PubMed -  citations and abstracts in the fields of medicine, nursing, dentistry, veterinary medicine, health care systems, and preclinical sciences.

PubMed Central - a database of articles in the fields of medicine, nursing, dentistry, veterinary medicine, health care systems, and preclinical sciences.

PubMed Clinical Queries - specialized searches for COVID-19 articles, clinical study categories, and medical genetics

Agency for Healthcare Research & Quality - lead Federal agency charged with improving the safety and quality of America's health care system

BioMed Central - hundreds of open access, peer-reviewed journals in all areas of biology and medicine.

Environmental Health & Toxicology - information resources covering toxicology, environmental health, outreach to underserved and special populations, HIV/AIDS, drugs and household products, and disaster/emergency preparedness and response.

NCBI Bookshelf - online books and reports in the life sciences and healthcare.

NIH Databases and Directories - extensive list of databases and directories in the health sciences.

Open Science Framework Preprints - "Preprints" are typically complete and public versions of articles before they have gone through peer review. 

PLoS - open access journals in the biological and medical sciences.

PTSDpubs - an extensive PTSD resource produced by the U.S. Department of Veterans Affairs although it is not limited to literature on PTSD among Veterans.

PubMed Dietary Supplement Subset - citations on a broad spectrum of dietary supplement literature including vitamin, mineral, phytochemical, ergogenic, botanical, and herbal supplements in human nutrition and animal models.