Cannabis

• An increased risk of developing psychosis
• Effects of cannabis on the brain
• Smoking cannabis after a diagnosis

An increased risk of developing psychosis

People who smoke cannabis regularly and from an early age increase their risk of developing psychosis. Researchers are not yet certain of the mechanisms involved, but projects around the world have consistently highlighted this increased risk of psychosis among regular cannabis users.

One study, for example, 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 about four times more likely to have psychotic symptoms by the time they were 26.

Another study followed 45,000 people who had joined the armed forces in Sweden over 15 years. The researchers found the risk of developing schizophrenia was 2.4 times higher for people who had smoked cannabis before they turned 18 than for non-users.

Nevertheless, the vast majority of cannabis users do not become unwell, just as the majority of people who drink alcohol in moderation never come to harm.

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Effects of cannabis on the brain

Researchers are trying to find out why cannabis increases the risk of psychosis for some people, and the effect the chemicals it contains have on the brain.

They think the psychoactive ingredient in cannabis that produces the 'high' – called THC (delta-9-tetrahydrocannabinol) – has a key role.

Old-fashioned cannabis resin (hash) contains about four per cent THC, but modern varieties such as skunk may contain up to 18 per cent.  

Researchers have found that people who smoke skunk regularly are almost seven times more likely to develop psychosis than those who use traditional cannabis resin or grass.

Several studies have shown that when healthy people are given an injection of pure synthetic THC, more than a third experience symptoms of psychosis.

A very recent brain imaging study of a small number of people (who had no experience of psychosis and smoke an occasional joint) showed that THC increased the brain's response to normally insignificant stimuli. Studies are continuing, but researchers think this might explain why smoking cannabis with high THC content can contribute to the development of psychosis: the chemical changes the way different parts of the brain are activated which means people begin to think normal experiences have a particular or special importance, and can therefore become paranoid

The same study showed the other main constituent of cannabis – CBD (cannabidiol) – had the opposite effect on the brain: researchers think CBD may act like antipsychotic medication.

Ongoing research is also focusing on how the action of THC on the brain influences brain chemicals like dopamine, thought to be involved in the development of psychosis, and glutamate, a neurotransmitter that has a part to play in learning and memory – and the way these chemicals interact.

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Smoking cannabis after a diagnosis

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

Some people who have been given a diagnosis say cannabis makes them feel better and counteracts the unpleasant side effects of antipsychotic medicine. CBD, one of the two major ingredients of the drug, can make people feel less anxious. Research also shows that people with psychosis use cannabis for the same reasons as other people – to relax and ‘get high’.

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

Health professionals and researchers are developing and testing talking therapies to offer support to people with psychosis to help them stay off cannabis. Motivational interviewing, for example, has been tested by professionals working in early intervention services and shown to help people cut down on the number of joints they smoke in the short term, but not in the longer-term term.