Background Publication and outcome reporting bias are well known problems when conducting a systematic review. Negative results are less likely to be published and under reporting or selective reporting are particularly common for adverse effects. One way to attempt to overcome these biases is to include unpublished studies or data. Systematic review guidance such as The Cochrane Handbook currently recommend searching beyond the published article by contacting experts or authors and by searching conference abstracts, the grey literature and trial registries (including industry trial registries).

Methods We sought to identify the proportion of systematic reviews of adverse effects which search for unpublished data and the success rates of identifying unpublished data for inclusion in a systematic review.

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 Data were extracted on the types of adverse effects and interventions evaluated, sources searched, how many unpublished studies were included and type of unpublished data included.

Results From 9129 DARE abstracts, 348 met our inclusion criteria. Most 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 Over a third (136/348, 39%) of all the reviews searched a specific source for unpublished data, such as conference abstracts or trial registries. However, less than half of these reviews (62/136, 46%) included unpublished data in their review. The most popular sources searched were conference abstracts, contacting authors and ClinicalTrials.gov.

Overall over a fifth of all the reviews included some unpublished data (78/348, 22%). Although most of these reviews searched specific sources of unpublished data (62/78, 79%), others did not but included sources which contain unpublished studies in addition to published (such as Embase or CENTRAL) (16/78, 11%).

The present survey was unable to accurately determine which sources give the highest yield in terms of unpublished data. However, the results indicate that industry trial registers and the Food and Drug Administration (FDA) website may have the highest success rates.

Conclusion The majority of reviews of adverse effects do not search specifically for unpublished data and of those that do less than half are successful. Given the potential for publication and outcome reporting bias more reviews should attempt to identify unpublished data and research to uncover the most successful sources of unpublished data should be made a priority.