Besides PubMed, there are a number of other databases where you might find articles related to your topic. If you are conducting a Systematic Review, you must search at least two databases to find all relevant literature on your topic. The list below shows literature databases that can be useful.
"The term ‘grey literature’ is often used to refer to reports published outside of traditional commercial publishing. Review authors should generally search sources such as dissertations and conference abstracts" (Higgins et al. 2019, section 4.3.5). Grey literature may include research reports, conference papers, dissertations and theses, clinical trials, government documents, census data, standards, patents, and other research outputs.
Grey literature has traditionally been considered somewhat difficult to locate, but it is important to consult these unpublished studies to reduce the risk of publication bias in results. Read the VU LibGuide "Grey literature for health sciences".
Reference: Higgins, J. (2019) 'Searching for studies' in Higgins, J. and Thomas, J., eds., Cochrane handbook for systematic reviews of interventions [online], Version 6, available: http://handbook.cochrane.org/ [accessed 27 May 2020].
Handsearching involves the page-by-page examination of relevant journal issues, conference proceedings and other publications for relevant studies. In addition, the checking of reference lists of journal articles and other documents retrieved from a search.
Why is handsearching important?
Locates relevant items poorly indexed or not indexed at all. Some databases do not comprehensively index all content in journal issues, or may not index at all supplements, special issues, or conference abstracts
Allows researchers to scan content quickly for relevant studies from high-impact journals
Ensures that relevant studies are not overlooked. (HLWIKI Canada)
Citation indexes track references authors include in the reference lists of their publications. They provide a means to search for and analyze the literature in a way not possible through simple keyword searching.
Web of Science includes the Science Citation Index; Social Sciences Citation Index; Arts & Humanities Citation Index.
Coverage of over 10,000 high-impact journals in the sciences, social sciences, and arts and humanities, as well as international proceedings coverage for over 120,000 conferences.