ARTICLES THE LANCET • Vol 363 • May 15, 2004 • www.thelancet.com 1579 Summary Background Use of illicit drugs, particularly cannabis, by young people is widespread and is associated with several types of psychological and social harm. These relations might not be causal. Causal relations would suggest that recreational drug use is a substantial public health problem. Non-causal relations would suggest that harm-reduction policy based on prevention of drug use is unlikely to produce improvements in public health. Cross-sectional evidence cannot clarify questions of causality; longitudinal or interventional evidence is needed. Past reviews have generally been non-systematic, have often included cross- sectional data, and have underappreciated the extent of methodological problems associated with interpretation. Methods We did a systematic review of general population longitudinal studies reporting associations between illicit drug use by young people and psychosocial harm. Findings We identified 48 relevant studies, of which 16 were of higher quality and provided the most robust evidence. Fairly consistent associations were noted between cannabis use and both lower educational attainment and increased reported use of other illicit drugs. Less consistent associations were noted between cannabis use and both psychological health problems and problematic behaviour. All these associations seemed to be explicable in terms of non- causal mechanisms. Interpretation Available evidence does not strongly support an important causal relation between cannabis use by young people and psychosocial harm, but cannot exclude the possibility that such a relation exists. The lack of evidence of robust causal relations prevents the attribution of public health detriments to illicit drug use. In view of the extent of illicit drug use, better evidence is needed. Lancet 2004; 363: 1579–88 See Commentary page 1568 Introduction The use of illicit drugs amongst young people seems to be widespread and may be increasing. 1 Cannabis is the most widely used illicit substance, although use of psychostimulants also appears quite common; use of opiates seems less common. Most of these drug users do not access drug treatment services and the consequences of their drug use are unclear. Physical health problems aside, there are concerns that illicit drug use, particularly cannabis use, could cause psychological and social problems. 2 Cannabis use has been shown to be associated with psychological health problems, use of other illegal drugs, reduced educational attainment, and antisocial behaviour. 2 The causal basis of these associations has not been established. If associations are non-causal, harm- reduction policies based on the prevention of drug use are likely to be ineffective. Conversely, a causal association could mean that “recreational” illicit drug use, in view of its apparent extent, represents an important, and substantially hidden, public health problem. Causal explanations for associations between drug use and psychosocial harm compete with three alternative explanations: reverse causation, where drug use is a consequence, rather than a cause, of psychosocial problems; bias, where the association is an artifact of study methodology; and confounding, when drug use is associated with other factors that predispose to psychosocial problems. A causal relation between drug use and psychosocial harm could plausibly be mediated by two principal mechanisms: directly, through neurophysiological pathways, or indirectly, through involvement in the criminal culture and commerce associated with use of an illegal substance. 3,4 Past reviews of the relevant evidence have often been non-systematic and have used restricted search strategies. Much evidence is cross-sectional and derives from highly selected samples. Such evidence is limited as a basis for inferring true causal relations and their possible relevance to public health. We therefore undertook a systematic review of general population, longitudinal studies relating illicit drug use by young people to subsequent psychological and social harm. Methods Search strategy and selection criteria We searched the general electronic databases MEDLINE, EMBASE, CINAHL, PsycLIT, and Web of Science, and the specialist databases of the Lindesmith Center, DrugScope, US National Institute on Drug Abuse and Substance Abuse and Mental Health Services Administration, and Addiction Abstracts, with an agreed battery of search terms (available from the authors) in July, 2000. This search was updated in July, 2001, and again in June, 2003. Addiction Abstracts was hand- searched for the period not covered by the electronic database. Key individuals in the specialty of addictions Psychological and social sequelae of cannabis and other illicit drug use by young people: a systematic review of longitudinal, general population studies John Macleod, Rachel Oakes, Alex Copello, Ilana Crome, Matthias Egger, Mathew Hickman, Thomas Oppenkowski, Helen Stokes-Lampard, George Davey Smith Departments of Primary Care and General Practice (J Macleod PhD, R Oakes BSc, T Oppenkowski MSocSci, H Stokes-Lampard MRCGP) and Psychology (A I Copello PhD), University of Birmingham, Birmingham, UK; Academic Psychiatry Unit, Keele University, Stoke on Trent, UK (Prof I Crome MD); Department of Social Medicine, University of Bristol, Bristol, UK (Prof M Egger MD, Prof G Davey Smith DSc); Department of Social and Preventive Medicine, University of Berne, Berne, Switzerland (Prof M Egger); and Centre for Research on Drugs and Health Behaviour, Social Science and Medicine, Imperial College, London, UK (M Hickman PhD) Correspondence to: Dr John Macleod, Department of Primary Care and General Practice, Primary Care Clinical Sciences and Learning Centre building, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (e-mail: j.a.macleod@bham.ac.uk) For personal use. 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