Solving the puzzle of whether cannabis is associated with, or causes, psychosis.

A recent issue of the American Journal of Psychiatry (January 1, 2022) contained new research data and an incisive editorial that advances our understanding of whether cannabis can cause, or is merely associated with, psychosis.

The association of cannabis with psychosis was first noted by the Scottish psychiatrist Thomas Clouston after visiting the Cairo asylum in 1896, where 40 out of the 253 patients had insanity attributed to hashish. Studies since then have conclusively documented cannabis’s association with schizophrenia-like psychosis.

For example, in 1987, 45,750 Swedish army conscripts were asked about their drug use. Those who had used cannabis more than 50 times were six times more likely to develop schizophrenia over the next 15 years than those who had never used it.

(1) Analysis of multiple research studies shows an average two-fold increase in psychotic outcomes for typical cannabis users and an almost four-fold increase for the heaviest users compared to non-users, though this remains only an association between cannabis and schizophrenia and not proof of causation.

(2)Those who use very high potency marijuana (e.g., “skunk”) on a daily basis have been found to be five times more likely than non-users to suffer from psychotic disorders.

(3) On the other hand, a Dutch survey found that those who prefer cannabis with the highest CBD content do not experience as great an increase in psychotic experiences, which appears to demonstrate the protective nature of CBD.

(4)The question remains whether cannabis causes schizophrenia, schizophrenics are more likely to use cannabis, or does some more fundamental genetic condition increase the risk of both cannabis use and schizophrenia? Recent data from Canada may have begun to tease apart the chicken-and-egg problem of causality by supporting the idea that all young cannabis users are put at increased risk of psychosis.