Listed below are a few commonly used academic research databases for systematic reviews and other evidence syntheses in the health sciences. A rule of thumb is to incorporate at least 3 academic research databases in your search strategy.
Gray literature includes theses, dissertations, conference proceedings, government reports, clinical trials, preprints, and other information that is not controlled by commercial publishers. The inclusion of gray literature in your search can help minimize the effect of Publication Bias, which is the tendency for studies that have statistically significant results to get published in journals and other commercial publications.
When searching for gray lit, please document 1) search terms used (and what fields/boxes you searched), 2) date run, 3) how many results were retrieved, and 4) how many results were finally included in the review.
In addition to the Web of Science Core Collection (which includes theses, dissertations, conference proceedings), PubMed (which includes preprints), and Google/Google Scholar, below are a few sources that cover different kinds of gray literature: