The U.S. Food and Drug Administration’s latest guidelines recommend people ages 1 to 70 get 600 international units of vitamin D per day, and people older than 70 get 800 IUs a day. Many experts say adults can tolerate more vitamin D without much danger – up to 4,000 international units – although there is debate about how much is too much. Extremely high doses of vitamin D supplements can cause constipation, bone damage, kidney stones and other conditions.

The American College of Rheumatology (ACR) recommends that patients take vitamin D and calcium supplements if they take any dose of corticosteroids, also called glucocorticoids. And although there are no firm guidelines in place, the ACR also tells doctors to consider annual vitamin D testing for patients on corticosteroids for more than three months.

Some doctors already make a habit of checking vitamin D levels before patients start corticosteroids. Stanley Cohen, MD, a practicing rheumatologist and a clinical professor in the department of internal medicine at the University of Texas Southwestern Medical School, began testing his patients' vitamin D levels more than three years ago when he saw several surveys that suggested vitamin D deficiency might be prevalent among these patients.

Dr. Cohen sees almost 90 patients each week; roughly 20 to 30 percent of them are taking oral steroids. “We find low vitamin D levels quite frequently. In about 30 to 40 percent of the patients we test, the vitamin D levels are low and we have to treat them,” he says. Most patients can get their levels back to normal with an over-the-counter supplement, he notes, but if they are severely vitamin-D deficient, prescription-strength doses are needed.

Dr. Cohen says this new research further confirms the need to regularly check patients’ vitamin D levels. “We already knew that people on corticosteroids are at a higher risk for fractures,” he says. “This is another thing we need to be aware of. It’s very easy to treat this by monitoring vitamin D levels and replacing vitamin D.”

Indeed, a known downside of corticosteroids is that they increase the risk of fractures via a mechanism other than vitamin D deficiency. By decreasing the amount of calcium the body can absorb and increasing the amount of calcium that is excreted from the body, corticosteroids decrease bone formation and increase bone loss. Experts, however, aren’t certain why corticosteroids impact vitamin D levels.