Six RCTs (n=11,926) were included in the review. Quality assessment data were not reported. Sample sizes ranged from 604 to 3,989.
All studies combined (knowledge and comprehensive program interventions) using a combined outcome measure (alcohol and marijuana use) reported a pooled MUR of 0.95 (95% CI 0.91 to 1.00) in comparison with control.
In comparison with no exposure, comprehensive intervention programs resulted in a mean absolute reduction of seven days in marijuana use per month (MUR 0.93, 95% CI 0.92 to 0.94, range six to eight days; two studies) and 12 days of alcohol usage (MUR 0.88, 95% CI 0.87 to 0.89, range of 11 to 13 days; three studies)
Knowledge-only programmes resulted in a mean absolute usage of two days of alcohol usage per month (MUR 0.98, 95% CI 0.92 to 1.04, range four to six days; two studies). One study of a knowledge-based outcome assessed marijuana use
Sensitivity analyses did not significantly alter the review findings. The risk of publication bias could not be assessed due to the paucity of studies.