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Does smoking cannabis increase the risk of developing psychosis?

Professor Sir Robin Murray: Cannabis has been a very controversial issue, and for people of my generation we were brought up that really cannabis was pretty harmless.

But throughout the 90s, we kept seeing patients who had developed psychosis, particularly young men, and their parents or their sisters would say, ‘they smoke a lot of cannabis, could their psychosis have anything to do with this?' And we would say, ‘no, no I don’t think so, I think it is just a coincidence.’ But we saw this happening so often that we began to take note of it.

Now the way to study this is to take a large series of the general population, divide them into the cannabis smokers and the non-cannabis smokers and follow them up and there have been about 9 studies that have followed up general population samples to see do the cannabis smokers show an increased risk of psychosis in general, and schizophrenia in particular, and they do. If you have been a moderate cannabis smoker, you probably double your risk of developing psychosis; if you are a heavy cannabis smoker, or smoking a lot of high potency cannabis, you may increase your risk up 4 to 6 times.

The cannabis plant produces a whole lot of chemicals, but the 2 most important ones are THC or tetrahydrocannabinol, or CBD or cannabidiol.

Now THC, if we inject it into you or me in sufficient quantities, will make us euphoric and giggly, but subsequently, if you have a higher dose, you will develop paranoia and begin to hear and see things. So we know that THC can acutely induce a psychotic reaction.

On the other hand, CBD, or cannabidiol, seems to some extent to counteract the effects of THC, so if we give people CBD, and then give them THC, they don’t develop such a pronounced psychosis.

Now old-fashioned cannabis has both THC and CBD in it, so probably the chill out effect, the relaxation effect of old fashioned cannabis, grass or hash is partly due to the CBD.

Skunk or sensamilla, the more recent types of cannabis, have much more THC, maybe something like 16 to 18% instead of 4%, and they also have no CBD, or practically no CBD. So it seems they produce a sort of double hit, a double whammy of more THC, the more powerful psychotogen and less of the CBD, which seems to be protective.



This page was last updated 19 April 2013
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PLEASE NOTE: Since these films were made, the Institute of Psychiatry at King's College London has become the Institute of Psychiatry, Psychology & Neuroscience (from 1 September 2014).