The vitamin D supplement business has grown from sales of 75 million dollars in 2006 to 550 million in 2010. More and more, the question arises: does vitamin D affect my medication or does my medication affect my vitamin D? The 25-hydroxylase (CYP3A4) which converts vitamin D to 25(OH)D, is also an enzyme that metabolizes many drugs. Test tube studies indicate that as many of half of all therapeutic drugs are metabolized by CYP3A4, suggesting that vitamin D may have widespread effects on many drugs.
Recently Dr Kim Robien and colleagues of the University of Minnesota conducted a thorough review of the world’s literature. They identified 1225 reports to select 109 unique reports that met their quality hurdles to report on the subject.
On drugs that affect vitamin D, they found the following:
- The bile acid sequestrates Celestipo (trade names Colestid, Cholestabyl) and cholestyramine (Questran, Questran Light, Cholybar) could theoretically reduce vitamin D levels by interfering with intestinal absorption, but no study confirms that they do so.
- Orlistat (Xenical) decreases vitamin D levels by binding the pancreatic enzymes involved in absorbing fats and vitamin D is a fat-soluble vitamin.
- Statins increase vitamin D levels substantially.
- The TB drugs, rifampin and isoniazid, may decrease vitamin D levels and the authors, noting vitamin D may help treat TB, recommend that those on such drugs get their vitamin D levels checked.
- Hydroxychloroquine (Plaquenil) is used to treat malaria and lupus. It may reduce vitamin D levels.
- Antiepileptic drugs, such as phenobarbital, phenytoin, primadone, carbamazepine, oxcarbazepine, and felbumate, may lower vitamin D levels. Forty-six studies addressed this question but the authors were unable to find any good study of this effect.
- Steroids may lower vitamin D levels, but the authors were not convinced by the data. Certainly, patients on steroids have low 25(OH)D levels and osteoporosis but the authors could not say that was due to the direct effect of steroids on vitamin D levels.
- Immunosuppressants, like cyclosporine (Sandimmune) and tacrolimus (Prograf, Advagraf, Protopic) do not appear to alter vitamin D status.
- Chemotherapy agents used in cancer treatment do not appear to affect vitamin D levels, but the authors recommend that anyone on chemotherapy get their level checked.
- Highly active antiretroviral agents (HAART) may lower levels.
On the other hand, they found the following on vitamin D’s effect on specific drugs:
- Thiazides diuretics [such as Dyazide, chlorothiazide, hydrochlorothiazide (Microzide), Indapamide and metolazone (Zaroxolyn)], which are used to treat hypertension, probably do not affect serum calcium levels on those taking vitamin D. Only three case reports suggest thiazides and vitamin D may raise serum calcium and those case reports all had another possible cause of the hypercalcemia.
The authors conclude that little is actually known about vitamin D‘s interactions with drugs and caution is in order. They conclude,
“The area of drug–vitamin D interactions is a clear example of a situation where lack of evidence does not equate to ‘no harm.’ The available research to date has primarily focused on drugs that are commonly associated with osteoporosis (suggesting a potential effect on vitamin D metabolism) or where case reports of adverse outcomes have been reported in the medical literature. Given the increasing prevalence of vitamin D supplementation in the general population, continued evaluation of potential drug–vitamin D interactions is warranted.”
Furthermore, they recommend:
“Until further research is available, healthcare professionals should be aware of the potential for drug–vitamin D interactions, assess their clients’ use of dietary supplements, and monitor serum 25(OH)D concentrations where indicated with the ultimate goal of achieving adequate serum 25(OH)D concentrations while optimizing drug efficacy and minimizing drug toxicity.”