PubMed
PubMed comprises more than 23 million citations for biomedical literature from MEDLINE, life science journals, and online books. MEDLINE, the authoritative database of the NLM, is the principal component of PubMed's search service system.
a) Enter your search query. Select the appropriate filter for your question under Article types (e.g. systematic reviews, meta-analyses, randomized controlled trials, case reports, etc.) Note: Use MeSH subject terms instead of Article type filters to obtain case-control and cohort studies.
b) Enter search into Clinical Queries. [Clinical Queries uses specialized methodology search filters to retrieve clinically sound control trial studies regarding the therapy, diagnosis, etiology, and prognosis of diseases.]