PubChem provides information on the biological activities of small molecules. It is organized as three linked databases within the NCBI's Entrez information retrieval system. These are PubChem Substance, PubChem Compound, and PubChem BioAssay. PubChem also provides a fast chemical structure similarity search tool.

It's impossible to describe all the databases that are available from the National Library of Medicine and their inter-relationships. Links are provided from this site to all the other databases.