The U.S. Food and Drug Administration has expanded its recall of a commonly used type of heart drug because some of the products may contain a potentially cancer-causing chemical.

Last month, the FDA issued a voluntary recall of certain batches of the drug valsartan, which is used to treat high blood pressure and heart failure. The recalled drugs were manufactured by Zhejiang Huahai Pharmaceuticals in China and contain a chemical known as N-nitrosodimethylamine (NDMA), which has been shown to increase the occurrence of cancer in animals.

The agency has since expanded the recall twice to include additional products from more drug companies.

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The U.S. Environmental Protection Agency has deemed NDMA a probable human carcinogen, or a chemical that could increase the risk of cancer in humans. It is found in certain water supplies and food at very small levels that are considered reasonably safe for humans.

However, the amounts of NDMA found in the recalled batches of valsartan from the manufacturer in China exceeded these acceptable levels. Scientists from the FDA estimate that if 8,000 people took the highest valsartan dose (320 mg) from the recalled batches daily for four years, there may be one additional case of cancer over the lifetimes of those 8,000 people.

Patients can see a list of the recalled medications, as well as a list of products containing valsartan that have not been recalled, on the FDA's website. Only products contaminated with NDMA are being recalled — not all valsartan tablets.

"It is important to know that not all valsartan products contained NDMA, so pharmacists may be able to provide a refill of valsartan medication from batches that that are not affected by the recall, or doctors may prescribe a different medication that treats the same indications," the FDA states.

Doctors and the FDA advised people who are taking valsartan from a recalled batch to continue taking their current medicine until their doctor or pharmacist provides a replacement or a different treatment option.

"The best remedy we are telling our patients is to contact their pharmacist and request they take an appropriate brand that does not have this impurity," Dr. Satjit Bhusri, a heart specialist at Lenox Hill Hospital in New York City, told HealthDay when the recall was first announced.

"There are many other substitutes for this medication, and that is an important discussion all patients should have with their physician," Bhusri said.

The FDA did not indicate how the drugs became contaminated. According to the EPA, NDMA — which was formerly used in rocket fuel, among other things — can be an unintended byproduct of wastewater treatment plants that use chemical compounds called chloramines for disinfection.