Cannabis users’ risk of developing psychotic disorders ‘appears to decrease with time once they stop using the drug’, according to a report from King’s College London’s Institute of Psychiatry, Psychology and Neuroscience.
There is extensive research that heavy cannabis use is associated with the development of psychosis, alongside poor mental health and sleep issues, with daily users and users of high-potency cannabis facing ‘particularly increased risk’, the researchers state.
However, the study – published in the Canadian Journal of Psychiatry – found that it took around 37 weeks of abstinence for a person’s risk of developing psychosis to return to the same level as someone who’d never used the drug. For some people who regularly used high-potency strains, however, there could still be an elevated risk even after more than 180 weeks.
Researchers used a multi-centre study of first-episode psychosis among almost 900 people across Europe and South America, and compared the findings to more than 1,200 people without psychosis. They analysed current and lifetime cannabis use, duration and frequency of use, potency of the drug used, time since cessation, and age of first use.
‘Our study, which is the first to examine whether the risk of a psychotic disorder goes down if a person ceases cannabis use, is vital,’ said joint senior author Dr Isabelle Austin-Zimmerman. ‘As the legalisation of cannabis continues to grow, so too will the number of people using the drug. Our study provides important evidence confirming that sustained cessation of cannabis use is associated with a decreased risk of psychosis.’
Mental health charities have previously warned that moves to legalise cannabis for recreational use in the UK risked ‘fuelling the nation’s mental health crisis’. ‘One third of all new patients diagnosed with psychosis in London would not have got ill if they hadn’t used high-potency cannabis,’ said chair of the Rethink Mental Illness charity’s clinical advisory group, Sir Robin Murray, with the organisation urging policy makers to make sure that any debates around legalisation reflected the ‘possible impact on the most vulnerable and those living with – or at risk of – severe mental illness’.
Meanwhile, drug seizures in England and Wales were up by 13 per cent in the year to March 2024, according to the latest Home Office figures. More than 80 per cent of the seizures were by police, with 19 per cent carried out by the Border Force.
Border Force seizures were 57 per cent up on the previous year, mainly driven by herbal cannabis seizures, the Home Office states. They also seized their largest ever quantity of powder cocaine, at more than 26 tonnes – a 75 per cent increase on 2022-23. The quantity of heroin seized, however, was down by 54 per cent to 441kg, the lowest quantity seized since the late 1980s.
Cannabis use cessation and the Risk of psychotic disorders: a case–control analysis from the first episode case – control EU-GEI WP2 study available here
Seizures of drugs in England and Wales, financial year ending 2024 available here