Table 3

Main findings for use or intention to use illicit drugs

PoolingOutcomeDesignReferencesSubgroupsNumber of subjects exp vs ctrl†Effect
measure
Effect size (95% CI)
or effect direction (p-value)‡
Heterogeneity p-value¶
Pooled analysesUse of illicit drugsRCTSlater et al 2006;10 Fang et al 2010;18 Newton et al 2010;11 Schwinn et al 2010;19 Lee et al 2010282701 vs 2769SMD, random effects−0.02 (−0.15 to 0.12)0.020*
Intention to use illicit drugsRCTPolansky et al 1999;25 Yzer et al 2003;26 Zhao et al 2006;27 Fang et al 201018771 vs 499SMD, fixed effects−0.07 (−0.19 to 0.04)0.840
Past-month use of methamphetamine4 ITS and 1 CBAMeth Project studies12–16age 12–1714 865 vs 7497OR, random effects1.16 (0.83 to 1.61)
age 18–24347 vs 632OR, random effects1.63 (0.70 to 3.79)
Past-year use of methamphetamine4 ITS and 1 CBAMeth Project studies12–16age 12–1717 105 vs 7497OR, random effects0.59 (0.43 to 0.81)**
age 18–241039 vs 632OR, random effects0.70 (0.34 to 1.45)
Single studiesLifetime, past 90-day, or past-30-day use of marijuanaRCT (community-media)Slater et al 201121NA (3236)OR, random effects0.60 (0.38 to 0.94)*
Cohort (mass-media)OR, random effects0.26 (0.19 to 0.35)***
Past-year use of marijuanaCohortHornik 200622NA (3529)OR, fixed effects1.21 (1.19 to 1.65)*
Intention to use marijuanaNA (2915)OR, fixed effects0.89 (0.79 to 1.00)§
Past 12-month episodes of cannabis intoxicationCohortScheier and Grenard et al 201023age 13–14NA (2515)mean difference, SEM−0.022
age 15–18mean difference, SEM0.144*
Past 30-day use of marijuana among high-sensation seekersITSPalmgreen et al 200124FayetteNA (3174)test for slope↓ (p=0.001)
Knox, first campaignNA (3197)test for slope↓ (p=0.001)
Knox, second campaigntest for slope↓ (p=0.002)
Past 30-day use of marijuana (girls, 8th grade)ITSCarpenter and Pechmann 20112NA (130 245)OR, fixed effects0.67 (0.52 to 0.87)**
Frequency of use of 10 types of drugsCBAMiller et al 200020567 vs 431mean difference, ANOVAfor LSD: ↑ (p<0.001)
for marijuana, cocaine, amphetamine, and heroin: ‘no longer significant’ differences
  • §p<0.10 *p<0.05 **p<0.01 ***p<0.001.

  • †NA=breakdowns of students exposed to the interventions were not available. Number of analysed subjects is between brackets.

  • ‡Whenever the effect size was not reported, ↓=decreased use or intention to use, and ↑=increased use or intention to use.

  • ¶Heterogeneity test for meta-analyses of RCTs.

  • ANOVA, analysis of variance; CBA, controlled before and after; Cohort, prospective cohort; ITS, interrupted time-series; RCT, randomised controlled trial; SEM, structural equation modelling; SMD, standardised mean difference.