What is the lethal dose of marijuana?

According to which US Government authority you want to believe, the lethal dose of marijuana is either about one-third your body weight, or about 1,500 pounds, consumed all at once.

In summary, enormous doses of Delta 9 THC, All THC and concentrated marihuana extract ingested by mouth were unable to produce death or organ pathology in large mammals but did produce fatalities in smaller rodents due to profound central nervous system depression.

The non-fatal consumption of 3000 mg/kg A THC by the dog and monkey would be comparable to a 154-pound human eating approximately 46 pounds (21 kilograms) of 1%-marihuana or 10 pounds of 5% hashish at one time. In addition, 92 mg/kg THC intravenously produced no fatalities in monkeys. These doses would be comparable to a 154-pound human smoking at one time almost three pounds (1.28 kg) of 1%-marihuana or 250,000 times the usual smoked dose and over a million times the minimal effective dose assuming 50% destruction of the THC by smoking.

Thus, evidence from animal studies and human case reports appears to indicate that the ratio of lethal dose to effective dose is quite large. This ratio is much more favorable than that of many other common psychoactive agents including alcohol and barbiturates (Phillips et al. 1971, Brill et al. 1970).

Acute Effects of Marihuana,  from Marihuana, A Signal of Misunderstanding - Table of Contents

4.  Nearly all medicines have toxic, potentially lethal
effects.  But marijuana is not such a substance.  There is no record in
the extensive medical literature describing a proven, documented
cannabis-induced fatality.

5.  This is a remarkable statement. First, the record on
marijuana encompasses 5,000 years of human experience.  Second, marijuana
is now used daily by enormous numbers of people throughout the world.
Estimates suggest that from twenty million to fifty million Americans
routinely, albeit illegally, smoke marijuana without the benefit of
direct medical supervision.  Yet, despite this long history of use and
the extraordinarily high numbers of social smokers, there are simply no
credible medical reports to suggest that consuming marijuana has caused a
single death.

6.  By contrast aspirin, a commonly used, over-the-counter
medicine, causes hundreds of deaths each year.

7.  Drugs used in medicine are routinely given what is called
an LD-50.  The LD-50 rating indicates at what dosage fifty percent of
test animals receiving a drug will die as a result of drug induced
toxicity.  A number of researchers have attempted to determine
marijuana's LD-50 rating in test animals, without success.  Simply
stated, researchers have been unable to give animals enough marijuana to
induce death.

8.  At present it is estimated that marijuana's LD-50 is around
1:20,000 or 1:40,000.  In layman terms this means that in order to induce
death a marijuana smoker would have to consume 20,000 to 40,000 times as
much marijuana as is contained in one marijuana cigarette.  NIDA-supplied
marijuana cigarettes weigh approximately .9 grams.  A smoker would
theoretically have to consume nearly 1,500 pounds of marijuana within
about fifteen minutes to induce a lethal response.

9.  In practical terms, marijuana cannot induce a lethal
response as a result of drug-related toxicity.

10.  Another common medical way to determine drug safety is
called the therapeutic ratio.  This ratio defines the difference between
a therapeutically effective dose and a dose which is capable of inducing
adverse effects.

         11. A commonly used over-the-counter product like aspirin has a
therapeutic ratio of around 1:20.  Two aspirins are the recommended dose
for adult patients.  Twenty times this dose, forty aspirins, may cause a
lethal reaction in some patients, and will almost certainly cause gross
injury to the digestive system, including extensive internal bleeding.

         12.  The therapeutic ratio for prescribed drugs is commonly
around 1:10 or lower.  Valium, a commonly used prescriptive drug, may
cause very serious biological damage if patients use ten times the
recommended (therapeutic) dose.

         13.  There are, of course, prescriptive drugs which have much
lower therapeutic ratios.  Many of the drugs used to treat patients with
cancer, glaucoma and multiple sclerosis are highly toxic.  The
therapeutic ratio of some of the drugs used in antineoplastic therapies,
for example, are regarded as extremely toxic poisons with therapeutic
ratios that may fall below 1:1.5.  These drugs also have very low LD-50
ratios and can result in toxic, even lethal reactions, while being
properly employed.

         14.  By contrast, marijuana's therapeutic ratio, like its LD-50,
is impossible to quantify because it is so high.

         15.  In strict medical terms marijuana is far safer than many
foods we commonly consume.  For example, eating ten raw potatoes can
result in a toxic response.  By comparison, it is physically impossible
to eat enough marijuana to induce death.

         16.  Marijuana, in its natural form, is one of the safest
therapeutically active substances known to man.  By any measure of rational analysis
marijuana can be safely used within a supervised routine of medical care.

DEA Judge Young's Ruling on Medical Marijuana