Should meta-analysts search Embase in addition to Medline?

It is widely accepted that meta-analysts should search multiple databases. The selection of databases is ideally based on the potential contribution of each database to the project or on the potential for bias if a database is excluded, as supported by research evidence. We explore whether searching Embase yields additional trials that influence a meta-analysis. We identified meta-analyses that searched Medline and Embase. A random-effects weighted mean method was used to estimate the intervention effect in articles indexed only in Embase compared with those indexed elsewhere. On average, Embase-unique trials yielded significantly smaller estimates by 29% (ratio of odds ratio [ROR] 0.71, 95% confidence interval [CI] 0.56-0.90) but influenced the pooled estimate by an average of only 6% (ROR 0.94, 95% Cl 0.88-0.99). Searching Medline but not Embase risks biasing a meta-analysis by finding studies that show larger estimates, but their prevalence seems low enough that the risk may be slight, provided the rest of the search is comprehensive. (C) 2003 Elsevier Inc. All fights reserved.