About six months after several major pharmacies pulled Zantac and its generic equivalents off their shelves, citing a potentially harmful contaminant in the heartburn medication, federal regulators are throwing their weight behind the drug's removal from the market. The Food and Drug Administration requested Wednesday that manufacturers immediately pull all prescription and over-the-counter versions of the drug.

The trouble with ranitidine, the active ingredient in Zantac and its equivalents, rests with a contaminant known as N-Nitrosodimethylamine, or NDMA. The agency says that over time, NDMA — a probable cancer-causing substance that's also found in various foods, including cured meats — shows up as an impurity in ranitidine in concentrations that exceed federal standards.

The FDA issued a warning last September about the drugs, prompting pharmacies, including CVS, Walgreens and Walmart, to halt sales of Zantac and its equivalents. Now, amid an ongoing investigation, the FDA says it has confirmed that the issue is linked with the way ranitidine naturally breaks down over time in normal storage conditions — not how it is manufactured.

"We didn't observe unacceptable levels of NDMA in many of the samples that we tested," Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research, explained in a statement Wednesday. "However, since we don't know how or for how long the product might have been stored, we decided that it should not be available to consumers and patients unless its quality can be assured."

The problem first surfaced last summer, when , a company that does safety testing on popular drugs, discovered the contaminant in alarming levels.

"Our tests in lab conditions seem to pretty clearly show that the drug is able to degrade into NDMA by itself, either in the tablet or actually in the human body," Valisure founder and CEO David Light recalled in a conversation with NPR last fall.

The FDA said Wednesday that it is sending letters to all ranitidine manufacturers recommending that they pull their products from the market. That includes Sanofi, the manufacturer behind Zantac, which says it already conducted a of the product in the U.S. and Canada as the FDA's investigation unfolded.

Federal regulators are also recommending that patients with over-the-counter versions of the drug stop taking them, and that those with prescriptions consult their doctors about switching. The FDA says that, to date, it has not found NDMA in the active ingredients in a host of similar medications, such as Pepcid, Prevacid and Prilosec.

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