Drug War Distortions: Cannabis and Mental Illness

In March 2005, several reports appeared in the news media regarding a causal link between cannabis and psychosis. The National Post of Canada reported on March 10, 2005 ( "Study Links Pot-Smoking, Psychosis") that "Daily marijuana users are 1.6 to 1.8 times more likely to develop psychosis compared to non-users, a group of New Zealand scientists reported after following the health and development of 1,200 men and women born in 1977 for 25 years. The findings, published in the March edition of the journal Addiction, make it increasingly difficult for pot advocates to dismiss a growing body of evidence linking regular cannabis use with increased risks of psychosis, an outcome mental health experts have long suspected. 'This makes it more definitive,' says Harold Kalant, a professor of pharmacology at the University of Toronto. 'Most people accept that the link has been established beyond anyone's ability to dismiss it.'"

As is often the case, there is less here than meets the eye. The article, "Tests of Causal Linkages Between Cannabis Use and Psychotic Symptoms," by David M. Fergusson, L. John Horwood and Elizabeth M. Ridder, though important, has flaws which bring its findings into question.

The first point that needs to be noted is that the researchers did not perform or review actual diagnoses of these subjects nor did they report on actual cases of mental illness. The study looked only at self-reports of possible symptoms. Indeed, the authors in their Addiction article were much less hyperbolic in their conclusion:
"Although each of these lines of evidence is subject to uncertainty and debate, the weight of the evidence clearly suggests that the use of cannabis (and particularly the heavy use of cannabis) may alter underlying brain chemistry and precipitate the onset of psychosis/psychotic symptoms in vulnerable individuals. The present study adds to this evidence by showing: (a) it is unlikely (although not impossible) that the association between cannabis use and psychotic symptoms in a population sample was due to confounding factors, and (b) the predominant direction of causality is likely to involve a path from cannabis use to psychotic symptoms rather than a path from psychotic symptoms to cannabis use."

Yet, are even these cautious conclusions justified by the data?

The number of symptoms being reported was actually rather low: non-users reported on average a maximum of 0.69 symptoms, while the daily users reported on average a maximum of 1.95 symptoms. What also went unreported by the news media was the fact that other user groups in the study, including the weekly users, showed a decline in the number of reported symptoms over time. Data from the report is printed below.

Table 1 Mean psychotic symptoms (number of subjects) by frequency of cannabis use (past 12 months) at 18, 21 and 25 years.
	Frequency of cannabis use (past 12 months)
Age (years)	Never	Less than monthly	At least monthly	At least weekly	Daily	P
18	0.64 (598)	0.95 (242)	1.07 (82)	1.93 (70)	1.64 (33)	<0.0001
21	0.69 (538)	1.00 (215)	1.14 (100)	1.48 (94)	1.61 (64)	<0.0001
25	0.60 (559)	0.89 (232)	0.93 (76)	1.15 (81)	1.95(55)	<0.0001

The researchers relied on a psychological screening instrument called the Symptom Checklist-90 to measure these symptoms. (The full checklist and the psychological constructs represented by the different problems/complaints can be viewed at the Medical Algorithms Project.) The symptoms examined by the researchers were:

hearing voices that other people do not hear
the idea that someone else can control your thoughts
other people being aware of your private thoughts
having thoughts that are not your own
having ideas and beliefs that others do not share
the idea that something is seriously wrong with your body
never feeling close to another person
the idea that something is wrong with your mind
feeling other people cannot be trusted
feeling that you are watched or talked about by others

As noted by Bruce Mirken and Mitch Earleywine in their article "Psychosis, Hype and Baloney" (AlterNet, March 7, 2005):
"[T]he article gives no indication that respondents were asked to distinguish between feelings experienced while high and feelings experienced at other times. Thus, we are left with no indication at all as to whether these supposed psychotic symptoms are long-term effects or simply the normal, passing effects of marijuana intoxication. While it's possible the researchers had these data and didn't see a need to report them, the failure to do so is downright bizarre. It's like 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.
"Even if these were long-term effects, the researchers 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. Consider: 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. It's a sign of a rational person realistically assessing his or her situation.
"The same goes for 'feeling other people cannot be trusted.' Just ask Robin Prosser, the Montana medical marijuana patient arrested last summer on possession charges by the cops who came to save her life after she'd attempted suicide because she was in unbearable pain after running out of medicine.
"Fergusson reports very little raw data, so we don't know which symptoms came up most often, or whether the differences in average levels of symptoms between users and non-users came from a few people having a lot of symptoms or a lot of people having a couple symptoms. The heavy-user group, with the highest levels of supposed psychosis, reported an average of less than two symptoms each. So it is entirely possible that the entire case for marijuana 'causing' psychosis is based on marijuana smokers having the completely reasonable feelings that they have beliefs different from mainstream society [or] should be a tad suspicious of others."

Distortion 18: Cannabis and Mental Illness