What do we really know about the link between cannabis and psychosis?

Frequent use of cannabis has long been associated with an increased risk of psychosis, but the amount of the drug needed is far lower than you might think, writes Ian Hamilton

Cannabis use is not a niche activity: an estimated 200 million people use the drug across the world (Tom Ford)

The link between cannabis and mental health problems such as psychosis and schizophrenia have been discussed and investigated for decades. Over the years research has incrementally added to knowledge about the risks cannabis poses to mental health. One well established idea is that the more an individual uses cannabis and the greater the frequency of use there is a parallel risk of psychosis.

The quantity and frequency of use of any drug, including alcohol, have long been associated with an elevated risk of developing both physical and mental health problems, ranging from cancer to cognitive impairment. This received wisdom is challenged by new research that suggests even a small amount of exposure to cannabis is associated with the risk of psychosis and schizophrenia.

The researchers carried out a review of over 500 studies into cannabis and psychosis conducted between 2010 and 2020, pooling the data from these and carrying out a meta-analysis, which revealed that it was not just heavy users of cannabis that were at risk, low frequency users were, too. This is a significant finding as it not only overturns the long-held view of the potential psychological risks of cannabis use, it has implications for policy makers, public health and specialist treatment interventions.