PubMed comprises more than 28 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include linksPubMed comprises more than 28 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites. to full-text content from PubMed Central and publisher web sites.