Any substance which when absorbed into a living organism may modify one or more of
its functions. The term is generally accepted for a substance taken for a therapeutic
purpose, but is also commonly used for abused substances. Synonymous with medicine,
pharmaceutical.
Source:
PAC, 1993, 65, 2003
(Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993))
on page 2040