Health effects of cannabis

Every major investigation held about cannabis has found that the harms presented by cannabis use have been exaggerated, and that prohibition creates more harm than it prevents.

Even in 2019 we hear that “cannabis effects the brain” and this is used to justify an age limit of 20, shutting out adults aged 18 and 19 from being able to access cannabis legally and forcing them to obtain cannabis from those who operate outside the law. The truth is that the latest science debunks claims that cannabis significantly harms the brain. It’s also worth noting that the lead researchers for both the Dunedin Longitudinal Study and the Christchurch Health and Development Study, Professors Richie Poulton and David Fergusson respectively, both support cannabis law reform as both acknowledge any harms from cannabis are made worse by its prohibition.

In particular, the claim that the medical and psychological effects of Cannabis are so “dangerous and harmful” that we must not change the cannabis laws, has increasingly been seen to be unsupportable, particularly in light of the known effects of tobacco and alcohol. As the 1979 Sackville Royal Commission into the Non-Medical Use of Drugs in South Australia found:

… even a cursory glance at the modern history of Cannabis shows a repeated pattern of widely believed myths which often fly in the face of the available evidence. It seems as discredited beliefs (such as Cannabis being an addictive narcotic causing violent crime and insanity) are rejected, they are replaced by new myths (for example, that even casual use carries serious health risks to the user)… It is apparent that the debate has been more concerned with values and community attitudes than with the objective ascertainment of facts

The World Health Organisation in 1998 released a report that agreed with the New Zealand Ministry of Health’s report Cannabis: the public health issues 1995-1996 in acknowledging that the consumption of alcohol and tobacco are more harmful than the use of cannabis.

The United States Institute Of Medicine in March 1999 released a report rejecting the ‘gateway’ theory that says cannabis leads to the use of harder drugs, recognised that cannabis is less harmful than alcohol or tobacco, and acknowledged that cannabis can be a useful medicine for many people.

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

Relatively few adverse clinical effects from the chronic use of marijuana have been documented in humans. However, the criminalization of marijuana use may itself be a health hazard, since it may expose the users to violence and criminal activity.

“Marijuana Use and Mortality” April 1997 American Journal of Public Health

There are no long lasting ill-effects from the acute use of marijuana and no fatalities have ever been recorded … there seems to be growing agreement within the medical community, at least, that marijuana does not directly cause criminal behaviour, juvenile delinquency, sexual excitement, or addiction.

Dr J. H. Jaffe, The Pharmacological Basis of Therapeutics. L.Goodman and A Gillman, eds. 3rd edn. 1965.

Cannabis is remarkably safe. Although not harmless, it is surely less toxic than most of the conventional medicines it could replace if it were legally available. Despite its use by millions of people over thousands of years, cannabis has never caused an overdose death.

Professor Lester Grinspoon, M.D., Associate Professor of Psychiatry, Harvard Medical School, before the Crime Subcommittee of the Judiciary Committee, U.S. House of Representatives, Washington, D.C., October 1, 1997


In over 10,000 years of documented and widespread use, there has never been a documented proven death resulting from cannabis use. Hundreds of thousands of New Zealanders use cannabis regularly, and if it were as toxic as some prohibitionists purport, our hospitals would be full of cannabis patients. Instead, considering the prevalence of use, they are conspicuous by their absence.

THC is non-toxic because it precisely fits into a specific neurotransmitter system rather than interrupting or interfering with chemical reactions in the nervous system. This is not to say that THC does not effect parts of the body, but that it does not damage the brain or body.

THC plugs into ‘anandamide’ receptors, which have only recently been discovered and located in areas of the brain that cannabis has long been known to effect – the higher thinking processes, emotions, perceptions, motor coordination, short term memory, plus the CB1 receptors of the immune system, the reproductive organs, and the lungs.

Cannabis has long been used by creative people as a source of inspiration and to enhance sensory experiences. It has long been a useful and effective medicine, and we are only now discovering why. Cannabis is helpful to people with neurological disorders such as Multiple Sclerosis because anandamide affects motor coordination. It is useful for people suffering AIDS wasting syndrome and nausea associated with cancer treatments because anandamide stimulates the feeding response and suppresses nausea. Cannabis is useful for glaucoma because anandamide regulates eye pressure. Cannabis is useful for asthmatics because anandamide triggers a coughing response in the lungs. Cannabis can help some people with immune disorders such as AIDS by stimulating certain parts of the immune system mediated by the newly-discovered CB1 receptors.

