wooden poles and plastic cones) on which they were required to perform a number of manoeuvering and parking tasks requiring good psychomotor control and judgement. Six consecutive laps of the 1.1 mile track were completed on the first test trial. Three hours after smoking, subjects were tested again on half as many laps.

The low marijuana dose and placebo conditions were not different in terms of the number of oles or "road cones" hit. Both the higher marijuana dose and the alcohol condition produced small increases in "hits", which were reliably different from placebo, but not significantly different from each other, The higher cannabis dose resulted in a slight (7%) but consistent decrease in driving speed. This latter effect is in agreement with other less formal data. 144, 340  Driving speeds in the other drug conditions were not significantly different from those of the placebo conditions. Efficiency of handling was noted by a within-car observer. Awkward or superfluous driving manoeuvers ("rough handling") tended to be greater under both drug conditions, but only the alcohol scores were significant. No substantial drug effects on the driving measures were detected on the second trial three hours after drug administration.

Thirteen of the 16 subjects were experienced in driving in normal traffic after smoking cannabis or drinking alcohol, while three had never done so. Of the 13 experienced subjects, all but one reported having driven when feeling at least as "high" as they felt when getting the lower cannabis dose, while seven reported having driven when feeling at least as "high" as they did after getting the higher cannabis dose. Eleven of the 13 subjects had driven when feeling as "high" as they felt after the alcohol dose. In both the alcohol and upper cannabis conditions, the subjects rated their driving ability as lower than they did after placebo. Moreover, they felt that driving took more effort after either drug, and that normally they would be less likely to drive when feeling as they did.

Faking poor performance under the alcohol condition seems unlikely in the present study. To begin with, subjects' attitudes were carefully explored in interviews; none displayed any sign of hostility towards alcohol, and all were intermittent drinkers. Furthermore, some subjects thought they were receiving both drugs in the alcohol or high-dose marijuana conditions. In addition, the alcohol impairment among the marijuana users on these tasks was comparable to that displayed by a similar group of 12 non-marijuanausing alcohol subjects in a secondary study, in which cannabis was not mentioned.

Considerably more experimental work is needed to elucidate the exact nature of the effects of cannabis on driving skills, including the examination of a variety of other performance and risk measures, as well as dose-response and drug-interaction effects (especially with alcohol). Important questions regarding high-speed freeway decisions and manoeuvering, for example, as well as susceptibility to distraction and reaction to unexpected events under conditions of relaxation, stress, fatigue, or boring long-distance driving should be explored. Some of these conditions are not easily amenable to controlled experimentation. On the basis of the Commission study, one cannot assert that cannabis does or will cause automobile accidents or the contrary. It does demonstrate, however, that marijuana, in acute doses within the range commonly consumed by some individuals in North America today, can produce measurable impairment of short duration in some driving tasks which are, in some respects, not unlike the effects of alcohol in quantities commonly used. More specific comparisons of the effects of these drugs on driving cannot be made on the basis of these data. Additional investigation is clearly indicated.

Previous claims that cannabis has no effect on driving skills (based largely on a misinterpretation of Crancer's work), and poorly supported statements that experienced cannabis users can "come down", and completely control, or compensate for all the effects of the drug when necessary must clearly be modified.658  On the other hand, there is no evidence from available experiments for the notion that social cannabis use produces a disasterous loss of judgement or psychomotor control. Our research suggests that until further data are available. driving while under the influence of cannabis should be avoided.

As described in an earlier section, a pilot study which suggested that cannabis might increase visual glare-recovery time by several seconds received wide publicity.199 Considerable concern has been expressed that night-time driving problems might be caused by such a sensory effect. However, the trend reported in the original report was not statistically significant. An investigation by the Commission of the effects of cannabis on recovery of dim-light visual acuity after bright glare603 and a more comprehensive study by Moskowitz 455 have also failed to find any major changes. To date, no adverse effects of cannnabis on visual glare-recovery, after-image retention, or dark-adaptation time have been demonstrated.

