Close monitoring is essential during isoniazid prophylaxis.

A three-year prospective study during the entire course of preventive therapy with isoniazid (INH) for recently infected hospital employees was undertaken to evaluate the risk of developing INH-related hepatic injury. Results of clinical and laboratory tests at 1, 2, 6, and 12 months were correlated to ascertain the significance and value of monitoring procedures. Thirty-two (32%) of 100 employees developed elevated SGOT levels: one (1%) patient developed clinical hepatitis in the first month of therapy; four (4%) subjects with SGOT elevations (82-378 units) became clinically symptomatic at one to four months and INH was discontinued; one (1%) individual with minimal increases in serum bilirubin and SGOT levels was symptomatic at eight months and the drug was stopped. Twenty-four (24%) otherwise asymptomatic subjects who developed SGOT elevations (70-480 units) were followed at closer intervals to complete INH therapy without further progression of liver function abnormality. The intervals between the beginning of treatment and the elevation of SGOT levels varied widely, with elevations occurring throughout the 12-month course of isoniazid therapy.