Can Cannabis Cause Schizophrenia?

If you logged in to the front page of The Huffington Post today you might have seen an article by Rupert Wolfe Murray called Can Cannabis make you Crazy?

Rupert asked his readers to consider the other side of the legalisation argument, not the one that is advocated by Richard Branson.

On the same day that this was published BBC Radio 4’s Life Scientific interviewed psychiatrist Professor Sir Robin Murray MD DSc FRCP FRCPsych FMedSci FRS about his life’s work – studying schizophrenia (psychosis) and trying to understand why some people have schizophrenia and others don’t.

He gave valuable insight into the potential effects of cannabis on mental health.

Dr. Murray says psychosis means you don’t have the ability to “filter out stimuli” and this makes everything significant to the person with the condition. “Why are these people wearing red jumpers? Could it be that somebody has hired them to follow me?” This is caused by the dopamine levels in the brain being too high which gives everything significance and gives the person a feeling of great importance…perhaps they believe they have been chosen by God for a particular mission, or “could it be that all these things are happening to me because they’re against me and they’re all out to get me?” These are grandiose illusions and paranoid illusions.

Social causes of schizophrenia

Murray found out that psychosis is not just a brain disease – living in a city significantly increases the risk of schizophrenia (the bigger the city, the greater the risk) and migrants are six times more likely to develop the condition than long term residents. This suggests that the problem has social causes, as well as biological. Can drugs cause schizophrenia?

He’s also outspoken about the mental health risks of smoking cannabis, based both on his scientific research and direct experience working at the Maudsley Hospital in South London.

Dr Murray reports seeing psychosis in patients who had not shown any developmental difficulties and who did well in their youth. They didn’t fit the usual model of the person that develops psychosis. “Since about 2002 it’s become apparent that smoking cannabis, and certainly the more modern variants of cannabis increases your risk, there are a number of causal risk factors that act together to push people over the threshold into psychosis, it’s clear that drug abuse is one of these.”

More potent strains of cannabis

Murray points out that all studies that have been published, with one exception, have investigated the effects of THC in more old-fashioned forms of cannabis such as resin, which contains about 4-5% THC, but modern-day skunk contains about 14-18% THC. He believes that smoking 4 or 5 joints a day for five years at this potency will “certainly increase your risk of psychosis”.

“The evidence seems to be that if you’re smoking skunk every day then the risk goes up probably about 6-fold.”

He points out that drug abuse is among a number of risk factors for psychosis and that you would have to have a predisposition towards psychosis in order for it to be triggered.


If you think you might be addicted to cannabis we recommend that you consult with your G.P and visit Narcotics Anonymous online. This self-help group is similar to Alcoholics Anonymous but has a focus on drug addiction. Your local Narcotics Anonymous groups can offer immediate help and support.

If you want to immerse yourself in an intensive treatment programme then residential rehab could be the right choice for you. You can email or call us to speak to someone about your addiction before you commit to any treatment.

If you are worried that you or a family member may be developing schizophrenia then we recommend that you visit your G.P as soon as possible. You can also find lots of information at Mind, a mental health charity.

See: Can Cannabis make you Crazy

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