Six trials (4,679 participants, range 41 to over 2,500) were included.
Primary care based interventions (three fair-quality trials): One trial found a reduced risk of marijuana use in adolescents who received a primary care-based intervention at 12 months in one cohort (Czech Republic, adjusted risk ratio 0.47, 95% CI 0.32 to 0.71) but not in another cohort (USA). Two other primary-care based trials found no statistically significant differences in drug use between intervention and control at six to 12 months follow-up.
Computer-based interventions (two good-quality and one fair-quality trial): All three trials found statistically significant effects that favoured the intervention in numbers of self-reported marijuana use and use of prescription drugs for non-medical purposes at one or two years follow-up. Mean marijuana use occasion scores ranged from 0.0 to 0.10 single use in the past 30 days in the intervention group and from 0.11 to 0.20 in the control groups. Prescription drugs use for non-medical purposes occasions in the intervention groups ranged from 0.0 to 0.09 in the past 30 days at both the one- and two-year follow-up compared with 0.10 to 1.6 at 1 year and from 0.11 to 3.6 at 2 years in the control groups. One study found that girls in the intervention group reported fewer occasions of inhalant use than those in the control group (p<0.024). All three trials generally found no reduction in depressed mood at 12 or 24 months.
One trial found no effect on marijuana-related consequences or driving under the influence of marijuana (primary care health professional or computer delivery). No other studies reported data on health, social and legal outcomes and none reported data on adverse events.