One in four new cases of psychotic conditions such as schizophrenia could be the direct result of smoking extra-strong varieties of cannabis, a major new study concludes.

The finding suggests that about 60,000 people in Britain are currently living with conditions involving hallucinations and paranoid episodes brought on by abuse of high-potency cannabis, known as skunk, and more than 300,000 people who have smoked skunk will experience such problems in their lifetime.

The six-year study, the first of its kind in Britain, calculates that daily users of skunk are five times more likely to suffer psychosis than those who never touch it.

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Study:

Proportion of patients in south London with first-episode psychosis attributable to use of high potency cannabis: a case-control study

Between May 1, 2005, and May 31, 2011, we obtained data from 410 patients with first-episode psychosis and
370 population controls. The risk of individuals having a psychotic disorder showed a roughly three-times increase in
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). Use of skunk-like cannabis every day conferred 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 our geographical area was 24% (95% CI 17–31), possibly because of the high
prevalence of use of high-potency cannabis (218 [53%] of 410 patients) in our study.

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