THE CANNABIS AND PSYCHOSIS CONNECTION QUESTIONED: A COMMENT ON FERGUSSON ET AL. 2005

A critique of a New Zealand cohort study on cannabis and psychosis argues that the study's use of ten items from the Symptom Checklist 90 to assess psychosis is flawed because the items focus heavily on paranoid ideation, which is a known acute effect of marijuana intoxication. The authors did not ask respondents to distinguish feelings while intoxicated from those at other times, nor did they consider that paranoid feelings may be a rational reaction for people using an illegal, socially frowned-upon substance. The daily user group reported an average of less than two symptoms each, raising questions about whether the findings reflect mental illness or normal reactions to intoxication and social context.

Study at a glance

Abstract

Fergusson and colleagues assert that their results from a New Zealand cohort ‘add to the growing body of evidence that regular cannabis use may increase risks of psychosis’[1]. In fact, their study raises more questions than it answers. One obvious question is raised by their use of ten items from Symptom Checklist 90 as the only assessment tool for symptoms of psychosis. The items assessed focus heavily on paranoid ideation, e.g. ‘feeling other people cannot be trusted’, ‘feeling you are being watched or talked about by others’, and ‘having ideas or beliefs that other do not share.’ This is of concern because it is well known—widely reported in the literature [2] and commonly referenced in popular culture for decades [3,4]—that paranoid feelings are a relatively frequent effect of acute marijuana intoxication. Fergusson et al. give no indication that respondents were asked to distinguish between feelings experienced while intoxicated and feelings experienced at other times. Thus, we are left with no clue as to whether these are long-term effects actually indicative of mental illness or simply the normal, passing effects of acute intoxication. This is akin to reporting that people who go to bars are more erratic drivers than people who don’t, without bothering to look at whether they’d been drinking at the time their driving skills were assessed. In addition, Fergusson et al. seem not to have considered that what might be an indication of psychosis in other circumstances could be an entirely normal reaction for people who use marijuana. Someone using a substance that is both illegal and socially frowned-upon almost by definition has ‘ideas or beliefs that others do not share.’ This is not a sign of mental illness, but rather an indication of a rational, thinking person realistically assessing his or her situation. Considering the widespread use of undercover officers in drug stings, the same can be said for ‘feeling other people cannot be trusted’. Fergusson does not report which symptoms appeared most often, or whether the differences in average levels of symptoms between users and non-users came from a few people having many symptoms or many people having a few symptoms. This raises yet more questions, as the daily user group, with the highest levels of supposed psychosis, reported an average of less than two symptoms each. Based on the data reported, it is entirely possible that the case for marijuana ‘causing’ mental illness is based solely on marijuana smokers having the completely reasonable feelings that they have beliefs different from mainstream society and thus should be a tad suspicious of others.