Chronic Diphenylhydantoin Therapy Does Not Reduce Plasma 25‐hydroxy‐vitamin D

A randomized controlled study was performed to investigate the effect of 2 years' monitored diphenylhydantoin (DPH) therapy on plasma 25-hydroxyvitamin D (25-OHD) in non-epileptic, non-institutionalized subjects. Mean +/- SEM plasma 25-OHD of 18 DPH-treated subjects at the end of 2 years' drug treatment was 59 +/- 8 nmol/l (23.6 +/- 3.2 ng/ml), which was not decreased compared to that of eighteen control subjects (54 +/- 8 nmol/l, 21.6 +/- 3.2 ng/ml). In addition, mean plasma 25-OHD had not changed 1 month after ceasing DPH. The treated group had a higher mean serum alkaline phosphatase (SAP) during DPH treatment, attributable to hepatic enzyme induction. It is concluded that therapeutic doses of DPH without other anticonvulsants do not have a clinically significant effect on plasma 25-OHD.