The Food and Drug Administration is warning that some batches of the popular heartburn medication Zantac contain low levels of a chemical that could cause cancer.

According to the agency, low levels of a chemical called NDMA were found in some ranitidine medicines. Ranitidine is used to treat heartburn, and sold under the brand-name drug Zantac

FDA said NDMA is classified as a probable human carcinogen based on results from laboratory tests. NDMA is a known environmental contaminant found in water and foods, including meats, dairy products, and vegetables.

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The agency said it has been investigating NDMA and other similar impurities that have been found in blood pressure and heart failure medicines since last year. FDA has recommended numerous recalls of heart failure drugs as a result.

The FDA said it's still evaluating whether the levels found in antacids would pose a health risk to the millions of people who take them. So far, FDA said the the levels it has found from preliminary tests “barely exceed amounts you might expect to find in common foods.”

There are no recalls of Zantac or any other medications that contain ranitidine, but the agency said if people want to stop taking Zantac, there are other heartburn medications on the market that have different, unaffected ingredients.