From what is now known about the effects of marihuana, its use at the present level does not constitute a major threat to public health. However, this statement should not lead to complacency. Marihuana is not an innocuous drug. The clinical findings of impaired psychological function, carefully documented by medical specialists, legitimately arouse concern. These studies identify marihuana-related problems which must be taken into account in the development of public policy. Unfortunately, these marihuana-related problems, which occur only in heavy, long-term users, have been over generalized and overdramatized.

Two percent of those Americans who have ever used marihuana are now heavy users and constitute the highest risk group. Strong evidence indicates that certain emotional changes have taken place among predisposed individuals as a result of prolonged, heavy marihuana use. The clinical reports in the literature describing transient psychoses, other psychiatric difficulties, and impairment of cognitive function subsequent to use of marihuana and of other drugs do not prove causality but cannot be ignored.

The causes of these emotional difficulties are much too complex to justify general conclusions by the public or the press. The mass media have frequently promoted such clinical reports to appear as far reaching events affecting the entire population. The clinician sees only the troubled Population of any group. In evaluating a public health concern, the essential element is the proportion of affected persons in the general group. The people responsible for evaluating public health problems must concern themselves with the proportion of people out of the total population who are affected by any specific condition. The highest risk groups should be identified as the source of primary concern. A recognition that a majority of marihuana users are not now a matter of public health concern must be made so that public health Officials may concentrate their attention where it will have maximum impact.

The concept of relative risk is crucial to an evaluation of the impact of marihuana on public health. We believe that experimental or intermittent use of this drug carries minimal risk to the public health, and should not be given overzealous attention in terms of a public health response. We are concerned that social influences might cause those who would not otherwise use the drug to be exposed to this minimal risk and the potential escalation of drug-using patterns. For this group, we must deglorify, demythologize, and deemphasize the use of marihuana and other drugs.

The Commission reemphasizes its concern about the small minority of heavy, long-term marihuana users who are exposed to a much greater relative risk of impaired general functioning in contemporary America. Public health officials should concentrate their efforts on this group. Fortunately, the group has to date not grown sufficently in size to warrant its being considered a major public health concern.

We retiterate, too, the public health implications of an increase in the at-risk population. We suspect that such an increase is most likely if a sudden shift in social policy significantly increased availability of the drug. One of the factors we consider in Chapter V when evaluating the various social policy options and legal implementations is the effect of each policy on incidence and patterns of use. Regardless of how heavy this particular variable will weigh in that process, we must state that a significant increase in the at-risk population could convert what is now a minor public health concern in this country to one of major proportions.