No database covers 100% of published biomedical research. To avoid missing important publications, your best bet is to search multiple databases. MEDLINE (via either the PubMed or Ovid interfaces), Embase, the Cochrane Database of Systematic Reviews, and the Cochrane Register of Controlled Trials are four core databases to include in biomedical systematic reviews. You may also wish to include specialty nursing, psychology, or social science databases depending on the scope of your review.
To avoid publication bias, it is important to search for unpublished literature on your topic. Useful data may be available in meeting abstracts, white papers, and clinical trial registries. Links to several major grey literature sources are below.