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

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 structured 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 comparison to the control group, those randomized to motivational interviewing reduced their of use of cigarettes, alcohol and cannabis, mainly through moderation 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 cigarettes, alcohol and cannabis, respectively. For both alcohol and cannabis, the effect was greater among heavier users of these drugs and among heavier cigarette 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.