Potent cannabis users 'at higher risk of psychosis' – briefing

Users of potent cannabis are three times more likely to suffer from psychosis than non-users, according to a new study of hundreds of people in London.

Researchers at King's College London found that using potent cannabis was linked to almost a quarter of new psychosis cases they studied. However, their results also found that the use of hash, a milder form of the drug, was not associated with any increased risk of psychosis.

Experts suggest the study could advance our understanding of the relationship between the drug and mental health:

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What is psychosis?

The NHS describes it as a mental health problem that causes people to "perceive or interpret things differently from those around them". This can involve paranoia, hallucinations or delusions. It is not a condition in itself, but is triggered by other conditions such as schizophrenia or bipolar disorder.

What have previous studies found?

The Royal College of Psychiatrists says there is "sufficient evidence to show that those who use cannabis, particularly at a younger age, such as around the age of 15, have a higher than average risk of developing a psychotic illness, such as schizophrenia or bipolar disorder". These studies suggest that the more cannabis someone uses, the more likely they are to develop a psychotic illness. The available research also shows that those who have a family history of a psychotic illness may be at a higher risk of developing psychosis following the regular use of strong cannabis.

What does the latest study say?

King's College London's Institute of Psychiatry, Psychology and Neuroscience found that the risk of psychosis depends not only on the frequency of use, but also on the potency of the cannabis used. The study, which will be published in the Lancet Psychiatry later this week, analysed 410 patients with psychosis and 370 healthy participants. The risk was said to be three times higher for people who used potent cannabis than for non-users. This increased to five times higher for people who used it every day.

Does cannabis cause psychosis?

Suzi Gage, who has written a Phd on the topic, points out that the exposure for each participant in the King's study has been assessed retrospectively and may therefore include over or under reporting. The authors also note that the association is not necessarily causal. "There's lots of consistent evidence associating cannabis use with psychosis and schizophrenia, but as the saying goes, correlation isn't causation," writes Gage in The Guardian. Nevertheless, she says the new research is "an important step on the road to understanding the nature of the association between cannabis and psychosis".

How is 'potent' cannabis defined?

The BBC says that generally skunk, a stronger type of cannabis, contains more tetrahydrocannabinol (THC), the main psychoactive ingredient, than other types, making it more "potent". While hash (cannabis resin) contains cannabidiol or CBD, which can actually act as "an antidote to the THC, counteracting psychotic side effects". Dr Marta Di Forti, lead author on the research, told BBC Radio 4's Today programme that skunk was becoming more widespread. "In London, it's very difficult to find anything else," she said. Di Forti called for a "clear public message" to cannabis users and said GPs should be encouraged to ask how often and what type of cannabis their patients use.