The ruling by US Drug Enforcement Administration Judge Francis Young, in response to a petition by NORML in the USA to reschedule marijuana from Schedule 1 to 2 (to allow marijuana to be prescribed as a medicine) was the largest and most comprehensive study yet undertaken into marijuana’s toxicity and medical efficacy. He found:

  1. 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.
  2. 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.
  3. By contrast aspirin, a commonly used, over-the-counter medicine, causes hundreds of deaths each year.
  4. 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.
  5. 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.
  6. In practical terms, marijuana cannot induce a lethal response as a result of drug-related toxicity.

“Opinion and recommended ruling, findings of fact, conclusions of law and decision of DEA administrative law judge”, Sept 6 1988

Marijuana is one of the least toxic substances in the whole pharmacopoeia

Professor Lester Grinspoon, Harvard Medical School, USA

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.

There’s just nothing there. They were all fine.

“No Increase in Carcinogen-DNA Adducts in the Lungs of Monkeys Exposed Chronically to Marijuana Smoke”, 1992, Fundamental And Applied Toxicology, Dec 63 (3): 321-32.

The Merck Manual of Diagnosis and Therapy (1987) after reviewing an extensive body of research, concluded that “the chief opposition to the drug rests on a moral and political, and not toxicologic, foundation.”

The Lungs

The vast majority of cannabis smokers do so moderately, responsibly and infrequently. The main harm facing moderate cannabis users results from the delivery mechanism. Smoking is likely to be harmful to the lungs, although cannabis itself is non-toxic and does not damage the lungs like tobacco does.

Smoking enough of any plant matter is likely to be harmful to the lungs. Burning plant cellulose produces tars that include some carcinogenic substances, carbon monoxide, hot gases, and ash and solid particulates.

Cannabis smoke is not the same as the smoke produced from other plants such as tobacco.
Cannabis smokers consume far less smoke than tobacco users. Each joint is on average half the weight of a tobacco cigarette, and marijuana smokers rarely smoke as much as a tobacco user. Thus, the amount of irritant material inhaled almost never approaches that of tobacco users.

Users in our matched-pair sample smoked marijuana in addition to as many tobacco cigarettes as did their matched non-using pairs. Yet their small airways were, if anything, a bit healthier than their matches. We must tentatively conclude either that marijuana has no harmful effect on such passages or that it actually offers some slight protection against harmful effects of tobacco smoke

Cannabis in Costa Rica: A Study of Chronic Marijuana Use; Institute of Human Issues.

Researchers at the University of California (UCLA) School of Medicine conducted an 8-year study into the effects of long-term cannabis smoking on the lungs. 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… No differences were noted between even quite heavy marijuana smoking and non-smoking of marijuana.

American Journal of Respiratory and Critical Care Medicine, Volume 155

Some critics exaggerate the dangers of marijuana smoking by fallaciously citing a study by Dr. Tashkin which found that daily pot smokers experienced a “mild but significant” increase in airflow resistance in the large airways greater than that seen in persons smoking 16 cigarettes per day. What they ignore is that the same study examined other, more important aspects of lung health, in which marijuana smokers did much better than tobacco smokers. Dr. Tashkin himself disavows the notion that one joint equals 16 cigarettes.

A more widely accepted estimate is that marijuana smokers consume four times as much carcinogenic tar as cigarettes smokers per weight smoked. This does not necessarily mean that one joint equals four cigarettes, since joints usually weigh less. In fact, the average joint has been estimated to contain around half a gram of cannabis, or around half the weight of a cigarette, making one joint equal to two cigarettes.

It should be noted that there is no exact equivalency between tobacco and marijuana smoking, because they affect different parts of the respiratory tract differently: whereas tobacco tends to penetrate to the smaller, peripheral passageways of the lungs, cannabis tends to concentrate on the larger, central passageways. One consequence of this is that cannabis, unlike tobacco, does not appear to cause emphysema.

