CVS pulled Zantac and its own generic form of the popular heartburn medication from its shelves this weekend over concerns that the products may contain small levels of nitrosodimethylamine (NDMA), a possible cancer-causing chemical linked to liver damage. CVS, the largest pharmacy chain in the country, said Saturday it was suspending sales “out of an abundance of caution.”

The decision by the pharmacy giant adds to a flurry of worldwide concern about the drug. Major manufacturers of the generic form, ranitidine, have announced recalls, and other countries have requested that companies halt distribution of the drug or issued recalls. The U.S. Food and Drug Administration has been investigating the possible risk to patients, as well.

A box of Maximum Strength Zantac tablets at a pharmacy Monday in Miami Beach, Fla. CVS has halted sales of the popular heartburn treatment and the store generic version after warnings by U.S. health regulators. Wilfredo Lee/Associated Press

The FDA has stopped short of calling for people to stop taking the drug but has advised patients to talk to their doctors about taking an alternative prescription to ranitidine, an anti-heartburn medication that’s commonly used in prescription and over-the-counter medicines to treat issues like ulcers and acid reflux.

Exposure to low levels of NDMA does not pose an acute risk to patients. The chemical is classified as a probable carcinogen and is present in the environment and in food, such as grilled meat.

Sandoz, a division of the pharmaceutical giant Novartis, halted distribution of ranitidine in mid-September and then announced it was recalling lots after the impurity was detected above acceptable levels last week. Apotex Corp., another manufacturer, recalled ranitidine tablets last week on a “precautionary basis” after learning from the FDA and other global regulators that the drug can contain the impurity, regardless of the manufacturer. Drug recalls are typically executed by the individual manufacturers and subsequently announced by the FDA.

The FDA has been asking companies to test the levels of NDMA in their drugs and send samples to the agency.

“The agency is working with international regulators and industry partners to determine the source of this impurity in ranitidine. We are also working to understand what happens to NDMA levels in the body after administration of ranitidine,” FDA spokesman Jeremy Kahn said. “The FDA will take appropriate measures based on the results of the ongoing investigation.”

Problems with ranitidine were first flagged in June by Valisure, an online pharmacy that analyzes drugs to verify their chemical contents before sending them to consumers. Valisure chief executive David Light said that his company analyzed ranitidine in part because a co-founder of the company had an infant daughter who was taking the drug for acid reflux. He said they were disturbed to find the carcinogen present in multiple formulations of ranitidine.

“We feel that there should be a complete recall. This should be taken off the market, period, regardless of manufacturer,” Light said.

Sanofi, the pharmaceutical company that manufactures ranitidine under the Zantac trade name, has not recalled its drug.

“The FDA reported that the levels of N-nitrosodimethylamine (NDMA) in ranitidine in preliminary tests barely exceed amounts found in common foods,” Ashleigh Koss, a spokeswoman for Sanofi, told The Washington Post via email. “We are working closely with the FDA and are conducting our own robust investigations to ensure we continue to meet the highest quality safety and quality standards. At Sanofi, we remain committed to being transparent with our patients and consumers and will share an update when one is available.”

Globally, Canada and France have already recalled Zantac, the BBC reports, while GlaxoSmithKline has recalled its ranitidine products in India and Hong Kong, according to USA Today (GlaxoSmithKline does not sell ranitidine products in the United States).

Other pharmacy chains like Walgreens and Rite-Aid and distributors like Walmart have already suspended Zantac sales at their stores. CVS did not immediately respond to a request for comment on the timeline of its recall or whether the company has ever halted sales of a product that has not officially been recalled by its maker.