Werewolf in London? Or maybe it’s a Skunk.

Cannabis is now the most popular illicit drug in the world. Several US states have legalized cannabis for medical or recreational use and more are in the process of doing the same. Numerous prospective epidemiological studies have reported that use of cannabis is a modifiable risk factor for schizophrenia-like psychosis. In 2012, the Schizophrenia Commission in the UK concluded that research to quantify the link between cannabis use and serious mental illness should be pursued.

Between May 1, 2005, and May 31, 2011, researchers culled data from 410 patients with first-episode psychosis and 370 controls. The risk of individuals having a psychotic disorder was approximately three-fold higher among users of “skunk-like” cannabis, compared with those who never used cannabis (adjusted odds ratio [OR] 2•92, 95% CI 1•52–3•45, p=0•001). Further, daily use of skunk-like cannabis resulted in the highest risk of psychotic disorders, compared with no use of cannabis (adjusted OR 5•4, 95% CI 2•81–11•31, p=0•002).

The population attributable fraction of first episode psychosis for skunk use for the geographical area of south London was 24% (95% CI 17–31), possibly because of the high prevalence of high-potency cannabis (218 [53%] of 410 patients) in the study.

Clearly, and as seen elsewhere, availability of high potency cannabis in south London most likely resulted in a greater proportion of first onset psychosis than in previous studies where the cannabis is less potent.

Why Does this Matter?

Changes in marijuana potency and the increased prevalence of use by adolescents and young adults increases the risk of serious mental illness and the burden on the mental health system.

Chronic, relapsing psychotic illness produced by cannabis is similar to that produced naturally in Schizophrenia. However, treatment responses are not the same. Indeed, skunk use appears to contribute to 24% of cases of first episode psychosis in south London. Our findings show the importance of raising awareness among young people of the risks associated with the use of high-potency cannabis. The need for such public education is emphasized by the worldwide trend of liberalization of the constraints on cannabis and the fact that high potency varieties are becoming increasingly available.

Finally, in both primary care and mental health services, developing a simple screening instrument as simple as yes-or-no questions of whether people use skunk or other drugs will aid public health officials to identify epidemiological maps and “hot spots” of increased drug use and to develop interdiction, education and prevention efforts.