To examine the link between symptoms of hyperactivity-inattention and conduct disorder in childhood, and the initiation of tobacco and cannabis use, controlling for other behavioral symptoms, temperament and environmental risk factors.

The sample (N = 1107 participants, aged 4 to 18 years at baseline) was recruited from the population-based longitudinal Gazel Youth study with a follow-up assessment 8 years later. Psychopathology, temperament, environmental variables, and initiation of tobacco and cannabis use were self-reported. Event time analyses were performed to assess the effects of childhood disruptive symptoms on age at first use of tobacco and cannabis.

Proportional hazard models revealed that participants with high levels of childhood symptoms of both hyperactivity-inattention and conduct disorder were at highest risk of early tobacco initiation (in males: hazard ratio [HR] = 2.05; confidence interval [CI]: 1.24–3.38; in females: HR = 2.01; CI: 1.31–3.09), and, in males, of early cannabis initiation (HR = 1.95; CI: 1.04–3.64). Temperament, through activity in both males and females and negative emotionality in females, was also associated to early substance use initiation.

Children who simultaneously have high levels of symptoms of hyperactivity-inattention and conduct disorder are at increased risk for early substance initiation. These associations may guide childhood health professionals to consider the liability for early substance initiation in high-risk groups.