Cannabis

• Cannabis
• When people who are unwell smoke cannabis

Cannabis

People who smoke cannabis regularly greatly increase their risk of developing psychosis. Research around the world has highlighted this risk.

One study has followed the lives of 1,000 men and women in New Zealand since they were born. Researchers discovered that people who smoked cannabis when they were teenagers were more likely to have symptoms of psychosis at age 26 than their peers who abstained. The earlier people started using cannabis, the more likely they were to have symptoms of psychosis as a young adult. People who had been regular cannabis users at 15 were more than 4 times more likely to have psychotic symptoms by the time they were 26.

Nevertheless, the vast majority of cannabis users do not become unwell, just as the majority of those who drink alcohol moderately never come to harm. Those with a family history of psychosis are more vulnerable, and there is some early evidence that people who have a particular type of a gene called ‘COMT’ are especially likely to develop psychosis. This gene determines how fast the chemical dopamine is broken down in the brain. Scientists think abnormal amounts of the brain chemical dopamine is somehow instrumental in the development of psychosis (see What causes psychosis? page).

The psychoactive ingredient in cannabis that produces the ‘high’ is called THC (delta-9-tetrahydrocannabinol). THC concentrations in old-fashioned cannabis resin (hash) are about four per cent, but modern varieties such as skunk may contain up to 18 per cent THC.  

Research has shown that when healthy people are given an injection of pure synthetic THC, more than a third experience symptoms of psychosis when they are under the influence of the equivalent to a joint of street-strength skunk.

Researchers at the IoP have also found that people who smoke skunk are almost seven times more likely to develop psychotic illnesses than those who use traditional cannabis resin or grass.

Studies are continuing to find out more about how cannabis contributes to the development of psychotic symptoms. These include imaging studies to find out how THC and CBD (cannabidiol) – the other main constituent of cannabis – interact. There is some suggestion that CBD may ameliorate the effect of THC on the brain.

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When people who are unwell smoke cannabis

Research around the world has shown that people with schizophrenia are twice as likely to use cannabis than people who do not have the illness.

Some people say cannabis makes them feel better and counteracts the unpleasant side effects of anti-psychotic medicine. CBD, the other major ingredient for inducing altered behaviour, can make people feel less anxious.

Nevertheless, people with psychosis who continue to smoke cannabis tend to have a more severe symptoms than those who stop.

Health professionals are developing and testing talking therapies to offer support to people with psychosis to help them stay off cannabis.