RESEARCH REPORT © 2004 Society for the Study of Addiction Addiction, 99, 39–52 Blackwell Science, LtdOxford, UKADDAddiction1360-0443© 2003 Society for the Study of Addiction to Alcohol and Other Drugs 98Original ArticleJim McCambridge & John StrangEfficacy of single-session motiva- tional interviewing Correspondence to: Jim McCambridge National Addiction Centre (The Maudsley/Institute of Psychiatry) Addiction Sciences Building 4 Windsor Walk Denmark Hill London SE5 8AF UK E-mail: J.McCambridge@iop.kcl.ac.uk Submitted 2 August 2002; initial review completed 16 September 2002; final version accepted 4 July 2003 RESEARCH REPORT The efficacy of single-session motivational interviewing in reducing drug consumption and perceptions of drug-related risk and harm among young people: results from a multi-site cluster randomized trial Jim McCambridge & John Strang National Addiction Centre (The Maudsley/Institute of Psychiatry), London, UK ABSTRACT Aim To test whether a single session of motivational interviewing (discussing alcohol, tobacco and illicit drug use) would lead successfully to reduction in use of these drugs or in perceptions of drug-related risk and harm among young people. Design Cluster randomized trial, allocating 200 young people in the natural groups in which they were recruited to either motivational interviewing (n = 105) or non-intervention education-as-usual control condition (n = 95). Setting Ten further education colleges across inner London. Participants Two hundred young people (age range 16–20 years) currently using illegal drugs, with whom contact was established through peers trained for the project. Intervention The intervention was adapted from the literature on motivational interviewing in the form of a 1-hour single-session face-to-face interview struc- tured by a series of topics. Measurements Changes in self-reported cigarette, alcohol, cannabis and other drug use and in a range of drug-specific perceptions and other indicators of risk and harm. Measurement at recruitment and follow-up interview 3 months later. Findings A good follow-up rate (89.5%; 179 of 200) was achieved. In compar- ison to the control group, those randomized to motivational interviewing reduced their of use of cigarettes, alcohol and cannabis, mainly through mod- eration of ongoing drug use rather than cessation. Effect sizes were 0.37 (0.15– 0.6), 0.34 (0.09–0.59) and 0.75 (0.45–1.0) for reductions in the use of ciga- rettes, alcohol and cannabis, respectively. For both alcohol and cannabis, the effect was greater among heavier users of these drugs and among heavier cig- arette smokers. The reduced cannabis use effect was also greater among youth usually considered vulnerable or high-risk according to other criteria. Change was also evident in various indicators of risk and harm, but not as widely as the changes in drug consumption. Conclusions This study provides the first substantial evidence of non-treatment benefit to be derived among young people involved in illegal drug use in receipt of motivational interviewing. The targeting of multiple drug use in a generic fashion among young people has also been supported. KEYWORDS: alcohol, brief intervention, cannabis, cigarette smoking, drugs, motivational interviewing, young people.