The Effects of Childhood ADHD Symptoms on Early-onset Substance Use: A Swedish Twin Study

Research has documented that children and adolescents with attention-deficit/hyperactivity disorder (ADHD) are at increased
risk of substance use problems. Few studies, however, have focused on early-onset substance use. This study therefore investigated
how the two symptom dimensions of ADHD (hyperactivity/impulsivity and inattention) are associated with early-onset substance
use, the role of persistent ADHD for the association, and to what extent the association is influenced by genetic and environmental
factors. Twins (1,480 pairs) in the Swedish Twin Study of Child and Adolescent Development were followed from childhood to
adolescence. ADHD symptoms were measured at age 8–9 and age 13–14 via parent-report, whereas substance use was assessed at
age 13–14 via self-report. Results revealed that hyperactive/impulsive symptoms predicted early-onset “sometimes” tobacco
use (adjusted odds ratios, 1.12, for one symptom count), controlling for inattentive symptoms and conduct problem behaviors.
There is no independent effect of inattentive symptoms on early-onset substance use. Children with persistent hyperactivity/impulsivity
(defined as scoring above the 75th percentile at both time points) had a pronounced risk of both early-onset tobacco and alcohol
use (adjusted odds ratios from 1.86 to 3.35, compared to the reference group). The associations between hyperactivity/impulsivity
and early-onset substance use were primarily influenced by genetic factors. Our results indicated that hyperactivity/impulsivity,
but not inattention, is an important early predictor for early-onset substance use, and a shared genetic susceptibility is
suggested to explain this association.