The gold standard, required by Cochrane Collaboration, Joanna Briggs and Institute of Medicine (U.S.) is to search several databases when attempting to identify all published evidence for a systematic review.  This ensures that no evidence is missed,  aids searchers to overcome variation in indexing, coverage, and search algorithms, and provides greater likelihood to minimize biases. 

When conducting a literature search for a systematic review the following databases should be considered:

• MEDLINE, EMBASE and Cochrane Central Register of Controlled Trials (CENTRAL) are three key databases to search for published studies in research, related to finding evidence to support clinical decision making.
• CINAHL or PsycINFO should always be used for topics related to nursing, allied health, or behavioral sciences and mental health.
• Specialized databases should be used if the topic is broader than clinical medicine or nursing, or too narrow and the journals in which this research is published don’t get indexed in mainstream databases like MEDLINE or EMBASE.  Some examples of specialized databases are: ERIC (Education Resources Information Center), PEDro (Physiotherapy Evidence Database), IPA (International Pharmaceutical Abstracts), Global Health, Sociological Abstracts, PAIS (Public Affairs Information Service), Cambridge Scientific Abstracts (CSA), LILACS (Latin American and Caribbean Health Sciences Literature).
• Web of Science or Scopus and Google Scholar should be incorporated in the process of searching for published studies to ensure that all unique references not indexed in MEDLINE or EMBASE are discovered.

Additional strategies and tactics to consider when searching multiple databases:

• Adapt search terms for each database while keeping the conceptual structure of the original search.  Review the controlled vocabulary headings for each database to identify appropriate terms. Often headings that have a similar scope or definition vary slightly in the terminology used or differ in granularity from one database to another.
• Keep in mind that the search syntax will be different with every database.  Review each database’s unique syntax before performing the search.
• Many of the more specialized databases do not have the advanced search interfaces needed to conduct complex searches, thus the searches need to be simplified. The loss in precision from the simplified search is often made up for by the fact that the databases contain a smaller number of citations for review.
• Ask key subject experts if they know of any databases specific to the topic of interest.