Most systematic reviews of adverse effects did not include unpublished data

Objectives: We sought to identify the proportion of systematic reviews of adverse effects which search for unpublished data and the success rates of identifying unpublished data for inclusion in a systematic review.

Study design and setting: Two reviewers independently screened all records published in 2014 in the Database of Abstracts of Reviews of Effects (DARE) for systematic reviews where the primary aim was to evaluate an adverse effect or effects. Data were extracted on the types of adverse effects and interventions evaluated, sources searched, how many unpublished studies were included, and source or type of unpublished data included.

Results: From 9,129 DARE abstracts, 348 met our inclusion criteria. Most of these reviews evaluated a drug intervention (237/348, 68%) with specified adverse effects (250/348, 72%). Over a third (136/348, 39%) of all the reviews searched, a specific source for unpublished data, such as conference abstracts or trial registries, and nearly half of these reviews (65/136, 48%) included unpublished data. An additional 13 reviews included unpublished data despite not searching specific sources for unpublished studies. Overall, 22% (78/348) of reviews included unpublished data/studies.

Conclusion: Most reviews of adverse effects do not search specifically for unpublished data but, of those that do, nearly half are successful.