children with more severe ADHD would be more likely to receive medication treatment) is likely relevant.44 Given that medication treatment may be biased toward more severe cases,45 the present findings may indeed represent a protective effect if the group treated with stimulant medication had forgone that medication and developed substance use problems at a higher rate. While we look forward to forthcoming data from randomized controlled studies, such as the Multimodal Treatment Study of ADHD, even these results are qualified given that families from nonmedication treatment arms may obtain medication treatment after randomization. Third, we were limited in the substance outcomes available for meta-analysis in the present literature. Other substance use measures, including frequency or quantity of use, may be meaningful outcomes to examine given limitations with history of use (yes/no), in particular if measurement of substance use is after high school age.46 Fourth, the issue of comorbidity in ADHD is likely to be salient15 Several studies included in the 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
