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.
An annual survey of 3,720 adolescents obtained self-reports of past-year cannabis use and psychotic symptoms over four years, from age 13 to 16. The findings demonstrated a clear association of cannabis use frequency with increased psychotic symptoms, and not vice versa. Cannabis use in any given year was found to predict an increase in psychotic symptoms a year later, and not the other way around. (5) What’s more, another new study of nearly 80,000 members of the general American population shows that those with cannabis use disorder during the previous year have a 2.5-fold increase in the rate of formally diagnosed schizophrenia-like psychotic disorder. (6)
The excellent editorial by Ganesh and D’Souza explores the current state of scientific research into the causality of schizophrenia by cannabis. They conclude that applying complex genetic, consequentialist, and counterfactualist models to the emerging data is beginning to demonstrate that cannabis use has more influence over eventually developing schizophrenia than the reverse. (7)
While scientists argue about the fine details of causality and association, many parents with children who developed schizophrenia after regular cannabis use are thoroughly and loudly convinced that cannabis caused their child’s psychosis. Perhaps the emotions of devastating loss and pain are causing them to jump to this conclusion before the more cautious scientists. But the gap between assuming and scientifically proving cannabis causes schizophrenia-like psychosis is narrowing. It does not take much abundance of caution to warn youth, frequent cannabis users, and users of high THC produces containing minimal CBD of the risks they are taking with their mental health.
Cannabis can be used safely by most adults (see „Five Signs Cannabis is Being Used Too Frequently”). But being in denial of the real risks is not safe use.