Is cannabis use a risk to health?

A look at the evidence


Claims of various degrees of harmfulness and danger associated with the smoking of cannabis are often cited by prohibitionists and those who favour regulated legalisation alike.

see Pot Smokers Just As Healthy - Study: National Post (Canada), 11 June 2001

Whilst many people agree that prohibition is unjust and ineffective, they differ in their opinions on the needs for regulations. Whilst few would want unnecessary regulations and limitations legislated onto cannabis once legal, few also would want to see no regulations installed if indeed necessary. It is therefore vital that we attempt to reach some sort of conclusion on the harm or potential harm through individual or widespread cannabis use.

I have personally read many reports from scientific and empirical studies on cannabis use, as well as some of those based upon laboratory tests carried out on mice, rats, rabbits and monkeys, using concentrated and synthetic THC - tetrahydrocannabinol - one of the main active ingredients found in the parts of the cannabis plant used recreationally and medically, particularly the tops and heads.

My own studies of the evidence from both sides has led me to the following conclusions:

1) All of the allegations of harm are based upon dubious work, laboratory experiments not involving cannabis and not involving tests on humans, and unreliable anecdote often exaggerated and g by drug workers.

2) Cannabis is indeed "remarkably safe" and free from danger, barring of course the obvious dangers of being hit over the head with a large lump of resin.

"We.. say that on the medical evidence available, moderate indulgence in cannabis has little ill-effect on health, and that decisions to ban or legalise cannabis should be based on other considerations.": The Lancet, vol 352, number 9140, November 14 1998

As it is nonsensical to attempt to prove any substance to be completely harmless under all circumstances, I am tackling this issue by listing the various harm allegations and counteracting them with quotes from and references to the experts.

We must be careful to distinguish the empirical evidence from studies of the health effects of herbal and pure cannabis on people who smoke it and anecdotes or hypotheses base on studies using chemical THC and other extracts on mice and monkeys in the laboratory.

The Report of the World Health Organisation, so often cited by those who claim cannabis to be a health risk, says this:

"A great many assumptions have been made in extrapolating from health effects observed in laboratory animals to the probable health effects of equivalent doses and patterns of use in humans. In addition, there may be problems in extrapolating studies with pure THC to human experience with crude cannabis preparations. The plant material contains many other compounds, both cannabinoid and non-cannabinoid in nature and the possibility must always be considered that differences between experimental and clinical observations may be due in part to the effects of these other substances."


Alun Buffry, BSc., Dip Com (Open)


Myth: Cannabis is toxic / poisonous

From: DEA's Own Administrative Law Judge Ruled Cannabis Should Be Reclassified Under Federal Law,

DATED: SEP 6 1988

Findings of Fact:

"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."


In the journal FUNDAMENTAL AND APPLIED TOXICOLOGY, Dr. William Slikker, director of the Neurotoxicology Division of the National Center for Toxicological Research (NCTR), described the health of monkeys exposed to very high levels of cannabis for an extended period:

"The general health of the monkeys was not compromised by a year of marijuana exposure as indicated by weight gain, carboxyhemoglobin and clinical chemistry/hematology values."

(TOXICOLOGY LETTERS, No Increase in Carcinogen-DNA Adducts in the Lungs of Monkeys Exposed Chronically to Marijuana Smoke, 1992, Dec;63 (3): 321-32.

THE ARKANSAS TIMES (Refer Madness. 16 Sept 1993) asked Dr. Merle Paule of NCTR about evidence of cannabis toxicity and the health of the monkeys in the study, Dr. Paule said,

"There's just nothing there. They were all fine."

Myth: Cannabis intoxicates

This is really a matter of semantics, as, strictly speaking, a non-toxic substance cannot 'intoxicate'.

"intoxication" is usually and often detectable simply by a detrimental effect upon motor and cognitive skills; these are covered below.

Myth: Cannabis is addictive

Here we must distinguish between firstly, addictiveness and dependency, and secondly, between medical and psychological dependency.

Medical dependency is not really the issue here, since it is perfectly natural and acceptable for a person to be dependent upon a medicine to ease their suffering, given that the medicine is at least reasonably and acceptably safe.

TRENDS IN PHARMACOLOGICAL SCIENCES: Neurobiology of Marijuana Abuse. 1992, 13:201-206. pg. 203:

"research shows cannabis has limited potential for development of...psychological dependence due to the weak reinforcing properties of Delta-9-THC."

BRAIN RESEARCH JOURNAL: Chronic cannabinoid administration alters cannabinoid receptor binding in rat brain: a quantitative autoradiographic study. 1993, 616:293-302. pg. 300.

"cannabinoid dependence and withdrawal phenomena are minimal."

The Shafer Commission (USA) of 1970 said:

"Marijuana does not lead to physical dependency, although some evidence indicates that the heavy, long-term users may develop a psychological dependence on the drug"

The Panama Canal Zone Military Investigations (US Military, 1929) said:

"There is no evidence that Marihuana as grown and used [in the Canal Zone] is a 'habit-forming' drug."

In 1997, (R. v Clay), Ontario Justice John McCart (Canada) ruled, "Cannabis is not an addictive substance." B.C. Justice F.E. Howard in a similar case confirmed his findings in 1998.

US Department of Health and Human Services, 1991:

"Given the large population of marijuana users and the infrequent reports of medical problems from stopping use, tolerance and dependence are not major issue at present."

