The largest-ever study into the health effects of different types of cannabis products highlights concerns about stronger forms of the drug. The study found that as the strength or potency of cannabis products have increased over the years, so have rates of people being treated for cannabis addiction. The new study, published in The Lancet Psychiatry, suggests that people who use high potency cannabis are more likely to experience addiction than those using low potency products. It also suggests that people using high potency cannabis are more likely to experience a psychotic disorder.

As the strength or potency of cannabis products has increased internationally over the years, so have rates of people being treated for cannabis addiction, say the authors of a new study.

Researchers from the Addiction and Mental Health Group at the University of Bath (UK) systematically analysed the relationship between the types of cannabis people use and their addiction and mental health problems. Their work draws on 20 studies involving almost 120,000 people.