BackgroundAdolescents are vulnerable to risky behaviours that are likely to co-occur. We examined whether happiness, awareness of alcohol- or smoking-related harm or the size of friendship networks would be longitudinally associated with young people’s risky behaviours.MethodsWe used available cases (N=1,729) from adolescents aged between 10 and 15 who participated in waves 2 and 3 of the UK Longitudinal Household Study that has annually collected population representative data from 40,000 UK households. The outcome variable was patterns of cigarette and alcohol use among adolescents (1= persistent non-use; 2= ex-use; 3= initiation; 4= persistent use) that we derived by tabulating current alcohol or cigarette use at waves 2 and 3. Explanatory variables were scores on participants’ perception of overall happiness, awareness of harm due to alcohol and cigarette use, and supportive friendship network size, collected at wave 2. Covariates were participants’ sex, age, base level of self-reported health status, reported religious affiliation, and household social position. All estimates were corrected for the complex survey design and non-response. Multinomial logistic regression was used to test assumed associations by taking persistent cigarette and alcohol use as the reference category.ResultsFindings showed higher happiness scores were longitudinally associated with adolescents’ persistent non-use (RRR=1.06, 95% CI=1.01-1.13). Awareness of alcohol or cigarette use-related harm was longitudinally associated with persistent non-use (RRR=1.24, 95% CI 1.15-1.35) as well as initiation of alcohol or cigarette use (RRR=1.21, 95% CI=1.11-1.32).ConclusionJoint interventions to promote happiness and harm awareness could help young adolescents from engaging with drinking alcohol or smoking cigarettes.