Abstract: We included six trials with 1176 adolescents that measured outcomes at different follow-up periods in this review. Three studies with 732 adolescents compared brief interventions (Bls) with information provision only, and three studies with 444 adolescents compared Bls with assessment only. Reasons for downgrading the quality of evidence included risk of bias of the included studies, imprecision, and inconsistency. For outcomes that concern substance abuse, the retrieved studies only assessed alcohol and cannabis. We generally found moderate-quality evidence that, compared to information provision only, BIs did not have a significant effect on any of the substance use outcomes at short-, medium-, or long-term follow-up. They also did not have a significant effect on delinquent-type behaviour outcomes among adolescents. When compared to assessment-only controls, we found low- or very low-quality evidence that BIs reduced cannabis frequency at short-term follow-up in one study (standardised mean difference (SMD) -0.83; 95% confidence interval (CI) -1.14 to -0.53, n = 269). BIs also significantly reduced frequency of alcohol use (SMD -0.91; 95% CI -1.21 to -0.61, n = 242), alcohol abuse (SMD -0.38; 95% CI -0.7 to -0.07, n = 190) and dependence (SMD -0.58; 95% CI -0.9 to -0.26, n = 190), and cannabis abuse (SMD -0.34; 95% CI -0.65 to -0.02, n = 190) at medium-term follow-up in one study. At long-term follow-up, BIs also reduced alcohol abuse (SMD -0.72; 95% CI -1.05 to -0.40, n = 181), cannabis frequency (SMD -0.56; 95% CI -0.75 to -0.36, n = 181), abuse (SMD -0.62; 95% CI -0.95 to -0.29, n = 181), and dependence (SMD -0.96; 95% CI -1.30 to -0.63, n = 181) in one study. However, the evidence from studies that compared brief interventions to assessment-only conditions was generally of low quality. Brief interventions also had mixed effects on adolescents' delinquent or problem behaviours, although the effect at long-term follow-up on these outcomes in the assessment-only comparison was significant (SMD -0.78; 95% CI -1.11 to -0.45). We found low- or very low-quality evidence that brief school-based interventions may be more effective in reducing alcohol and cannabis use than the assessment-only condition and that these reductions were sustained at long-term follow-up. We found moderate-quality evidence that, when compared to information provision, brief interventions probably did not have a significant effect on substance use outcomes. It is premature to make definitive statements about the effectiveness of brief school-based interventions for reducing adolescent substance use. Further high-quality studies examining the relative effectiveness of BIs for substance use and other problem behaviours need to be conducted, particularly in low- and middle-income countries.

Plain-language summary: We included six studies in this review, with 1176 adolescents overall. The mean age of adolescents was 16.9 years. We were interested in studies with short-, medium-, and long-term follow-up periods to assess whether any effects were due to the brief intervention. The studies compared brief intervention programmes with two major kinds of comparison or control groups: 1) an information provision only (general health promotion materials and harm reduction information) group and 2) an assessment-only group, where adolescents received no intervention but were evaluated on substance use and other behaviour at follow-up appointments at different time periods following delivery of the intervention. Three studies with 732 adolescents compared brief interventions with information provision only, while the other three, with 444 adolescents, compared brief interventions with assessment only. Trials were either conducted in the United States or the United Kingdom. Delivery of the interventions was individual or group face-to-face feedback across high schools and further education colleges. All interventions were up to four sessions in length. Our primary outcome was abstinence or reduction of substance use behaviour, and our secondary outcomes were engagement in criminal activity related to substance use and engagement in delinquent-type behaviours related to substance use. For outcomes that concern substance use, the studies assessed use of alcohol and cannabis. When compared to information provision, brief interventions are probably not more efficacious in reducing substance use or delinquent behaviour. When compared to assessment-only controls, the interventions may have some significant effects on substance use and behaviours. At short-term follow-up, brief interventions significantly reduced cannabis frequency in one study. At medium-term follow-up, brief interventions significantly reduced frequency of alcohol use, alcohol abuse and dependence symptoms, and cannabis abuse symptoms in one study. At long-term follow-up, brief interventions significantly reduced alcohol abuse, cannabis frequency, and cannabis abuse and dependence symptoms in one study. The pattern of results indicates that adolescents who received a brief intervention generally did better in reducing their alcohol and cannabis use than adolescents who received no intervention at all. However, adolescents who received a brief intervention did not seem to do better in reducing their alcohol and cannabis use than adolescents who received information-only interventions. It is therefore premature to make definitive statements about the effectiveness of brief school-based interventions for reducing adolescent substance use. Overall, the evidence was of moderate or low quality, with two outcomes found to have very low quality of evidence. There were three major issues across the studies: 1) there was no blinding of adolescents, 2) there was uncertainty as to whether participant allocation to study groups was concealed, and 3) a small total number of adolescents and number of events. None of the included studies reported information about funding source or conflicts of interest.