| Cimetidine inhibits the action of vitamin D-hydroxylase (a hepatic
mixed-function oxidase) in the rat. Therefore, the hypothesis was
tested that this H2 receptor antagonist would affect vitamin D
metabolism in humans. Nine adult patients were treated with 400 mg
cimetidine orally twice daily during a period from winter to summer,
when days were becoming longer. Serum levels of 25-hydroxyvitamin D,
24,25-dihydroxyvitamin D and 1,25-dihydroxyvitamin D were monitored
before treatment, after 4 weeks of treatment, and 1 month after
cessation of treatment. No seasonal increase in the level of 25-
hydroxyvitamin D was observed during the period of treatment, but the
level rose significantly after withdrawal of the drug. The other
hydroxylates of vitamin D were not affected. Levels of albumin, total
calcium, phosphorus and alkaline phosphatase remained normal. The
data suggest that short-term treatment with cimetidine could
potentially perturb vitamin D metabolism in man.
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