Effect of increasing doses of phenytoin on the plasma 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D concentrations.

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Abstract
The circulating 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25-(OH)2D] concentrations were studied in a patient receiving increasing doses of phenytoin. The plasma 1,25(OH)2D concentrations were independent of the dose of phenytoin administered, as well as of the drug plasma concentrations. The plasma 25(OH)D concentrations were, on the other hand, increased by low phenytoin concentration but rapidly declined when the dose of phenytoin was increased and/or as the length of time of exposure to the drug increased. A linear relationship (R = 0.9651, P less than 0.05) was found between the plasma 25(OH)D concentrations and the dose/plasma phenytoin concentration ratio, suggesting that chronic phenytoin administration may have a dose-related effect on the circulating 25(OH)D concentrations.