The debate about cannabis and psychosis can be noisy with well-rehearsed arguments from both sides - but the problem with saying ‘cannabis makes you psychotic’ is that too many people have used it and too few have become psychotic

A recent study has reignited the debate about the link between cannabis and psychosis. Although the authors of the research are cautious about their findings and how they should be interpreted stating ‘The ready availability of high potency cannabis in south London might have resulted in a greater proportion of first onset psychosis cases being attributed to cannabis…’ this hasn’t stopped less prudent headlines and interpretations of the study findings.

The debate about the relationship between cannabis and psychosis can be noisy with well-rehearsed arguments from those supporting and those opposing the association between the two factors. Clearly this matters as any health related information needs to be perceived as credible by those it is aimed at, in this case the target group is young people. The potential danger with lazy position statements of ‘cannabis makes you psychotic’ is that too many people have used cannabis and too few have become psychotic and therefore the message doesn’t resonate with the experience of most young people.

Politicians have form on this issue stretching as far back as the early twentieth century with the production of the film Reefer Madness in America, a clear piece of propaganda but with the same themes that continue to this day regarding the relationship cannabis has with mental health. In 2009 Gordon Brown ignored his own scientific advisors and stated that he wanted to protect young people from the mental health risks of using cannabis and developing psychosis particularly, reclassifying it from a class C drug to a class B one, this was prior to the general election in what many perceived as a political move.

This was the third change to the classification of cannabis in five years, which one study showed had the opposite effect on admissions to hospital for cannabis psychosis, in that the rate of admissions reduced when cannabis had a lower classification. All of which adds to the potentially conflicting signals policy makers were sending about the risk to health of using cannabis.


It is not just politicians who are to be found wanting, research has been opaque as we are some way from understanding what exactly the relationship is and how exposure to cannabis can affect an individual’s mental health, including the risk of developing psychosis.

All research in this area has to rely on what we observe in a given population and over time, this means there are a number of variables such as frequency of cannabis use, type of use, dose etc. that need to be considered. Adding to that are age, gender, genetics other drug use, and so the list goes on. As an example it might be that people who choose to smoke cannabis are different to those who don’t, it is possible that people who develop problems with their mental health early in their life are more likely to experiment with cannabis and other drugs as a way of ameliorating unpleasant feelings and thoughts they have.

The most recent study on this topic is interesting and adds new knowledge as it explores what effect the type of cannabis a person is exposed to and how frequently they use it has on their risk of developing psychosis. The researchers were interested in a specific group of people, namely those who developed psychosis for the first time comparing them with people from the same south London area who had never used cannabis. The study investigated the effects of using high potency cannabis such as skunk comparing this to those using lower potency cannabis resin and those who had not been exposed to cannabis at all.

In short, there are two important findings from this investigation. First, they found an elevated risk of developing psychosis for the group that had used skunk every day, and second, the risk of psychosis was no greater for those people who had used cannabis resin (hash) every day when compared to those who had not used cannabis at all. This would add to previous work in the field which suggests that higher potency cannabis which has raised concentrations of tetrahydrocannabinol (THC) and lower cannabidiol (CBD) increases the risk of developing psychotic symptoms, and that lower potency cannabis such as resin which has reduced concentrations of THC and raised CBD may not increase the risk by comparison of developing psychosis. Paradoxically, some research suggests that CBD may offer a protective effect against psychosis. This could account for the reduced risk in the group that used cannabis resin as it is likely to have had higher concentrations of CBD.

This research is significant as we are at an interesting juncture internationally in relation to the status of cannabis and how it is regulated, with several American states decriminalising the use of cannabis and other regimes around the world adopting similar stances. Adding new knowledge which could inform such policy is vital; all we need now are receptive policy makers and legislators, as it’s clear cannabis does matter.

Ian Hamilton is a lecturer in mental health in the Department of Health Sciences, University of York, with an interest in the relationship between substance use and mental health, having formerly been a mental health nurse in South London, working with people who had a severe mental health problem and used drugs or alcohol problematically