Main results
Six studies involving 1139 participants were included in this review. Overall the quality of evidence was moderate in the information
provision comparison, and low or very low in the assessment only comparison. Reasons for downgrading the quality included risk of
bias of the included studies, imprecision and inconsistency. Our findings suggested that compared to information provision only, brief
interventions (BIs) did not have a significant effect on any substance use (three studies, 732 participants, standardised mean difference
(SMD) -0.06; 95% confidence interval (CI) -0.20 to 0.09) or delinquent-type behaviour outcomes among adolescents (two studies,
531 participants, SMD -0.26; 95% CI -0.54 to 0.02). When compared to assessment-only controls, BIs had some significant effects
on substance use and delinquent-type or problem behaviours, but high levels of heterogeneity existed between studies and it was not
always possible to pool the results. When the comparison was with assessment-only conditions, studies of individual interventions that
measured BI effectiveness reported significantly reduced substance use in general and in two studies reduced frequency of alcohol use
specifically. When the data were pooled, BIs reduced cannabis frequency (SMD -0.22; 95% CI -0.45 to -0.02) across three studies (n
= 407). Cannabis quantity was also reduced by BIs in comparison to assessment only (SMD -60.27; 95% CI -66.59 to -53.95) in one
study (n = 179). However, the evidence for studies that compared brief interventions to assessment-only conditions was generally of
low quality. Brief interventions also had mixed effects on participants’ delinquent or problem behaviours.