Prodrug: A precursor (forerunner) of a drug. A prodrug must undergo chemical conversion by metabolic processes before becoming an active pharmacological agent. For example, sulfasalazine is a prodrug. It is not active in its ingested form. It has to be broken down by bacteria in the colon into two products -- 5-aminosalicylic acid (5ASA) and sulfapyridine -- before becoming active as a drug.