How safe is isoniazid?

The complications of isoniazid (INH) were studied in 1033 patients, who had received INH for at least 18 months, with or without other drugs. Hepatitis developed in 25 patients; this was attributed to rifampicin, (15 cases); infectious hepatitis (three cases); INH alone, (three cases); IHN possibly exacerbating chronic liver disease, (two cases); and multiple drug treatment, (two cases). Central nervous system disorders (mainly peripheral neuropathy) due to INH occurred in 12 patients, all of whom were over the age of 40 years. Hypersensitivity to INH developed in 12 patients. Some difficulties in distinguishing hepatitis due to rifampicin from that due to INH are discussed. When the risk of hepatitis was compared with the risks of developing, or dying from, tuberculosis, it was found that the benefits of INH chemoprophylaxis outweighed the risks, particularly in patients who were less than 50 years of age.