antivitamins Substances that interfere with the normal metabolism or function of vitamins, or destroy them. Dicoumarol in spoiled sweet clover antagonizes the function of vitamin K, thiaminase in raw fish destroys vitamin B1, the drug methotrexate antagonizes folic acid metabolism (this is part of its mechanism of action in treating cancer), the drug isoniazid antagonizes the action of vitamin B6.

antimetabolite (anti-mi-tab-ŏ-lyt) n. a drug that interferes with the normal metabolic processes within cells by combining with the enzymes responsible for them. Some drugs used in the treatment of cancer, e.g. fluorouracil, methotrexate, and mercaptopurine, are antimetabolites. Side-effects can be severe, involving blood cell disorders and digestive disturbances. See also cytotoxic drug.

antimetabolite Compound that inhibits a normal metabolic process, acting as an analogue of a normal metabolite. Some are useful in chemotherapy of cancer, others are naturally occurring toxins in foods, frequently causing vitamin deficiency diseases by inhibiting the normal metabolism of the vitamin. See also antivitamin.