The effect of low-dose phenobarbitone on three indices of hepatic microsomal enzyme induction.

The effects of low-dose phenobarbitone on three indices of hepatic enzyme induction were studied. Eight healthy volunteers took phenobarbitone 7.5 mg daily for 4 weeks followed by 15 mg daily for 4 weeks; five subjects took 30 mg daily for a further 2 weeks. Phenobarbitone 15 mg daily produced a significant rise in antipyrine clearance (P less than 0.05). Phenobarbitone 30 mg daily produced a further rise, but probably because of the reduced numbers of subjects, this did not achieve significance (P = 0.06). Urinary 6-beta-hydroxycortisol and D-glucaric acid levels did not change significantly and remained within the range seen in subjects not taking enzyme-inducing drugs. We conclude that phenobarbitone 7.5 mg daily produces little (if any) enzyme induction whereas 15 mg, or more, may have the potential to produce drug interactions through enzyme induction.