BACKGROUND: Early and risky adolescent alcohol use have each been associated with adult alcohol consumption. However, it remains unclear whether these behaviours independently predict later-life substance use when considered jointly, and research examining links with substances other than alcohol is limited. This study addresses these gaps by examining longitudinal associations between age at first alcohol and risky adolescent alcohol use, with alcohol, tobacco, cannabis and other drug use in early adulthood, and aims to identify critical periods for public health interventions.

METHODS: Growing Up in Ireland is a nationally representative cohort (recruited aged 9 [Wave 1], born 1997-1998). Survey-weighted logistic regression examined whether age at first alcoholic drink and risky alcohol use at age 17 (Alcohol Use Disorders Identification Test scores) independently predict high-risk alcohol (AUDIT > 15), tobacco, cannabis and other drug use at age 20. Models were adjusted for age, sex, academic ability, personality, psychological factors, socioeconomic status, familial, peer and neighbourhood substance use.

RESULTS: The study included 4554 participants (49.8% female). Early alcohol use was common, with 27% reporting use aged 14 or younger. By age 20, 14% reported high-risk alcohol, 38% tobacco, 24% cannabis and 28% other drug use. Older age at first alcohol was associated with dose-response reductions in the odds of high-risk alcohol, tobacco, cannabis and other drug use at age 20, relative to those initiating alcohol at 14 or younger. Adolescents with high-risk alcohol use had double the odds of tobacco (adjusted odds ratio (aOR) 2.1, 95% confidence intervals (CI) 1.3-3.30) and other drug use (aOR 2.5, 95% CI 1.6-4.1) and an 11-fold increase in the odds of continued high-risk alcohol use (aOR 11.5, 95% CI 7.0-18.6) at age 20, relative to adolescents with low-risk alcohol use.

CONCLUSIONS: Age at first alcohol and risky adolescent alcohol use independently predict substance use in early adulthood when considered jointly in extensively adjusted models. These findings highlight the continued urgent need for public health interventions that address factors associated with early alcohol use and support adolescents who use alcohol in a high-risk manner given their elevated risk of progression to more serious substance use as adults.