REHASHING THE EVIDENCE ON PSYCHOSIS AND CANNABIS

As cabinet ministers in the UK fall over themselves to tell all about
their cannabis-taking younger days, Gordon Brown's Government begins
its review of the classification of cannabis, with the probable
outcome of relabelling it a class B drug of misuse. Possession would
then become an offence likely to lead to arrest and perhaps a jail
sentence. Cannabis was downgraded to class C in 2004, which meant
that the penalties for possession, production, or supply were reduced.

The Advisory Council on the Misuse of Drugs will examine the evidence
for harms caused by cannabis including those associated with the
increasingly available strains such as skunk. In January, 2006, after
its last review, the Advisory Council recommended that the class C
status for cannabis should remain, but that resources should be put
into education about the risks of cannabis and into further research
on its effects on mental health.

As pointed out by Merete Nordentoft and Carsten Hjorthoj in a
Comment, "published in this week's Lancet is the most comprehensive
meta-analysis to date of a possible causal relation between cannabis
use and psychotic and affective illness later in life". In their
systematic review, Theresa Moore and colleagues found "an increase in
risk of psychosis of about 40% in participants who had ever used
cannabis", and a clear dose-response effect with an increased risk of
50 200% in the most frequent users.

In 1995, we began a Lancet editorial with the since much-quoted
words: "The smoking of cannabis, even long term, is not harmful to
health." Research published since 1995, including Moore's systematic
review in this issue, leads us now to conclude that cannabis use
could increase the risk of psychotic illness. Further research is
needed on the effects of cannabis on affective disorders. The
Advisory Council on the Misuse of Drugs will have plenty to consider.
But whatever their eventual recommendation, governments would do well
to invest in sustained and effective education campaigns on the risks
to health of taking cannabis.