Vitamin D Drug Interactions: What the Evidence Shows
Vitamin D is one of the most widely supplemented nutrients — estimates suggest a large fraction of adults in many countries have low serum 25(OH)D levels, and supplementation is common at doses ranging from 400 IU to several thousand IU daily. At those doses, vitamin D interacts with several drug classes in ways that matter clinically. The interactions run in two directions: vitamin D affecting drug action, and drugs reducing vitamin D levels.
Thiazide diuretics and hypercalcemia
The most clinically significant vitamin D interaction for most supplement users involves thiazide diuretics — hydrochlorothiazide (HCTZ), chlorthalidone, indapamide, metolazone — among the most commonly prescribed blood pressure medications. Thiazides reduce urinary calcium excretion: the kidney retains more calcium than it normally would. Vitamin D increases intestinal calcium absorption: more calcium is pulled in from food and supplements. Both mechanisms raise circulating calcium, and together they can push it above the normal range — a condition called hypercalcemia.
Symptoms of hypercalcemia range from mild (fatigue, constipation, increased thirst) to serious (kidney stones, impaired kidney function, cardiac arrhythmia at higher levels). The risk is highest with higher vitamin D doses and in people who also take calcium supplements — three converging inputs on blood calcium. This interaction is documented in the vitamin D prescribing information and noted in the NIH ODS vitamin D fact sheet.
Drugs that deplete vitamin D
Several drugs lower vitamin D levels rather than amplifying its effects. Corticosteroids — prednisone, methylprednisolone, dexamethasone, and others — reduce intestinal calcium absorption and may impair conversion of vitamin D to its active form (calcitriol). Long-term corticosteroid use is a well-established cause of secondary osteoporosis partly for this reason. Long-term users are often monitored for vitamin D status, and supplementation is frequently part of managing corticosteroid-induced bone loss.
Anti-epileptic drugs — particularly phenytoin (Dilantin), phenobarbital, carbamazepine, and primidone — induce hepatic cytochrome P450 enzymes (primarily CYP2R1 and CYP24A1) that increase the breakdown of vitamin D and its metabolites. The result is lower serum 25(OH)D in people on long-term anti-seizure therapy. Phenytoin has an additional mechanism: it may directly reduce intestinal calcium absorption. Long-term use of these medications is associated with lower bone density and higher fracture risk, with vitamin D depletion as a contributing factor.
Orlistat (a fat absorption inhibitor used for weight management) and cholestyramine (a bile acid sequestrant used to lower cholesterol) both reduce absorption of fat-soluble vitamins, including vitamin D. Both reduce how much dietary and supplemental vitamin D reaches the bloodstream.
All of the interactions above are documented in the NIH ODS vitamin D health professional fact sheet.