("Drug Abuse and Drug Abuse Research, Rockville, MD, (1991) p C3

Myth: Cannabis smoking damages the lungs

Researchers at the University of California (UCLA) School of Medicine have announced the results of an 8 - year study into the effects of long-term cannabis smoking on the lungs. In Volume 155 of the American Journal of Respiratory and Critical Care Medicine, Dr. D.P. Tashkin reported "Findings from the present long-term, follow-up study of heavy, habitual marijuana smokers argue against the concept that continuing heavy use of marijuana is a significant risk factor for the development of [chronic lung disease. ..Neither the continuing nor the intermittent marijuana smokers exhibited any significantly different rates of decline in [lung function]" as compared with those individuals who never smoked marijuana. Researchers added: "No differences were noted between even quite heavy marijuana smoking and non-smoking of marijuana."

Myth: Cannabis suppresses the immune system. Two studies in 1978 and 1988 showed that cannabis actually stimulated the immune system

From: "Exposing Marijuana Myths:(The Lindesmith Center)" "False: Marijuana Impairs Immune System Functioning "It has been widely claimed that marijuana substantially increases users' risk of contracting various infectious diseases. First emerging in the 1970s, this claim took on new significance in the 1980s, following reports of marijuana use by people suffering from AIDS.


"The principal study fueling the original claim of immune impairment involved preparations created with white blood cells that had been removed from marijuana smokers and controls. After exposing the cells to known immune activators, researchers reported a lower rate of transformation in those taken from marijuana smokers.

"However, numerous groups of scientists, using similar techniques, have failed to confirm this original study. "In fact, a 1988 study demonstrated an increase in responsiveness when white blood cells from marijuana smokers were exposed to immunological activators.

"Studies involving laboratory animals have shown immune impairment following administration of THC, but only with the use of extremely high doses. For example, one study demonstrated an increase in herpes infection in rodents given doses of 100 mg/kg/day -- a dose approximately 1000 times the dose necessary to produce a psychoactive effect in humans.

"There have been no clinical or epidemiological studies showing an increase in bacterial, viral, or parasitic infection among human marijuana users. In three large field studies conducted in the 1970s, in Jamaica, Costa Rica and Greece, researchers found no differences in disease susceptibility between marijuana users and matched controls.

"Marijuana use does not increase the risk of HIV infection; nor does it increase the onset or intensity of symptoms among AIDS patients. In fact, the FDA decision to approve the use of Marinol (synthetic THC) for use in HIV-wasting syndrome relied upon the absence of any immunopathology due to THC.

"Today, thousands of people with AIDS are smoking marijuana daily to combat nausea and increase appetite. There is no scientific basis for claims that this practice compromises their immune responses. Indeed, the recent discovery of a peripheral cannabinoid receptor associated with lymphatic tissue should encourage aggressive exploration of THC's potential use as an immune-system stimulant."

Marijuana Myths, Marijuana Facts": Lynn Zimmer Ph.D. and John P. Morgan M.D.: "At the 1981 conference on marijuana sponsored by the World Health Organisation and Canada's Addiction Research Foundation, reviewers of the research literature on immunity reported "There is no conclusive evidence that cannabis predisposes man to immune dysfunction". A few years late, in approving THC (Marinol) for use as a medicine, the FDA found no convincing evidence that THC caused immune impairment. In 1992, the FDA approved Marinol as an appetite stimulant specifically for AIDS patients, who have serious immunosuppression."Marijuana Myths, Marijuana Facts": Lynn Zimmer Ph.D. and John P. Morgan M.D. ISBN 0-9641568-4-9; page 107.Munson and Fehr (1983) note 15, page 338

Food and Drug Administration, "Unimed's Marinol (Dronabinol) Lau, R.J. et al "Phytohemagglutinin-Induced Lymphocyre Transformations in Humans Receiving Delta-9-Tetrahydrocannabinol," Science 192, 805-07 (1976)Dax, EM. Et al., "The Effects of 9_ENE-Tetrahydrcannabinol on Hormone Release and Immune Function," Journal of Steroid Biochemistry 34: 263-70 (1989)Myth: Cannabis causes impotency / infertility

From: "Exposing Marijuana Myths: (The Lindesmith Center)" page 93;"Studies of men in the general population have also failed to find differences in the testosterone levels of marijuana users and nonusers. "There is no convincing evidence of infertility related to marijuana consumption in humans. "There are no epidemiological studies showing that men who use marijuana have higher rates of infertility than men who do not. Nor is there evidence of diminished reproductive capacity among men in countries where marijuana use is common."

Abel, E.L., et al, "Marijuana and Sex: A Critical Survey," Drug and Alcohol Dependence 8: 1-22 (1981)

Ehrenkranz, J.R.L. and Hembee, WC., "Effects of Marijuana on Male Reproductive Function," Psychiatric Annals 16: 243-49 (1986)

Cushman, P, "Plasma Testosterone Levels in Healthy Male Marijuana Smokers," American Journal of Drug and Alcohol Abuse 2: 269-75 (1975)

UK Royal Commission, The Wootton Report, UK, 1968: "Having reviewed all the material available to us we find ourselves in agreement with the conclusion reached by the Indian Hemp Drugs Commission appointed by the Government of India (1893-94)

Indian Hemp Drugs Commission, 1894

"The commission has come to the conclusion that the moderate use of hemp drugs is practically attended by no evil results at all. ... ...moderate use of hemp... appears to cause no appreciable physical injury of any kind,... no injurious effects on the mind... [and] no moral injury whatever."