The use and potency of cannabis is increasing worldwide, with rising levels of dependence and cannabis-induced psychosis, particularly in North America. Two new peer-reviewed studies, both drawing on data from Cannabis & Me, the largest survey of its kind, have identified key risk factors linked to paranoia in cannabis users.

The first study, published in BMJ Mental Health, examined the relationship between people’s reasons for first using cannabis and their later patterns of use.

A total of 3,389 former and current cannabis users aged 18 and over completed the survey, which captured their motivations for first and continued use, their weekly consumption in THC units, and their mental health.

Findings show that respondents who first used cannabis to self-medicate for pain, anxiety, depression, or minor psychotic symptoms reported significantly higher paranoia scores compared with those who tried cannabis out of curiosity or for fun, who had the lowest average paranoia and anxiety levels.

Professor Tom Freeman, Director of the Addiction and Mental Health Group at the University of Bath and a senior author on the study, said:

A key finding of our study is that people who first used cannabis to manage anxiety or depression, or because a family member was using it, showed higher levels of cannabis use overall. In future, standard THC units could be used similarly to alcohol units, for example, to help people track their cannabis consumption and better manage its effects on health.

Dr Giulia Trotta, a Consultant Psychiatrist and Researcher at King’s IoPPN and the study’s first author said:

This comprehensive study is the first to explore the interplay between childhood trauma, paranoia, and cannabis use among cannabis users from the general population. We have not only established a clear association between trauma and future paranoia, but also that cannabis use can further exacerbate the effects of this, depending on what form the trauma takes. Our findings will have clear implications for clinical practice as they highlight the importance of early screening for trauma exposure in individuals presenting with paranoia.

Respondents reported consuming an average of 206 THC units per week - equivalent to around 10–17 joints if using cannabis with 20% THC, the level typical of strains currently available in London. However, those who began using cannabis to cope with anxiety, depression, or because others in their household were already using it, consumed significantly higher weekly amounts.

In a second study, published in Psychological Medicine, researchers explored the relationship between childhood trauma, paranoia, and cannabis use. Over half (52%) of respondents reported experiencing some form of trauma. Those who had experienced trauma, particularly physical or emotional abuse, reported higher average levels of paranoia.

Crucially, the researchers found that cannabis use can exacerbate the impact of childhood trauma on paranoia, with different types of trauma linked to differing effects.

Professor Marta Di Forti, Professor of Drug Use, Genetics and Psychosis at King’s IoPPN, and senior author on both studies, added:

There is extensive debate globally about the legality and safety of cannabis use. Our findings show that while some individuals turn to cannabis as a way of coping with pain or trauma, this may carry significant risks for their mental health and wellbeing. Policymakers should consider these risks carefully when debating legalisation.

The Cannabis & Me project was funded by the Medical Research Council (MRC) and jointly led by the University of Bath and King’s College London.