Vitamin D metabolism in patients treated with phenytoin and phenobarbitone

There are many reports of abnormalities in calcium metabolism and even of frank osteomalacia in patients taking anticonvulsant drugs. Although enzyme induction has been suggested to be the cause of increased requirements for vitamin D leading to these changes, this has still to be proved. We have studied the metabolism of [3H]cholecalciferol and 25-hydroxycholecalciferol in subjects taking anticonvulsant drugs. The rate of removal of cholecalciferol from the plasma was no different from that in control subjects and no abnormality was detected in the production of disappearance of 25-hydroxycholecalciferol. However, plasma concentrations of 25-hydroxycholecalciferol fell more rapidly during the winter in epileptic subjects than in control subjects. Our results do not confirm that these changes are due to enzyme induction.