Some prescription medications can lead to a deficiency in vitamin D. Vitamin D can be consumed through food or created by the skin during exposure to sunlight but it must undergo a series of alterations within the body before it can be used. The vitamin is stored in fatty tissue and released when levels in the body drop, such as during periods of little to no sunlight.
Anti-seizure medications, including phenobarbital, phenytoin, primidone and valproic acid, may interfere with vitamin D. A type of cholesterol-lowering drug called bile acid sequestrants may also cause a vitamin D deficiency. Cholestyramine and cholestipol are two examples of this kind of drug. The tuberculosis drug rifampin and the weight loss drug orlistat, also known by its brand name Alli, may also be to blame for low levels of vitamin D. Corticosteroids may also negatively impact vitamin D levels in the body and this may increase the bone loss seen with this type of medication.
Orlistat and the drugs used to treat high cholesterol causes problems with vitamin D absorption because they affect body fat. The anticonvulsant drugs work in a different way, accelerating the conversion of vitamin D into an inactive form that cannot be used by the body. The precise mechanism behind the drop in vitamin D associated with corticosteroids and rifampin remains unknown but their effect can be detected as lowered levels of the active form of vitamin D circulating in the bloodstream.
The use of medications known to affect vitamin D levels in the body must be balanced with the risks of a deficiency. A severe vitamin D deficiency can lead to the disorders rickets and osteomalacia and even mild deficiencies may raise the risk of cancer, cause muscle weakness and pain, raise blood pressure and influence seasonal depression.
Doctors who prescribe drugs that can alter vitamin D activity will typically prescribe supplemental vitamin D or a multivitamin with this nutrient. Doses of up to 2000 IU each day are considered safe and effective. Additional vitamin D can be acquired through food sources, including cod liver oil, fortified milk and fatty fish. Sufficient sun exposure may be enough to counteract the negative effects of these medications.
Because vitamin D is stored in fat, a deficiency may not become apparent immediately after beginning these medications. Fat stores within the body will continue to release vitamin D until they are depleted, which may take weeks or months, depending on the level of vitamin D in the system beforehand. A physician should monitor anyone on medication known to block vitamin D metabolism to check whether or not a deficiency develops.