Scientific hypotheses can be tested by comparing the effects of one treatment over many diseases in a systematic review.

OBJECTIVES To describe the use of systematic reviews or overviews (systematic reviews of systematic reviews) to synthesize quantitative evidence of intervention effects across multiple indications (multiple-indication reviews) and to highlight issues pertaining to such reviews. STUDY DESIGN AND SETTING MEDLINE was searched from 2003 to January 2014. We selected multiple-indication reviews of interventions of allopathic medicine that included evidence from randomized controlled trials. We categorized the subject areas evaluated by these reviews and examined their methodology. Utilities and caveats of multiple-indication reviews are illustrated with examples drawn from published literature. RESULTS We retrieved 52 multiple-indication reviews covering a wide range of interventions. The method has been used to detect unintended effects, improve precision by pooling results across indications, and examine scientific hypotheses across disease classes. CONCLUSION Systematic reviews of interventions are typically used to evaluate the effects of treatments, one indication at a time. Here, we argue that, with due attention to methodological caveats, much can be learned by comparing the effects of a given treatment across many related indications.