Whatever the risks of cannabis smoking, the current laws make matters worse in several ways: (1) Recently enacted paraphernalia laws prevent the development and marketing of water pipes and other, more advanced technology that could significantly reduce the harmfulness of marijuana smoke; (2) prohibition encourages the sale of cannabis that has been contaminated or adulterated by insecticides, Paraquat, etc., or mixed with other drugs such as PCP, crack, heroin and even fly spray; (3) by raising the price of marijuana, prohibition makes it uneconomical to consume marijuana orally, the best way to avoid smoke exposure altogether; this is because eating typically requires two or three times as much marijuana as smoking. While many smokers would prefer to eat or drink cannabis preparations rather than smoke them, current laws state that any “cannabis preparation” is a Class B prohibited substance that carries a more severe punishment. The high price of cannabis also means there is usually no filter included in a joint, and smokers tend to inhale deeply and hold the breath in to get the maximum effect. If cannabis was sold at it’s true worth – for example, tobacco retails for less than $30 per ounce, including a two-thirds tax to the Government – smokers would not be forced to smoke in such a harmful way.


Health promotion strategies to cannabis users should encourage eating or drinking cannabis preparations, and the use of harm minimisation equipment such as vapourisers and waterpipes. Cannabis paraphernalia designed to minimise the harmful effects of smoking should be re-legalised by repealing the Misuse of Drugs (Prohibition of Cannabis and Methamphetamine Utensils) Notice 2003.

Addiction and Dependence

Cannabis is not addictive. Most people who smoke cannabis do so only occasionally. A small minority of New Zealanders – less than 1 percent – smoke cannabis on a daily or near daily basis. An even smaller minority develop a psychological dependence on marijuana. Most people who smoke marijuana heavily and frequently stop without difficulty. Some seek help from drug treatment agencies. Marijuana does not cause physical dependence, and if people experience withdrawal symptoms at all, they are remarkably mild.

There are many commonly used medicines that are very addictive, and the two legal drugs alcohol and tobacco are both addictive substances that together kill thousands of people every year. A June 1999 report by the Ministry of Health to the Ministerial Committee on Drug Policy stated that 224,000 New Zealanders are dependent on alcohol, and between 13,500 and 26,000 people are dependent on drugs such as heroin, morphine and homebake (opioids) in any six-month period.

A small minority of cannabis users can become dependent on cannabis. They need help in the form of treatment and counselling, rather than punishment. It makes no sense to punish other responsible cannabis users who are not dependent, merely to “send a message” to the few that have become dependent that they should stop being so.

There is a huge amount of research that proves the myth that cannabis is addictive to be at best a mistaken belief, or at worst an outright lie used to justify current policies:

Cannabis can be used on an episodic but continual basis without evidence of social or psychic dysfunction. In many users the term dependence with its obvious connotations, probably is mis-applied…

The USA Merck Manual of Diagnosis and Therapy (1987)

cannabinoid dependence and withdrawal phenomena are minimal.

Chronic cannabinoid administration alters cannabinoid receptor binding in rat brain: a quantitative autoradiographic study, 1993, Brain Research Journal, 616:293-302. p300.

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

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

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 Shafer Commission (USA) of 1970

research shows cannabis has limited potential for development of…psychological dependence

Neurobiology of Marijuana Abuse, 1992, Trends In Pharmacological Sciences, 13:201-206, p203

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, US Department of Health and Human Services, Rockville, MD, (1991) pC3

Cannabis is not an addictive substance.

R. v Clay ruling of Ontario Justice John McCart, 1997.

While it is true that THC and other cannabinoids are fat-soluble and linger in the body for prolonged periods, they do not normally affect behaviour beyond a few hours except in chronic users, and this lingering effect is why cannabis has no noticeable withdrawal symptoms. Upon stopping, cannabis users are ‘weaned’ off THC, whereas an opiate or nicotine addict suffers an abrupt and intense physical withdrawal.

Most impairment studies have found that the adverse effects of acute marijuana use wear off in 2-6 hours, commonly faster than alcohol. The one notable exception was a pair of flight simulator studies by Leirer, Yesavage, and Morrow, which reported effects on flight simulator performance up to 24 hours later. The differences, described by Leirer as “very subtle” and “very marginal,” were less than those due to pilot age. Another flight simulator study by the same group failed to find any effects beyond 4 hours. Similar “hangover” effects have been noted for alcohol. Chronic users may experience more prolonged effects due to a build-up of cannabinoids in the tissues. Some heavy users have reported feeling effects weeks or even months after stopping. However, there is no evidence that these are detrimental to safety.

