Several important study limitations should be emphasized. First, although the current meta-analysis improved substantially on the heuristic meta-analysis of Wilens et al,12 it is still relatively modest in terms of the number of studies included. In addition to implementing standard procedures to combat the file drawer problem, we independently contacted several research groups with longitudinal studies of children with ADHD to inquire about potential unpublished data. Although these efforts resulted in the addition of several studies with unpublished data, several investigators did not respond to or declined these requests. Second, the inference that stimulant medication treatment is unrelated to later substance outcomes is based on correlational data. That is, in the absence of random assignment to different treatment groups (eg, with and without stimulant medication), observed group differences may reflect unmeasured confounds. The potential role of intervention selection bias (eg, 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
