n. a drug of choice for the treatment of tuberculosis. Use of the drug can cause a form of neuritis by blocking the function of pyridoxine (vitamin B6) in metabolizing glutamic acid to form the neurotransmitter gamma-aminobutyric acid. Isoniazid is structurally related to a known monoamine oxidase inhibitor (MAOI), iproniazid, and was reputed to have some antidepressant activity, although it is not clinically used in this role and is not an MAOI. Also called isonicotinic acid hydrazide (INH).