Background

Although use of illicit drugs shows varying degree of heritability, the influence of shared and unique environmental factors predominate among adolescents. We explored factors predicting use of cannabis and other illicit drugs among Finnish adolescent twins. Methods

We used longitudinal data from the FinnTwin12–17 study with baseline at age 11–12 and follow-up at ages 14 and 17½, including 4138 individuals. The outcome was self-reported ever use of cannabis or other illicit drugs at age 17½. The potential predictors were measures reported by the twins, their parents or teachers. As individual factors we tested smoking, alcohol use, behavioral and emotional problems; as peer factors: number of smoking friends and acquaintances with drug experience; as family factors: parental substance use, socio-economic status and pre-natal exposure to nicotine. We used logistic regression models, controlling for twinship, age and sex, to compute odds ratios (OR) for each potential predictor. To adjust for within-family confounds, we conducted conditional logistic regressions among 246 twin pairs discordant for drug use. Results

13.5% of subjects had initiated use of cannabis or other illicit drugs by age of 17½. When adjusted for within-family confounds, smoking, drinking, and aggressiveness, as well as smoking and drug use among peers predicted use of illicit drugs. In the final regression model, the significant predictors were female sex, early smoking onset, drinking to intoxication, having smoking peers and acquaintances with drug experience, father's weekly drinking to intoxication, and aggressive behavior among boys. Smoking initiation by age of 12 was the most powerful predictor among individuals (OR = 26, p < 0.001) and within discordant pairs (OR = 22, p < 0.001). Conclusions

Early onset smoking is a powerful predictor for subsequent use of illicit drugs among Finnish adolescents, but the causal nature of this relationship needs to be clarified.