a substance that competes with, replaces, or inhibits a specific metabolite of a cell and thereby interferes with the cell's normal metabolic functioning. An antimetabolite is similar in structure to a metabolite, or enzymatic substrate, which is normally recognized and acted upon by an enzyme to form a substance required by the cell. Although the antimetabolite may resemble the substrate enough to be taken up by the cell, it does not react in the same way with the enzyme—either the enzymatic reaction is inhibited or the antimetabolite is converted by the enzyme into an aberrant component.

Many antimetabolites are used for therapeutic purposes. Sulfanilamides are antimetabolites that disrupt bacterial, but not human, metabolism and are used to eradicate bacterial infections in humans. Other antimetabolites such as methotrexate and 5-fluorouracil are used to treat a variety of cancers.