Cognitive Function

This issue has been thoroughly investigated by the Health Committee for its 1998 report Mental Health Effects of Cannabis. The report concluded:

The weight of available evidence suggests that long-term heavy use of cannabis does not produce severe or gross impairment of cognitive function.

This finding is consistent with the findings of other major investigations into cannabis:

There were no significant differences in cognitive decline between heavy users, light users, and non-users of cannabis. There were also no male-female differences in cognitive decline in relation to cannabis use. The authors conclude that over long time periods, in persons under age 65 years, cognitive decline occurs in all age groups. This decline is closely associated with ageing and educational level but does not appear to be associated [with cannabis use]

Lyketsos et al, Cannabis Use and Cognitive Decline in Persons under 65 Years of Age, Johns Hopkins University School of Hygiene and Public Health, American Journal of Epidemiology, May 1999, 149:794-800

Some participants had smoked at least two to four large cigarettes (each containing 1/4 to 1/2 ounce of cannabis) over 16 hours a day for periods of up to 50 years … the most impressive thing… is the true paucity of neurological abnormalities.

Ethiopian Zion Coptic Church Study, 1980

Heavy cannabis consumers suffered no apparent psychological or physical harm… IQ’s of Zion Coptics increased after they began to use ganga

Schaeffer: A Neuropsychological Evaluation; A Case History

There were no indications of organic brain damage or chromosome damage among smokers and no significant clinical psychiatric, psychological or medical differences between smokers and controls.

US Jamaican Study 1970

No impairment of physiological, sensory and perceptual performance, tests of concept formation, abstracting ability, and cognitive style, and tests of memory

US Jamaican Study 1974

Mental Illness

Although cannabis may exacerbate existing conditions in pre-disposed individuals, it is not itself a cause of mental illness. The incidence of schizophrenia has declined during a period in which cannabis use has increased among young adults.

This issue has been thoroughly investigated by the Health Committee for it’s 1998 report Mental Health Effects of Cannabis. The report concluded:

Based on the evidence we have heard in the course of this inquiry, the negative mental health impact of cannabis appears to have been overstated, particularly in relation to occasional adult users of the drug.

This is line with the findings of other major investigations into cannabis. The LaGuardia Commission Report of 1944 concluded:

Cannabis smoking] does not lead directly to mental or physical deterioration… Those who have consumed marijuana for a period of years showed no mental or physical deterioration which may be attributed to the drug

The impact of cannabis prohibition on mental health: Under the current policy, cannabis is readily accessible to those with a mental illness. Schizophrenia is associated with lower levels of anandamide, and many patients are able to self-medicate with cannabis to in effect boost their levels of THC/anandamide. The mental health of all cannabis users are threatened more by the current law than by the use of cannabis itself. The criminal law generates paranoia, suspicion, alienation and anxiety.


Cannabis use is a symptom of ‘deviant’ behaviour, not a cause. Cannabis use at school is likely to impact on learning abilities. However while some are quick to point the blame at cannabis itself, it should be noted that the free availability of cannabis to adolescents is occurring now under a system of total prohibition. Despite – or because of – the law, cannabis is just as available as pizza, and more available to minors than alcohol.


The use of cannabis by young people should be minimised by regulating the availability of cannabis with a minimum purchase age of 18 years.


The concept of a marijuana amotivational syndrome first appeared in the late 1960s, as marijuana use was increasing among youth. Large-scale studies of high school students have generally found no difference in grade-point averages between marijuana users and non-users. Field studies in Jamaica, Costa Rica and Greece also found no evidence of an amotivational syndrome marijuana-using populations. The weight of scientific evidence suggests that there is nothing in the pharmacological properties of cannabis that alter people’s attitudes, values, or abilities regarding work.

No scientific evidence has been gathered to demonstrate … the development of an amotivational syndrome amongst users.

Health Committee, Mental Heath Effects of Cannabis 1998 p43

…a-motivation [is] a cause of heavy marijuana smoking rather than the reverse

Dr. Andrew Weil, cited in Rubin & Comitas, Ganja in Jamaica, 1975

Cannabis … does not cause a motivational syndrome.

