Could you be OD’ing on vitamin D? If you’re taking megadoses of D supplements, you just might be. Excessive vitamin D levels are linked to heart problems, according to a recent study presented at the American Heart Association Scientific Sessions.

Researchers examined blood tests of 132,000 patients over a 1.5-year period. The findings: People who had excessive levels of vitamin D in their blood were 2.5 times more likely to develop atrial fibrillation (AF) compared to those with normal levels, says study author T. Jared Bunch, M.D. AF causes the heart to beat too fast, too slow, or irregularly, increasing the risk of stroke and heart failure.

In the study, 1.4 percent of patients with normal vitamin D levels developed AF compared to 3.8 percent in the excessive range. Normal vitamin D levels are defined as 41 to 80 ng/ml (nanograms per milliliter); excessive levels are more than 100 ng/dl.

Low levels of D are associated with conditions like heart disease and depression. Many doctors recommend taking a supplement to beef up stores—especially for those living in colder climates who bundle up in the winter and therefore don’t get much D-producing sun exposure. The problem? We often assume that taking more is better, so people pop multiple pills and end up taking a huge dose (like 10,000 IU/day).

How much is too much? Bunch says that men can safely take anywhere from 600 IU to 4,000 IU of vitamin D daily. Those diagnosed with AF and taking vitamin D should ask their doctor for a blood test to check if excessive levels of D are linked to their heart problems. For everyone else, “the only way to know how much vitamin D you need, or if you need any at all, is by taking a blood test,” Bunch says. Have your vitamin D levels checked when you go in for your first cholesterol screening, which should happen by or before your 40th birthday.