Does the regular use of skunk cause psychosis? A study in South London set out to test that link and found a high correlation between daily skunk use and first episode psychosis. However, no correlation was found in this study between psychosis and the daily use of the less powerful form of cannabis, hash.
Previous studies have suggested a link between cannabis use and psychosis. The UK’s 2012 Schizophrenia Commission claimed that cannabis use is the most preventable risk factor for psychosis. This present study is important because it attempts to clarify which type or frequency of cannabis use contributes to this risk factor.
The study looked at 410 patients with first-episode psychosis and compared them to a control sample of 370 individuals drawn from the general population in the study area. The study looked at the frequency and type of cannabis use. The researchers found that the risk of individuals having a psychotic disorder showed a roughly three-times increase in users of skunk, compared with those who never used cannabis. The researchers had allowed in their analysis for other differences between the patients who had developed a psychosis and the controls.
These results are in line with other studies that have shown a link between the heavy use of cannabis and schizophrenia-like psychosis. The importance of this study is in distinguishing between the types of cannabis being used, skunk or hash. Skunk has a greater concentration of THC than hash (15% as against 5%) and an almost complete absence of cannabidiol (compared to 4% in hash). Some studies have suggested that THC can produce psychosis-like experiences, while there is a suggestion that cannabidiol has the effect of preventing psychosis.
The study was reported in the UK press with headlines that claimed a causal link between the heavy use of skunk and the onset of psychosis. However, the researchers acknowledge that showing a correlation between two factors does not prove causation. It may be that there is another common factor that predisposes individuals to psychosis and to smoke skunk. Perhaps individuals experiencing the early stages of psychosis tend to use stronger cannabis as self-medication. However, the researchers point out that hash would be far more likely to be used as self-medication, given its anti-psychotic and calming effects. This leaves the likelihood, at very least, that there is indeed a causal link between the daily use of skunk and the onset of psychosis.
The study is published in the Lancet and is freely available to download.