Women receiving oral contraceptives have shown a significant increase in plasma vitamin A level. Excessive dosage of vitamin D induces hypercalcaemia and in some instances hypercalciuria. Hypercalcaemia in patients receiving digitalis may precipatate cardiac arrhythmias. Cholestyramine has been reported to reduce absorption of fat soluble vitamins: as such it may impair intestinal absorption of vitamin A and vitamin D. The administration of anti-convulsants has been shown to affect the vitamin D requirements in some patients. Pyridoxine may decrease the efficacy of levodopa in the treatment of parkinsonism. It should be used with caution for patients undergoing levodopa therapy. Ascorbic acid taken concomitantly with fluphenazine may necessitate an increase in dosage of fluphenazine.
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