Influence of phenobarbital and diphenylhydantoin on vitamin D metabolism and calcium retention in rats

Antiepileptic drugs and vitamin D supplementation

The incidence of biochemical signs of vitamin D deficiency and the effects of vitamin D supplementation were investigated in 83 children and 95 adults on chronic antiepileptic therapy and 40 mentally retarded controls living under comparable conditions. Low 25-hydroxyvitamin D and serum calcium, and elevated immunoreactive parathyroid hormone and alkaline phosphatase was a common finding in all groups, but in patients on antiepileptic drugs, signs of vitamin D deficiency were recorded more frequently. Supplementation of 125 microgram or 250 microgram vitamin D3 per week for 9 months normalized the laboratory findings in most patients; the effect of 37.5 microgram/week only slightly exceeded the influences of season observed in the controls and in epileptic patients without vitamin D. It is suggested that a dose between 37.5 and 125 microgram vitamin D3/week might be most suitable to avoid biochemical signs of vitamin D deficiency in children and adults on antiepileptic drugs.

Phenytoin inhibition of parathyroid hormone induced bone resorption in vitro

1 A study is reported of the effects of phenytoin and phenobarbitone on bone calcium mobilization by parathyroid hormone in vitro.2 In a therapeutic concentration (15 mug/ml), phenytoin significantly inhibited parathyroid hormone-induced calcium release from bone.3 The inhibitory effect of phenytoin on bone calcium mobilization could play a role in the maintenance of hypocalcaemia in epileptic patients on long-term anticonvulsant drug therapy.