R. v Clay, Ontario Justice John McCart (Canada), 1997.


Cannabis is associated with suicide and other ‘deviant’ behaviours only through correlation, not causation. People who attempt or commit suicide have a higher rate of using cannabis, but that does not mean cannabis has caused their distressed state. Rather, cannabis may be a therapeutic tool that offers some escape. The Health Committee’s 1998 report Mental Health Effects of Cannabis concluded:

Data collected by the Canterbury Suicide Project found that rates of cannabis abuse were higher amongst those making serious suicide attempts. However, further analysis suggested that again the involvement of cannabis was by association, as opposed to causation. Individuals who were predisposed towards a suicide attempt through a disadvantaged socio-demographic background or mental illness were also more likely to use cannabis. Evidence suggests that cannabis use is not a causal factor in suicide.

Blood Pressure and the Heart

According to the US National Academy of Sciences, the effects of marijuana on blood pressure are complex, depending on dose, administration, and posture. Marijuana often produces a temporary, “moderate” increase in blood pressure immediately after ingestion; however, heavy chronic doses may slightly depress blood pressure instead.

One common reaction is to cause decreased blood pressure while standing and increased blood pressure while lying down, causing people to faint if they stand up too quickly. There is no evidence that pot use causes persisting hypertension or heart disease; some users even claim that it helps them control hypertension by reducing stress. One thing THC does do is to increase pulse rates for about an hour, a condition known as tachycardia. This is not generally harmful and may even be beneficial since exercise does the same thing.

Tissue Damage

THC is actually chemoprotective and neuroprotective. THC may even be a cure for cancer. Last February researchers in Madrid announced they had destroyed incurable brain cancer tumours in rats by injecting them with THC, the active ingredient in cannabis. Most people don’t know anything about the discovery, since virtually no newspapers carried the story, but this isn’t the first time scientists have discovered that THC shrinks tumours.

In 1974, researchers at the Medical College of Virginia had been funded by the US National Institutes of Health to find evidence that marijuana damages the immune system. Instead, they found that THC slowed the growth of three kinds of cancer in mice–lung and breast cancer and a virus-induced leukaemia. The US Government quickly shut down the Virginia study and all further cannabis/tumour research.

Not only is the evidence linking cannabis smoking to cancer negative, but the largest human studies cited indicated that cannabis users had lower rates of cancer than non-users. What’s more, those who smoked both cannabis and tobacco had lower rates of lung cancer than those who smoked only tobacco-a strong indication of chemoprevention.

Marijuana Use and Mortality, American Journal Of Public Health, April 1997

The Immune System

It has been claimed that marijuana increases users’ risk of contracting infectious diseases. First emerging in the 1970s, this claim took on new significance following reports of medical marijuana use by people suffering from AIDS.

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.

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 later, 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.


Anti-drug campaigners often claim that children are permanently harmed by their mothers’ use of cannabis during pregnancy. It is claimed that cannabis is a cause of birth defects and development deficits. A number of studies have claimed reported low birth weight and physical abnormalities among babies exposed to marijuana in utero. However, when other factors known to affect pregnancy outcomes were controlled for – for example, maternal age, socioeconomic class, and alcohol and tobacco use – the association between marijuana use and adverse foetal effects disappeared.

Other studies have failed to find negative impacts from marijuana exposure. However, when negative outcomes are found, they tend to be widely publicised, regardless of the quality of the study. The Health Committee’s 1998 report Mental Health Effects of Cannabis found:

No conclusive evidence exists to demonstrate deleterious effects of cannabis use upon foetal mental development… The most recent review of literature on the effects of cannabis use on the foetus found that cannabis has no reliable impact on birth size, length of gestation, neurological development, or the occurrence of physical abnormalities. [cited: Zimmer & Morgan 1997]

The report also noted the research of Professor Richard Faull of the Anatomy Department of the University of Auckland, who informed the Committee of the results of his recent studies of cannabinoid receptors in the human brain. Cannabis, or more specifically THC, does have the potential to have a greater effect on the brain of the foetus than the adult. This does not mean damage.

THC safely plugs into anandamide receptors in the brain. These are located in areas of the brain that among other things influence emotions, perceptions, and the feeding response. All mammals have anandamide receptors. It may be no evolutionary accident that the foetus has a higher proportion of anandamide receptors than adults. It may be that the presence of additional anandamide receptors helps the foetus cope with the traumatic experience of being born.

