The three major U.S. pharmacy chains are removing versions of the heartburn drug Zantac from store shelves as health regulators investigate a probable carcinogen detected in the popular stomach drugs.

Walgreens Boots Alliance Inc. said Monday that it would pull all versions of the drug from its thousands of stores around the country. Rite Aid Corp. announced Monday that it would follow suit, removing branded and private-label versions of the drug. On Saturday, CVS Health Corp. said it would stop selling brand Zantac as well as its store-brand products.

Some Zantac products, known in generic form as ranitidine, have been recalled in the U.S. by manufacturers. Pharmacies are also beginning to halt sales of the drugs until more is known about the levels of a likely carcinogen that regulators around the world have found in the medicines.

Phil Caruso, a spokesman for Walgreens, told Bloomberg in an email Monday that the pharmacy chain would no longer be stocking the products. French pharmaceutical company Sanofi makes branded Zantac. Walgreens store-brand ranitidine was recalled last week by generic drugmaker Apotex Inc., which manufacturers the drug that is then packaged and labeled by the drugstore chain. Apotex also recalled ranitidine made for Walmart Inc. and Rite Aid Corp.

CVS said in a statement Saturday that it had suspended the sale of all branded Zantac and generics sold under the CVS label “out of an abundance of caution.” Sanofi’s Zantac and the generic version sold under the CVS name haven’t been recalled.

Novartis AG’s Sandoz unit recalled generic prescription ranitidine in the U.S. last week after detecting elevated levels of the carcinogen NDMA. The carcinogen is the same one that last year sparked a recall in about 30 countries of millions of blood-pressure pills called angiotensin II receptor blockers.

Novartis, GlaxoSmithKline Plc, and Dr. Reddy’s Laboratories Ltd. have halted global distribution of their versions of Zantac. French regulators, meanwhile, have ordered a recall of all Zantac and its generics available in pharmacies there.