Is smoking marijuana dangerous? A review of research over the last 20 years says it is

Assessing the risk of cannabis using the typical medical model of a control in trials is difficult as people are too aware of the effects of the drug

In England, four per cent of 11–15 year olds said they had used cannabis in the past month
In England, four per cent of 11–15 year olds said they had used cannabis in the past month

A newly published paper reminds health professionals, policy makers, and the public, of the potential acute and long-term risks of cannabis use.

In his report, Professor Wayne Hall at The University of Queensland Centre for Youth Substance Abuse Research, considers how the evidence has evolved over the past 20 years. He points out potential risks, both in the short term, and higher risks for three settings: long-term heavy use, use in adolescence, and during pregnancy.

Assessing the severity of risk from cannabis using the typical medical model of randomized control trials is difficult both ethically and in practice. Finding a fair placebo or other control is challenging both due to the euphoriant effect of cannabis and its well-known effect of causing ‘the munchies’. Short-term studies of any drug on pain are complicated by the well-recognized placebo effect on pain.

Similarly, longer term controlled studies needed to assess frequency of effects on risk of harm to the baby, cancer and mental disorders are ethically and clinically impractical. There are also obvious risks of relying on a classic tool of epidemiology – the prospective observational study i.e. comparing people who do or do not use cannabis. Many other aspects of lifestyle may be different in cannabis users, including level of tobacco and alcohol use, and use of other recreational drugs, all of which may contribute to observed risks associated with cannabis use.

Hall is therefore careful to make clear in his assessments the importance of taking into account associated lifestyle behavior and other explanations for risk. For example, he acknowledges that early concern about risk of lung damage from smoking cannabis is difficult to disentangle from associated tobacco use.

Hall’s report has led to published reaction from some users of cannabis that it is not harmful. However cannabis is a powerful drug and the use of any powerful drug is associated with risk of harm. The major bioactive chemical in the cannabis plant has been isolated as tetrahydrocannabinol (THC). The potential benefits of mimicking some of the beneficial effects of cannabis has led to major international efforts by biomedical researchers to understand the effects of THC. Thus to develop medically useful drugs to modulate the cell receptors in the brain and elsewhere in the body at which THC acts.

In the case of cannabis, obvious effects are on mood, behavior and appetite. Consumers of cannabis typically use it for recreational purposes. Some patients may use the drug for possible medical benefits – for example to reduce spasms in severe neurological disorders, and with the aim of improving quality of life during palliative care for advanced medical conditions. Adverse effects of cannabis may result from direct effects of tetrahydrocannabinol. They also may result from other chemicals in cannabis, and may occur indirectly due to damage caused by harmful chemicals inhaled when cannabis is smoked.

Decreased levels of anxiety and arousal resulting from cannabis use are in turn associated with reduced concentration, and hence risk of accidents when operating machinery, most obviously for drivers. Hall reports data that cannabis use around doubles the risk of fatal and non-fatal car accidents, stressing that this acute risk is even greater when alcohol is consumed at the same time. That adds cannabis to the category of recreational drugs that may not only harm users, but may also lead to injury or death of others through serious accidents.

Other effects depend on route of use of the drug. Cannabis when smoked generates carbon monoxide, which reduces oxygen supply to the tissues. This may be one of several reasons for reduced baby size in pregnant women who are regular users of smoked cannabis. Other reasons Hall identified included poorer maternal nutrition in cannabis users and reduced engagement with obstetrics services. He also notes two separate effects of cannabis on adolescence: Pre-natal exposure to cannabis may lead to impaired academic development at least to the age of 14; and persistent use of cannabis during adolescence is associated with leaving school at an early age and other markers of impaired educational attainment.


Other chemical components of the smoke generated when the plant is smoked are likely to contribute to some of the long term risks of cannabis use, including an increased risk of lung cancer of eight per cent for each joint-year of cannabis smoking.

Psychiatric disorders associated with cannabis use may have several explanations. These include a direct consequence of overstimulation of natural brain receptors for cannabis – the cannabinoid receptors. An obvious confounding explanation in some patients is reverse causation – patients with dysphoric symptoms due to already incipient psychiatric illness, such as schizophrenia, may turn to use of recreational drugs to relieve their mental malaise, only to develop obvious clinical psychiatric illness.

Donald Singer, Professor of Clinical Pharmacology and Therapeutics, Yale School of Medicine

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