Public Release: 

Better research needed to assess psychological and social effects of cannabis

NB. Please note that if you are outside North America, the embargo for LANCET press material is 0001 hours UK Time 14 May 2004.


Authors of a study in this week's issue of THE LANCET conclude that more evidence is required to determine the nature and extent of the association between cannabis use and negative psychosocial outcomes.

The use of cannabis and other illicit drugs by young people is linked to psychosocial harm including psychological health problems, the use of other illegal drugs, reduced educational attainment, and antisocial behaviour. However, whether these associations are causal (ie, cannabis use is a direct cause of psychological illness and antisocial behaviour) remains unclear.

John Macleod from the University of Birmingham, UK, and colleagues reviewed general population longitudinal studies reporting associations between illicit drug use by young people and psychosocial harm. They identified 48 relevant studies, of which the most reliable evidence came from 16 studies.

In general, consistent associations were identified between cannabis use and both lower educational attainment and increased reported use of other illicit drugs. Less consistent associations were found between cannabis use and psychological health problems and problematic behaviour.

Dr Macleod comments: "Despite widespread concern, we have found no strong evidence that use of cannabis in itself has important consequences for psychological or social health. This finding is not equivalent to the conclusion that use of cannabis is harmless in psychosocial terms; problems with the available evidence render it equally unable to support this proposition. Better evidence is needed in relation to cannabis, which is widely used, and in relation to other drugs that, although less widely used, might have important effects".

In an accompanying Commentary (p 1568) Franjo Grotenhermen from the Nova-Institut GmbH, Germany, states: "There is some reason to believe that cannabis contributes to psychosocial problems in adolescents and young adults, and no responsible adult would want young people to take drugs. There is no question that this issue is an important candidate for education and prevention, but there is a fierce debate on the place repressive measures should have in this context. There is little reason to believe that criminalisation has had a strong effect on the extent of cannabis use by young people. Moreover, prohibition itself seems to increase the harmfulness of drug use and cause social harm. By stopping all cannabis users from being treated as criminals, I believe this year's change by the British Government of its cannabis law (a declassification from class B to C) is a sensible attempt to balance the possible harms caused by cannabis and its prohibition... we must learn to deal with drugs and their possible dangers without fear".


Lancet 2004; 363: 1568, 1579-88

Contact: Abigail Dixon, Press Office, University of Birmingham;
T) 44-121-414-5134;
E) Dr Franjo Grotenhermen, Nova-Institut GmbH, D-50354 Huerth, Germany;
T) 49-221-9543-9228;

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