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Is cannabis harmful to mental health? Within the scientific community, there is consensus that most people who use cannabis will not develop significant mental health problems. Certain individuals, however, are more susceptible to negative effects.

It was long thought cannabis was a relatively harmless drug, and concerns about its use were overstated. Some psychiatrists reported that excessive use could lead to a psychotic state, including hallucinations, delusions, and thought disturbance.

But the first significant indication of a link between cannabis use and psychotic illness came only in 1987, from a large Swedish study that followed more than 50 000 subjects over 15 years. Reported cannabis use at the study’s start increased the likelihood of being diagnosed with schizophrenia in the next 15 years.

This finding did not spur much interest, and no similar studies were reported until 2002. Since then, studies have explored the association between cannabis and psychotic illness. In 2007, various studies concluded that daily cannabis use doubles the risk of psychosis.

A positive correlation between cannabis use and psychosis does not mean a direct causal link. Psychosis could cause cannabis use, rather than vice versa, or an unknown mediating factor could cause both cannabis use and psychosis. Other factors complicate the relationship. For example, the age at which cannabis use begins is important. People who started using it before the age of 16 have a higher risk of psychosis. Evidence says the maturing brain is more susceptible to the negative impact of cannabis.


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Research has shown that a particular variant of a gene called AKT1 mediates the risk of psychosis. For carriers of the C/C variant (roughly 20% of the population), the risk of psychosis increased seven-fold but only for the people who used cannabis every day. Different strains of cannabis pose more or less risk. A comparison of the two most common types of cannabis in Britain, hash and sinsemilla (“skunk”), found that using skunk implied a significant greater risk of psychosis, while hash did not.

Studies have explored the role of cannabis use in disorders such as depression and anxiety. While little sign of a link has been found, there is evidence for the addictive potential of cannabis. Roughly 10% of people who smoke cannabis develop dependence, which produces withdrawal symptoms when use is stopped.

It is generally accepted that frequent and prolonged cannabis use impairs cognitive functioning, but that these effects are reversible following abstinence of three months.

However, a recent study that followed people from birth to age 38 found that those who started using cannabis early, every day, and for several years had a permanent eight-point drop in IQ scores.

Much depends on how old you are when you start, your genes, and how much, how often, and what kind you use – and whether you get caught.

Amir Englund is a researcher at King’s College London. Robin MacGregor Murray is Professor of Psychiatric Research at the Institute of Psychiatry, King’s College London. www.project-syndicate.org


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