We will present results of a systematic review of the effectiveness of methylphenidate in the treatment of adult ADHD. Furthermore, we will discuss the influence of methods on the results of meta analyses.

We searched Medline, PsycINFO and CCTR to identify randomised controlled clinical trials comparing methylphenidate with placebo in the treatment of adult ADHD. Studies were summarized with meta-analytic methods. Subgroup analyses were conducted with respect to parallel group vs. cross-over trials and self vs. observer ratings. The relationship between dosage and effect size was explored by weighted regression analysis. The results were tested for publication bias, and several sensitivity analyses were conducted.

Nineteen studies met the inclusion criteria of which 17 were included in the meta-analysis. The overall effect size was significant at d=0.42 different from zero, but this was only half the size expected on the basis of a previous meta-analysis. No significant differences could be observed in the subgroup analyses. The regression analysis showed no significant influence of mean daily dose on effect size.

These results contradict findings of a previous meta-analysis and challenge guideline recommendations. Differences between populations of children and adults and methodological issues in meta-analyses are discussed.