Expert answers from
- An increased risk of experiencing psychosis
- The role of THC
- CBD – a future treatment?
- Smoking cannabis after a diagnosis of mental illness
People who smoke cannabis regularly and from an early age increase their risk of experiencing psychosis. Researchers are not yet certain of why this is the case and the mechanisms involved, but projects around the world have consistently highlighted this increased risk among regular cannabis users.
One team of researchers, for example, has analysed information given by 1,000 men and women in New Zealand at regular intervals since they were born. The researchers discovered that people who smoked cannabis when they were teenagers were more likely to have symptoms of psychosis when they were 26 than their peers who had 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. The research team collected information from them over a period of 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.
In teenage years, the brain is still developing and this may be why smoking cannabis regularly at this time can increase the risk of experiencing psychosis.
Nevertheless, the majority of cannabis users do not experience the symptoms of psychosis and many people who are given a diagnosis of a mental illness such as schizophrenia have never used cannabis. Researchers are trying to find out why cannabis increases the risk of psychosis for some people and not for others. One theory is that some people may be more prone to developing the symptoms of psychosis as a result of smoking cannabis because they have a certain set of genes that makes them more vulnerable.
Cannabis contains many different chemicals. Researchers think one of the psychoactive ingredients in cannabis that produces the 'high' – called THC (delta-9-tetrahydrocannabinol) – has a key role in the development of the symptoms of psychosis.
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.
Researchers think THC may change the way different parts of the brain are activated which means people can lose touch with reality, thinking normal experiences have a particular or special importance, hearing or seeing things that aren't there and feeling paranoid.
This may explain why smoking cannabis with a high THC content can contribute to the development of psychosis.
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 different chemicals interact.
Research has shown that the other main constituent of cannabis – CBD (cannabidiol) –seems to moderate the effect of THC. (Hash contains almost equals amount of THC and CBD while skunk contains virtually no CBD.)
Studies have shown that CBD, responsible for the calm and relaxation some people feel when they smoke cannabis, can act like antipsychotic medication. Scientists think CBD might potentially be a future option to treat people who have a diagnosis of schizophrenia or who experience the symptoms of psychosis, though many more research studies will need to be carried out to prove its value as a therapy. Researchers think CBD might also potentially help other mental health problems, such as anxiety and depression, as well as some physical health problems.
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 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 experience 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.
This page was updated on 21 April 2013.
There are currently no plans to update the page because existing funding for mentalhealthcare.org.uk ceases at the end of April 2013.
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Links on this page last checked: 21 April 2013
Next links check due: August 2013
- Risk of psychosis greater for people who regularly use high potency cannabis
- Click to download research summary
Other useful websites
A website from Liverpool-based HIT (formerly the Mersey Drug Training and Information Centre) that helps people assess their cannabis use, the impact on their lives and suggests ways of making changes.
An independent government-funded website giving free information about drugs by email,
text or phone (0800 77 66 00)