WASHINGTON (CBS) – Drug maker Novartis has halted distribution of generic versions of Zantac. The popular heartburn medication has been investigated for containing a potential cancer-causing ingredient.

A Novartis spokesman told CBS News “our internal investigation is ongoing to determine further details,” and the drug can remain on pharmacy shelves for now. Sanofi, the French pharmaceutical company that makes Zantac, said it had no plans to stop distribution or production.

Last week, the FDA said that ranitidine treatments like Zantac have been found to contain a “probable” carcinogenic impurity known as N-nitrosodimethylamine or NDMA. The presence of NDMA in common blood pressure drugs called valsartan was linked to dozens of recalls in the past year.

“The FDA reported that the levels of (NDMA) in ranitidine in preliminary tests barely exceed amounts found in common foods,” Sanofi said in a statement. “We are working closely with the FDA.”

How dangerous is NDMA? CBS News medical contributor Dr. Tara Narula explained it’s a byproduct that can be formed through several manufacturing processes, like the making of pesticides, rubber or the production of cured meats.

“It is out there; it shouldn’t be in our medications, however,” she said.

NDMA has been shown to cause cancer in animals, but Narula said we don’t know for sure how dangerous it is in humans.

There is no FDA recall of Zantac or its generic versions yet, and they’re not telling people to stop using it. Concerned patients can talk to their doctor about other drug options.

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