What is already known on this topic

Existing evidence suggests cannabis is a contributory (but not necessary) risk factor for psychosis.1 However, the increasing percentage of tetrahydrocannabinol (THC) in cannabis has further complicated this contentious debate. The risk attributable to cannabis with high levels of THC (high-potency cannabis) may require consideration separately from milder forms of cannabis. Indeed, high-potency cannabis has been cited as one possible cause for the incidence of schizophrenia doubling since 1965.2

Methods of the study

Di Forti and colleagues recruited 410 inpatients with a first episode of affective or non-affective psychosis (ICD 10) in South London between 2005 and 2011. A control sample of 370 healthy people was recruited in the same geographical area. Patients were significantly younger, more likely to be male, less likely to be Caucasian, had lower educational attainment and were more likely to have never worked. Data on cannabis use were obtained from the introductory questions to an abbreviated version of the Cannabis Experiences Questionnaire. Particular attention was given to cannabis type and frequency, cannabis type was classified as hash-like or skunk-like. Statistical analyses were performed using Stata V.13, using regression and population attributable fraction (PAF) for the cannabis variables.

What this paper add

• This is one of the largest samples exploring the relationship between high-potency cannabis and psychosis in the UK.

• This paper attributes the onset of psychosis to high-potency cannabis in 24% of their sample.

• The study controls for many confounders which differentiate healthy controls and patients and cannabis users and non-users, including age, gender, ethnicity, nicotine smoking, lifetime exposure to drugs other than cannabis, education and employment status.

Limitations

• While the control group comprises diverse ethnicities similar to the census for the area of London studied, the patients are significantly less likely to be Caucasian; 32% patients were Caucasian compared to 57% controls.

• An inherent assumption is that cannabis directly causes psychosis, this may lead to an over estimation of attributable risk since patients will be enriched by other psychosis risk factors. The PAF, does not require causality to be established, and provides an indication of the number of illness cases which could be prevented if cannabis was removed as a factor. Given that we know patients possess a number of risk factors to express psychosis it would not be possible to separate out the individual effect of cannabis from these other illness factors.

What next in research

• More than one risk variable needs to be taken into account when determining the attributable risk of cannabis for psychosis. For instance a study investigating mental health outcomes in non-Caucasian migrant groups, who have increased risk of psychosis,3 ,4 compared to Caucasian migrants and native matched ethnic groups all stratified for substance use.

• The attributable and population risk for high-potency cannabis relative to other types of cannabis (rather than no use) would be pertinent analysis to contribute to the on-going debate.

• Consideration needs to be given to what cannabis is available for consumption; use of high-potency cannabis may not reflect preference rather the type of cannabis accessible.

Do these results change your practices and why

Probably yes. Cannabis represents a modifiable risk for psychosis. The consumption of cannabis in any young person displaying psychological distress requires clinical discussion. The Cannabis Experiences Questionnaire offers clinicians the opportunity to ask young people about their subjective effects from cannabis in a non-judgemental manner; it offers a window into the young person's psychological vulnerabilities through the experiences they have had. Di Forti and colleagues highlight the need to ask young people about the type of cannabis they consume. Given that questions about the type and mode of cannabis consumption can be readily integrated into a clinical session this paper strongly suggests that it should become standard practice. Engaging a young person in a conversation about their cannabis experiences provides an opportunity to give information to assist in gaining control over their mental health. Keeping in mind results from this study, the type of cannabis consumed highlights the potential expediency with which intervention is required.