Although we believe it is sensible to advise pregnant women to abstain from using drugs – including cannabis – without their doctor’s supervision, the weight of scientific evidence indicates that cannabis has few adverse consequences for the developing human foetus.


There have been no epidemiological studies which have shown increased infertility in marijuana-using humans, and studies of overall reproductive rates have found no reduction in reproductive rates in countries where a higher rate of marijuana use is found.

Studies of men in the general population have also failed to find differences in the testosterone levels of marijuana users and non-users… 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.

Exposing Marijuana Myths, The Lindesmith Center, p93


The adverse effects of alcohol on driving performance are well documented, and opponents of cannabis use often claim that cannabis produced similar impairment.

Research into impairment and traffic accident data from several countries by the University of Toronto indicates marijuana, when consumed alone, “does not significantly increase a driver’s risk of causing an accident – unlike alcohol.” Both alcohol and cannabis affect driving performance but those who smoke marijuana tended to be much more cautious behind the wheel due to the heightened awareness of their impairment. By comparison, “subjects who received alcohol tend to drive in a more risky manner.”

A 1998 study by the University of Adelaide and the South Australian Transport Board found that “there is no evidence of any increase in the likelihood of being culpable for crashes amongst those injured drivers in whom cannabinoids were detected.”

In most cases where cannabis has been detected in the bloodstream of drivers involved in crashes, alcohol has also been present.

Professor Olaf Drummer, a forensic scientist the Royal College of Surgeons in Melbourne said in 1996 “Compared to alcohol, which makers people take more risks on the road, marijuana made drivers slow down and drive more carefully…. Cannabis is good for driving skills, as people tend to overcompensate for a perceived impairment.”

Simulated driving scores for subjects experiencing a normal social ‘high’ and the same subjects under control conditions are not significantly different. However, there are significantly more errors for alcohol intoxicated than for control subjects

Crancer Study, Washington Department of Motor Vehicles.

THC’s adverse effects on driving performance appear relatively small

U.S. Department of Transportation, National Highway Traffic Safety Administration (DOT HS 808 078), Final Report, November 1993

There is no controlled epidemiological evidence that cannabis users are at increased risk of being involved in motor vehicle or other accidents.

Australian Institute of Criminology Report, 1996, page 6


There has never been a single, controlled scientific study showing drug urinalysis improves workplace safety. Claims that drug testing works are based on dubious anecdotal reports or the mere observation of a declining rate of drug positives in the working population, which has nothing to do with job performance.

The few scientific studies that have been conducted have found little difference between the performance of drug-urine-positive workers and others. The largest survey to date, covering 4,396 postal workers nationwide, found no difference in accident records between workers who tested positive on pre-employment drug screens and those who did not.

The American Management Association recently surveyed the scientific evidence related to workplace drug testing, and found no significant benefit from the practise.

There is no evidence that cannabis prohibition reduces the risk of accidents. On the contrary, recent studies suggest that marijuana may actually be beneficial in that it substitutes for alcohol and other, more dangerous drugs.

Research by Karyn Model found that US states with decriminalisation had lower overall drug abuse rates than others; another study by Frank Chaloupka found decriminalised states have lower accident rates. In Alaska, accident rates held constant or declined following the legalisation of personal use of marijuana.

Crime and Violence

Cannabis use is not a cause of crime, other than use itself has been deemed a crime. Far from making cannabis users violent or inclined to commit crimes, cannabis tends to make its users passive and want to stay at home. The only link between cannabis and criminal activity is the criminal law.

Prohibition puts otherwise law-abiding citizens in contact with the criminal underworld with whom they must interact in order to purchase cannabis.

Prohibition also makes it more likely that a cannabis user may break other laws, because it brings the entire criminal justice system into disrepute, and alienates cannabis users from the rule of law. Once they have broken one law, which is blatantly unjust and unfair, they may become more likely to break other laws. Once cannabis users have a criminal record for cannabis use, the deterrent to not commit other crimes may not be so strong.

The Health Committee’s 1998 report Mental Health Effects of Cannabis commented:

Research, including that conducted by the 1972 Shafer Commission in the United States, has shown that cannabis itself does not induce violent behaviour. Recent studies using experimental controls to exclude pre-existing social factors have confirmed that cannabis does not stimulate violence. Instead cannabis was found to induce a sedative effect during the period of intoxication.

