Naming a couple, the best choice might depend on your use case (e.g. market data information, level of association, types of chemicals):

• ChEMBL https://www.ebi.ac.uk/chembl/

  They have a working API. They also have some support in Bioservices library - https://github.com/cokelaer/bioservices.

• Comparative Toxicogenomics Database http://ctdbase.org (associations can be downloaded in a single batch).

• DrugCentral https://drugcentral.org

• Supertarget and/or SuperDRUG2 https://bioinformatics.charite.de/supertarget/ | http://cheminfo.charite.de/superdrug2/

• DrugTargets http://drugtargets.insight-centre.org

Finally a link to an entry in BiOkeanos, in case you want to look for other similar databases:

• https://biokeanos.com/source/drugbank.