meta-analysis characterized comorbidity among ADHD probands, but few compared whether substance use outcomes based on stimulant medication status differed by comorbidity status or type. Given that externalizing disorders may confound the association between ADHD and substance use outcomes,47 future research must parse whether the null effects of stimulant medication treatment and later substance outcomes vary by (type of) comorbidity. We urge researchers to include detailed information on outcome by comorbidity so that this information may be examined in future meta-analytic reviews. Fifth, there is evidence that age of treatment initiation is a relevant construct in later substance outcomes because one study found that children who began taking stimulant medication before 8 years of age did not differ in nonalcoholic substance use compared with those without medication treatment, whereas those who began medication treatment after 8 years of age had increased substance abuse.11 We were unable to thoroughly assess the potential role of age of medication use onset, along with other important and relevant medication-related information thoroughly (type of medication, dosages, medication discontinuation, and treatment adherence) and, importantly, current treatment status. However, future research in this domain must carefully document and examine these potential moderators.
