A review of the adverse effects of isoniazid (INH) was undertaken and it was found that these effects could be divided into: (1) toxic, (2) idiosyncratic and (3) hypersensitivity reactions. Major toxic reactions are peripheral neuropathies at conventional dosage and seizures from overdosage. Pyridoxine is of benefit in these disorders. Other presumed toxic reactions are autonomic neuropathy, encephalopathy, psychosis, optic atrophy, and pyridoxine responsive anemia. Idiosyncratic reactions are INH-induced lupus erythematosus, rheumatic-like syndromes and various hematologic disorders. These respond promptly to withdrawal of the drug. Hypersensitivity reactions to INH include hepatitis, dermatitis, fever, angiitis and hemolytic anemia. These reactions promptly subside with withdrawal of the drug although in mild reactions this may not be necessary. Corticosteroid therapy or desensitization can be used if INH needs to be readministered.