Thirty-four RCTs, providing 43 effect sizes, were included in the review (n=10,632 participants). Sample sizes ranged from 29 to 2,318 patients. Quality scores ranged from 5 to 8 points out of a maximum possible score of 10 points. Duration of follow-up ranged from one week to 156 weeks.
The overall effect of the interventions was small (d=0.20, 95% CI 0.13 to 0.27). There was evidence of significant statistical heterogeneity (p<0.001).
Effects were greater for trials where the intervention was compared with an attention/placebo comparator (d=0.22, 95% CI 0.15 to 0.30; 35 comparisons) than those compared with an active comparator (d=0.10, 95% CI -0.06 to 0.25; seven comparisons).
Smaller effects were associated with interventions aimed at treating tobacco use (d=0.14, 95% CI 0.06 to 0.23; 13 comparisons) than those treating alcohol use (d=0.22, 95% CI 0.14 to 0.29; 28 comparisons), interventions that included entertainment (d=0.14, 95% CI 0.04 to 0.25; 13 comparisons), chat/discussion features (d=0.12, 95% CI -0.01 to 0.25; eight comparisons), interventions that involved moderate therapist contact (d=0.09, 95% CI -0.04 to 0.22; eight comparisons) and trials that evaluated abstinence as an outcome variable (d=0.14, 95% CI 0.05 to 0.24; 13 comparisons). There was no evidence of significant statistical heterogeneity between the subgroups for any of the moderator variables.
There were no significant correlations between treatment effect and number of sessions, weeks of follow-up, or methodological score.
Subgroup analyses excluding seven trials that used an active treatment as comparator: A greater effect was reported for trials that assessed alcohol (d=0.26) than trials that assessed tobacco (d=0.12). Smaller effects were associated with trials that measured abstinence (d=0.12) than trials that measured post-intervention substance use (d=0.28) or reduction in substance use (d=0.30). A greater effect was reported for offline programs (d=0.37) than web programs (d=0.18).