Solving the puzzle of whether cannabis is associated with, or causes, psychosis.
A recent issue of the American Journal of Psychiatry from 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 some more fundamental genetic condition increases 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)