Substance, structurally similar to a metabolite, which competes with it or replaces it, and so prevents or reduces its normal utilization.
Source:
PAC, 1993, 65, 2003. 'Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)' on page 2015 (https://doi.org/10.1351/pac199365092003)