Vitamin B6 (pyridoxine) supplementation during isoniazid (INH) therapy is necessary in some patients to prevent the development of peripheral neuropathy. In vivo pyridoxine is converted into coenzymes which play an essential role in the metabolism of protein, carbohydrates, fatty acids, and several other substances, including brain amines, INH apparently competitively inhibits the action of pyridoxine in these metabolic functions. The reported frequency of INH-induced neuropathy in various studies is reviewed and population groups at relatively high risk of developing this complication are identified. The routine use of pyridoxine supplementation to prevent peripheral neuropathy in high risk populations is recommended.