Driving Records

Waller has stressed that tests under experimental or laboratory conditions alone can give no definitive conclusions regarding highway crashes.648' Epidemiological data regarding actual driving conditions is needed as well. There have been several studies of the driving histories of identified marijuana users. McGlothlin and associates investigated the records of marijuana users who volunteered for an LSD experiment and found them to have traffic violation and accident rates which were not significantly greater than non-users.415 Klein's group found that chronic cannabis users admitted to more frequent traffic violation charges than non-users,340 but it was not ascertained whether or not these occurred while under the influence of the drug. In Haines and Green's study, none of 81 regular users who had driven when "high" (many of them regularly) "...has ever incurred physical harm or has been involved in an auto accident when stoned"."' Crancer and Quiring found arrested male marijuana users to have more traffic violations and accidents than a control group, while in the marijuana arrestees as a whole (including females) the violation rates were not significantly elevated.146  As the authors point out, arrested users are not representative of the general population of users in society and there is no way of determining whether or not the excess accidents occurred while the users were under the influence of the drug. Even within the group studied, the reasons for deviant driving are unclear. The authors state, "The most obvious causes of a poorer driving record might be increased driving exposure, physiological impairment from using drugs. and character disorder."

Several other studies strongly suggest that personality and social factors may account for poor driving records.171 Waller investigated the driving records of 231 persons convicted for illegal drug use (usually cannabis) in California, and found the group to have more documented traffic law violations. hut no more accidents than other drivers. Many of these delinquent drivers began "illegal drug use... only after they were already known as 'negligent operators'." Consequently, drug use was probably not a cause of the violations, but more likely reflected a general tendency to take risks and ignore the law.646 The author later concluded that, with other factors controlled marijuana use alone was probably not related to excessive crash risk.647

For maximum gain, future investigations of driving records should take into account such variables as age, sex and social characteristics of the subjects, pre-drug-use behaviour and personality, the frequency of cannabis and other drug use, overall driving exposure (both with the drug and without it), and the presence or absence of intoxication or drug-induced impairment at the time of the violation or accident of interest.

Accident Investigations

Several anecdotal reports, case histories or other stories implying marijuana involvement in accidents or poor driving have appeared in the literature.557,93,340,663 The significance of even verified isolated reports is difficult to ascertain. Waller points out in his recent review of drugs and driving:

Anecdotes or individual case histories can suggest relationships and mechanisms in specific instances. They can give no indications, however, as to the frequency with which relationships exist. The event described may be commonplace or quite rare. 648 [P. 1479]

Unfortunately, due to the lack of convenient and reliable chemical techniques for quantifying or even detecting cannabinoids in body fluid or tissues. it has not yet been possible to objectively compare the occurrence of cannabis use in drivers who have had accidents with those who have not. Such controlled surveys of fatal and non-fatal accidents, and of drivers both "at fault" and "victims", have been of major importance, for example, in clarifying the role of alcohol in traffic accidents (see Borkenstein's work 64 ). Similar studies focussed on the detection of cannabis and other drugs, as well as alcohol, in persons involved in accidents, should be initiated once the appropriate biochemical tests have been developed for cannabinoids. Recent advances in detecting cannabinoid metabolites in the urine may provide some qualitative assistance in this area.

In order for epidemiological studies of this type to yield reliable information about a drug's potential traffic hazards, the incidence of the use of the drug in the driver population must be fairly substantial. Such investigations are clearly not sensitive to driving conditions which are relatively infrequent at the time of the study. It would appear that in most parts of Canada the present coincidence of cannabis use and automobile driving in the general population is probably too low for the detection of possible cannabis-related traffic hazards using this research technique. Such studies might be feasible in parts of British Columbia or California, for example, or in other areas or countries where regular cannabis use is quite common.

Conclusions Regarding Traftic Hazards

In his 1971 review of the literature, Waller cautiously concludes that the use of marijuana may not be associated with a substantial increase in crash risk.648 Similarly. Nichols, in a major review of the broad area of drugs and highway safety prepared for the United States Department of Transport in 1971, concluded that the existing data do not confirm the hypothesis that drug use (other than alcohol) is presently a major factor contributing to highway crashes and fatalities.474  Both investigators stress that important gaps exist in our present knowledge, and that no definite statement can be made at the present time regarding the possible role of cannabis in accidents.

While Commission experimental findings do not invalidate the guarded conclusions of Nichols and Waller, both the demonstration of some, even limited, driving impairment due to cannabis, and the evidence suggesting that cannabis and alcohol have additive detrimental effects on certain psychomotor skills, indicate that a cautious approach should be taken in interpreting the present, very limited epidemiological data regarding cannabis and traffic accidents. Continuing changes in the frequency and patterns of use of cannabis (and other drugs, including alcohol), in addition to improved research techniques, may substantially alter the epidemiological picture in the future. These changes must be carefully monitored and documented.

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