Isoniazid is an antituberculous drug that is usually administered for nine to 12 months. The potential for clinically important interactions exists because this drug is a potent inhibitor of drug metabolism. Studies and case reports have shown that isoniazid inhibits the metabolism of several drugs, including phenytoin, carbamazepine, anticoagulants, benzodiazepines, and vitamin D. Furthermore, isoniazid inhibits both monoamine oxidase and diamine oxidase (histaminase). Additional study is required to document the clinical significance of other isoniazid interactions. Future investigations will identify new isoniazid interactions.