Every major study of cannabis has found no link between cannabis and crime or violent behaviour, including the Jamaican Study of 1970:

This study indicates that there is little correlation between the use of ganga and crime, except insofar as the possession and cultivation of ganga are technically crimes

The LaGardia sub-committee of New York 1944 said:

Marijuana is not the determining factor in the commission of major crime….The publicity concerning the catastrophic effect of marijuana smoking in New York City, is unfounded


The claim that there has been a 10-, 20- or 30-fold increase in marijuana potency since the 1970s is often used to discredit previous studies that showed minimal harm caused by the drug and convince users from earlier eras that today’s marijuana is much more dangerous. It is a myth based on biased government data.

Samples of cannabis from the early ’70s came from low-potency Mexican “kilobricks” whose potency had deteriorated to non-psychoactive levels of less than 0.5%. These were then compared to more recent samples of decent-quality cannabis, making it appear that potency had skyrocketed. The US Government’s own data show that average marijuana potency increased modestly by a factor of two or so during the seventies, and has been more or less constant ever since.

Mean Percentage THC of Seized Marijuana, USA, 1981-1993
Mississippi Potency Monitoring Project

Year % of THC
1981 2.28
1982 3.05
1983 3.23
1984 2.39
1985 2.82
1986 2.30
1987 2.93
1988 3.29
1989 3.06
1990 3.36
1991 3.36
1992 3.00
1993 3.32

The Health Committee’s 1998 report Mental Health Effects of Cannabis commented:

The DPFT, the Police and the Ministry stated that the potency of cannabis had not increased significantly over time… The Institute of Environmental Science and Research (ESR) provided information showing that the THC levels in New Zealand grown cannabis are not high by international standards.

In fact, there is nothing new about high-potency pot. During the sixties, it was available in premium varieties such as buddha sticks, as well as hashish and hash oil, which were just as strong as today’s seedless heads, but were ignored in government statistics. While the average potency of domestic pot did increase with the development of sinsemilla in the seventies, the range of potencies available has remained virtually unchanged since the last century, when extremely potent tonics were sold over the counter in pharmacies. In Holland, high-powered hashish and sinsemilla are currently sold in coffee shops with no evident problems.

Contrary to popular myth, greater potency is not necessarily more dangerous, due to the fact that users tend to adjust (or “self-titrate”) their dose according to potency. Even if potency had increased slightly since the 1970s, it would not mean that smoking marijuana had become more dangerous. In fact, since the primary health risk of marijuana comes from smoking, and because THC itself is non-toxic, higher potency products are less harmful because they allow people to achieve the desired effect by inhaling less smoke.

The ‘Gateway’ or ‘Stepping Stone’ Hypothesis

Cannabis does not cause people to use hard drugs. Cannabis is the most popular illegal drug in New Zealand, so people who have used less popular drugs, such as heroin, cocaine, and LSD, are likely to have also used marijuana, just as they are also likely to have used alcohol, tobacco, caffeine, and watched television. Correlation does not imply causation. Most marijuana users never use any other illegal drug. For the vast majority of people, marijuana is a terminus rather than a gateway drug.

The assumption that cannabis consumers run a higher risk of switching to hard drugs, especially heroin, is known as ‘the stepping-stone hypothesis’. This idea was first put forward in the forties in the USA and has since greatly influenced public opinion and drug policies. There is no inevitable relationship between the use of marijuana and other drugs. In the Netherlands, for example, although marijuana use among young people rose in the early 1990’s, cocaine use decreased. The Dutch policy of allowing marijuana to be purchased openly in government-regulated “coffee shops” was designed specifically to separate young marijuana users from illegal markets where heroin and cocaine are sold.

There is no physically determined tendency towards switching from soft to harder substances. Drug policy, however, appears to play a role. The more users become integrated in a subculture where hard drugs as well as cannabis can also be obtained, the greater the chance that they may switch to hard drugs.

The use of marijuana does not lead to morphine or heroin or cocaine addiction and no effort is made to create a market for these narcotics by stimulating the practice of marijuana smoking

The LaGardia sub-committee of New York 1944

Most marijuana users do not go on to use other drugs

Marijuana: Facts for Teens. U.S. Department of Health and Human Services. Washington, D.C. 1995, p.10.

Recommendation: Separation of the drug markets is essential and therefore should form the basis of the current cannabis policy.

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