The Great Pot Debate: Does Marijuana Improve Mental Function?

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REUTERS/Robert Galbraith

Marijuana buds, including their cost and degree of potency, are shown in a medical marijuana dispensary in Oakland, California June 30, 2010. Picture taken June 30, 2010. REUTERS/Robert Galbraith (UNITED STATES - Tags: SOCIETY)

I recently tried to find some consensus in the research on whether smoking marijuana makes you psychotic — some data indicate that pot-smokers are more likely to develop schizophrenia, while other studies find that marijuana actually enhances mood in people with schizophrenia (and millions of others). But although there are some intriguing trends, hinting at an answer, the data is still wildly conflicted.

Now there’s a new meta-analysis of 10 studies in Schizophrenia Bulletin. The statistical review involved data on 572 patients, and found that on average people with schizophrenia who had a history of smoking marijuana had better cognitive functioning on multiple tests. More surprisingly, the research found that the earlier the patients started smoking, the better their cognitive functioning. One study included in the analysis even found that more frequent marijuana use was linked with improved mental functioning.

Here’s why that finding is equivocal. On the face of it, it sounds like we should be prescribing medical marijuana to everyone with schizophrenia — the earlier the better. But the studies included in the review were observational, showing only correlations between marijuana use and cognitive function. Correlation, of course, is not cause.

Being able to buy marijuana, for instance, actually requires a reasonably high level of cognitive and social functioning (well, at least to a point): as I noted in my earlier article, simply being able to find someone who sells pot and appearing sane enough not to scare the seller away is challenging for most people who are impaired by schizophrenia.

So the results of the meta-analysis could be explained this way: it could be that only patients who are already high-functioning are able to buy pot, and smoking it doesn’t worsen their cognitive function. Alternatively, it could be that there’s a milder form of schizophrenia that is triggered by marijuana in some people with a genetic predisposition to the disorder.

However, the fact that schizophrenia rates have stayed stable over time, while cannabis use has increased massively in the developed world, suggests that this group is not large and that if cannabis isn’t what pushes people over the line, something else eventually will. (Otherwise, cannabis should increase schizophrenia rates.)

Still, the data do suggest that there is a subgroup of people with a family history of schizophrenia who appear to develop the disorder earlier if they smoke cannabis. It’s possible that these people would gain years of mental health if they simply avoided marijuana. It’s also true, in general, that heavy cannabis users with schizophrenia have more hospitalizations, suggesting a worsening of function.

So, how will we ever figure out what’s really going on? Controlled trials of marijuana smoking would be considered unethical by many — and no one would fund them anyway. Long-term follow-up of people with first episodes of psychosis who smoke pot and those who do not may shed some light on the question — at least, these studies would show whether marijuana makes the course of the disorder better or worse. The author of the current meta-analysis is no working on a study of 10-year outcomes of such patients, and we’ll report the results when they’re available.

In the meanwhile, the only thing it’s safe to say on this question is that people with a family history of schizophrenia should not smoke marijuana — and that the debate over this drug will surely continue.