Genetic control of drug levels in man: phenylbutazone.

Phenylbutazone was administered to seven pairs of identical (monozygotic) twins and to seven pairs of fraternal (dizygotic) twins. Individual half-lives ranged from 1.2 to 7.3 days. Subjects with identical genotypes (monozygotic twins) exhibited very similar phenylbutazone half-lives; significantly greater differences in half-life occurred in dizygotic twins. The previously established large variations among individuals in phenylbutazone metabolism appear to be genetically, rather than environmentally, determined.