Pharmacokinetics of isoniazid: influence of age.

The distribution of the acetylator phenotype of isoniazid was studied in 458 patients of different ages, and the influence of age on its apparent distribution volume, clearance and half-life was investigated in slow and rapid acetylators. The doses of isoniazid for the patients were determined according to the inactivation index method, as described by Vivien. Apparent distribution volume showed no difference between slow and rapid acetylators but it did decrease significantly with age. Clearance and half-life varied significantly in slow acetylators, and these variations led to a decrease in the necessary dose of isoniazid.