Vitamin D Supplementation: An Update

Christine Gonzalez, PharmD, CHHC

US Pharmacist

Drug Interactions

Vitamin D supplements may interact with several types of medications. Corticosteroids can reduce calcium absorption, which results in impaired vitamin D metabolism.^[6] Since vitamin D is fat soluble, orlistat and cholestyramine can reduce its absorption and should be taken several hours apart from it.^[6] Phenobarbital and phenytoin increase the hepatic metabolism of vitamin D to inactive compounds and decrease calcium absorption, which also impairs vitamin D metabolism.^[6]

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Cite this: Vitamin D Supplementation: An Update - Medscape - Oct 01, 2010.

Table 1. Forms of Vitamin D
Cholecalciferol (vitamin D₃) Naturally occurring form of vitamin D; made by the skin upon sun exposure; found in foods and most supplements
Calcidiol (25-hydroxyvitamin D₃) Prehormone made directly from cholecalciferol in the liver; low biologic activity, but major form circulating in the bloodstream; concentrations measured by typical blood test for vitamin D deficiency
Calcitriol (1,25-dihydroxyvitamin D₃) Activated form made from calcidiol in the kidneys and tissues; the most potent steroid hormone in the body
Ergocalciferol (vitamin D₂) Made in the laboratory by radiating fungus; not naturally occurring in the body; used in prescription vitamin D (Drisdol), available in 50,000-IU capsules

Table 2. Dosing and Blood Concentrations
Dosage	Expected Increase in Blood Concentration
100 IU	1 ng/mL
200 IU	2 ng/mL
400 IU	4 ng/mL
800 IU	8 ng/mL
1,000 IU	10 ng/mL
2,000 IU	20 ng/mL

Source: Reference 30.

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