Adverse health effects of non-medical cannabis use

Wayne Hall & Louisa Degenhardt
The Lancet (Vol 374)
October 17, 2009

publicationFor over two decades, cannabis, commonly known as marijuana, has been the most widely used illicit drug by young people in high-income countries, and has recently become popular on a global scale. Epidemiological research during the past 10 years suggests that regular use of cannabis during adolescence and into adulthood can have adverse effects. Epidemiological, clinical, and laboratory studies have established an association between cannabis use and adverse outcomes.

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We focus on adverse health effects of greatest potential public health interest—that is, those that are most likely to occur and to affect a large number of cannabis users. The most probable adverse effects include a dependence syndrome, increased risk of motor vehicle crashes, impaired respiratory function, cardiovascular disease, and adverse effects of regular use on adolescent psychosocial development and mental health.


Acute adverse effects of cannabis use include anxiety and panic in naive users, and a probable increased risk of accidents if users drive while intoxicated (panel 1). Use during pregnancy could reduce birthweight, but does not seem to cause birth defects. Whether cannabis contributes to behavioural disorders in the offspring of women who smoked cannabis during pregnancy is uncertain.

Chronic cannabis use can produce a dependence syndrome in as many as one in ten users. Regular users have a higher risk of chronic bronchitis and impaired respiratory function, and psychotic symptoms and disorders, most probably if they have a history of psychotic symptoms or a family history of these disorders. The most probable adverse psychosocial effect in adolescents who become regular users is impaired educational attainment. Adolescent regular cannabis users are more likely to use other illicit drugs, although the explanation of this association remains contested. Regular cannabis use in adolescence might also adversely affect mental health in young adults, with the strongest evidence for an increased risk of psychotic symptoms and disorders.

Some other adverse effects are associated with regular cannabis use (panel 2), but whether they are causal is not known because of the possible confounding effects of other drugs (tobacco for respiratory cancers; tobacco, alcohol, and other drugs for behavioural disorders in children whose mothers smoked cannabis during pregnancy). In the case of depressive disorders and suicide, the association with cannabis is uncertain. For cognitive performance, the size and reversibility of the impairment remain unclear. The focus of epidemiological and clinical research should be on clarifying the causative role of cannabis for these adverse health effects.

The public health burden of cannabis use is probably modest compared with that of alcohol, tobacco, and other illicit drugs. A recent Australian study estimated that cannabis use caused 0·2% of total disease burden in Australia—a country with one of the highest reported rates of cannabis use. Cannabis accounted for 10% of the burden attributable to all illicit drugs (including heroin, cocaine, and amphetamines). It also accounted for around 10% of the proportion of disease burden attributed to alcohol (2·3%), but only 2·5% of that attributable to tobacco (7·8%).