Zantac

(CNN) -- CVS Pharmacy and Walgreens are no longer selling Zantac, as well as generic version of the heartburn medication, over concerns that they might contain a substance that could cause cancer.

CVS, the largest pharmacy chain in the country, said over the weekend it was suspending sale of Zantac and other products which contain ranitidine until further notice. Walgreens said in a statement it is removing the medications from shelves while the US Food and Drug Administration continues its review of the products.

The FDA said earlier this month that it has learned that some ranitidine medications -- including those known by the brand name Zantac -- contain low levels of an impurity that could cause cancer.

Zantac and its CVS-branded generic counterpart have not been recalled, a statement from CVS said, and the FDA has not recommended that patients stop taking ranitidine products. Customers who have already bought the products can return them to CVS for a refund, CVS said.

Drugmaker Sandoz is recalling ranitidine hydrochloride capsules in the United States because of confirmed contamination with the probable human carcinogen.

Other medications under investigation

The nitrosamine impurity known as N-nitrosodimethylamine, or NDMA, has been classified as a probable human carcinogen based on lab tests, and this isn't the first time that it has been detected in a common medication.

Since last year, the FDA has been investigating NDMA and other impurities in blood pressure and heart failure medicines known as angiotensin receptor blockers or ARBs. Numerous recalls have been launched as the FDA found "unacceptable levels" of nitrosamines in several of those common drugs containing valsartan.

A study published last year in the medical journal BMJ found no "markedly increased short term overall risk of cancer" among users of the valsartan drugs contaminated with NDMA. Yet that study also noted that research into long-term cancer risk is needed.