People who turn to cannabis to cope with physical or mental health problems may face a significantly higher risk of paranoia compared with those who use the drug recreationally, according to new research from King’s College London’s Institute of Psychiatry, Psychology & Neuroscience (IoPPN) and the University of Bath.
The findings, published in BMJ Mental Health and Psychological Medicine, draw on data from Cannabis & Me—the largest study of its kind, involving more than 3,300 current and former cannabis users aged 18 and over.
Why people start matters
The first study examined motivations behind cannabis use and their long-term effects. Participants who reported using cannabis to self-medicate for pain, anxiety, depression, or minor psychotic symptoms scored markedly higher for paranoia compared to those who first tried it for fun, curiosity, or social reasons.
“Our study provides vital evidence on how the reason someone first starts using cannabis can dramatically impact their long-term health,” said Dr. Edoardo Spinazzola, research assistant at King’s IoPPN and lead author. He added that self-medicating users also had higher rates of anxiety and depression, often reaching levels warranting referral for counseling.
Heavier use among self-medicators
Researchers also tracked weekly consumption in standardized units of THC—the psychoactive component of cannabis. On average, respondents consumed the equivalent of 206 THC units weekly (roughly 10–17 joints at typical London strength). However, those who began using cannabis to ease anxiety or depression, or because of family exposure, consumed substantially more, averaging between 248 and 287 units per week.
Professor Tom Freeman, Director of the Addiction and Mental Health Group at the University of Bath, said standard THC units could eventually function like alcohol units, helping users monitor their intake and health risks.
Trauma link intensifies risk
The second study investigated how childhood trauma interacts with cannabis use and paranoia. Over half of participants reported some form of trauma, with physical and emotional abuse emerging as the strongest predictors of paranoia.
Cannabis use was found to exacerbate paranoia among trauma survivors, though the effect varied depending on the type of trauma. For instance, those exposed to emotional abuse or household discord reported significantly higher THC consumption and paranoia scores, while other forms of trauma showed weaker associations.
“This is the first study to explore how childhood trauma, paranoia, and cannabis use interact in the general population,” said Dr. Giulia Trotta, consultant psychiatrist at King’s IoPPN and lead author. “We’ve shown that cannabis can amplify the long-term psychological effects of trauma, with clear implications for clinical practice.”
Broader implications
Professor Marta Di Forti, senior author of both papers and clinical lead at the UK’s first cannabis clinic for psychosis, warned that the findings carry weight in ongoing debates about cannabis legalization.
“Many patients start using cannabis to cope with pain or distress, but this carries serious risks for mental health,” she said. “Policy makers should consider these risks carefully. Legalisation without adequate education and health support could place heavy burdens on individuals and healthcare systems.”
The Cannabis & Me project was funded by the Medical Research Council (MRC).
