Cannabis & psychosis


This week’s issue of The Lancet contains the most comprehensive meta-analysis to date of the link between cannabis and psychosis:

The evidence is consistent with the view that cannabis increases risk of psychotic outcomes independently of confounding and transient intoxication effects, although evidence for affective outcomes is less strong. The uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal studies such as those reviewed here. However, we conclude that there is now sufficient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life.

The journal’s podcast includes an interview with Stanley Zimmet, of the Department of Psychological Medicine at Cardiff University, who is one of the authors of the study. 

I’m inclined to think that the long-term effects of cannabis (and those of any other psychotropic drug) very much depend on the mental composition of the individual using the drug. Therefore, smoking cannabis may increase the risk of psychosis or affective disorders, if one is somehow already predisposed to such a condition. But finding a correlation between two factors does not mean that one causes the other.

On a related note, there’s an article in Slate, by a psychiatrist and a professor of psychology, who argue that addiction is not a brain disease (via Mind Hacks, where it is discussed further).


7 thoughts on “Cannabis & psychosis

  1. Well, of course addiction isn’t a disease – in the way it’s been redefined.
    With the original definition – physiological adaptation to a drug that makes the drug necessary for normal functioning – addition is obviously a maladaptive disorder. But the new definition implies no such thing.

  2. I don’t really see this as anything but an assumption of causation. Indeed, I would suspect that the causation is probably there but in the reverse of the studies assumption, i.e. those who have a preexisting psychosis, are somewhat more likely to become regular pot smokers. Sorry, but this study seems little more than another propaganda tool, for more scare tactics that few kids will buy into.
    A comment at the Wall Street Journal’s health blog puts it into perspective;
    Using US Drug policy data we can estimate how much risk a 40% increase entails. Marijuana use is only listed by Last Month Used, but being the most popular illicit drug, so overall usage numbers are close enough. The NHS**3 survey on drugs puts the lifetime use for any illicit drug at an average of 36% from 1979 – 2001* which puts 36% of the population at a 40% higher risk for schizophrenia and the other 64% for normal risk. Using the general population schizophrenia rate of .005 as the risk factor gives marijuana users a .006118 chance of schizophrenia as opposed to the .00437 odds for someone with a lifetime of drug abstinence. There’s your 40% from about 4/1000 to 6/1000.
    Even if preexisting psychosis doesn’t make it more likely that a person will smoke marijuana, this really doesn’t seem like much of a big deal. If on the other hand, people with preexisting psychosis are more likely to smoke it, then theses numbers are even more meaningless.
    The big problem that I have with b.s. studies like this one, is that there are very good reasons for kids not to smoke pot, or use other recreational drugs. The problem comes in when schools use scare tactics, based on the b.s. to try to keep kids off drugs, the kids realize that it’s b.s. and ignore the whole message. When they see their friends doing it, without going nuts, they assume that it’s ok. I think it’s much better to give kids the real scoop and trust them to make a well informed decision, such as waiting until their brains have fully developed or are closer to full development, to experiment with drugs. Or decide not to do it at all, because there are risks, bad things can happen. Let then know the real risks, instead of stuffing them full of crap that they can discount wholesale.

  3. I remember back in the old days, they use to blame LSD on psychotic behavior. There were all sorts of urban myths about the kid who tried LSD at a party and went c r a z y! I guess pot’s the new bad boy.

  4. I think that one of the key factors in any discussion of marijuana’s long-term effects should be the POTENCY of the weed’s THC content.
    Some of the selectively bred species of weed on sale in Amsterdam coffee houses have THC contents way up in the 20% range, which is enough to knock even a seasoned weed smoker on their ass.
    It wouldn’t be too much of a stretch to imagine some of those uber-strong marijuana species triggering chemical imbalances in the brain that would have been unheard of just a few years ago, before the selective breeeding horticulture of marijuana became such an exact science.

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