Over the last six months, dozens of batches of medications used to treat high blood pressure have been recalled by the Food and Drug Administration over concerns that they may be contaminated with a cancer-causing chemical. On Friday, the FDA said there is currently a national shortage of the widely prescribed medication valsartan, and that other drugs used to treat high blood pressure could also become in short supply due to the ongoing recalls.

Several generic valsartan products sold in the U.S. have been found to be contaminated with the chemical NDMA, which has linked to cancer.

“Overall, the risk to individual patients remains very small, although this doesn't diminish the significance of this episode or our concerns,” FDA Commissioner Dr. Scott Gottlieb and Dr. Janet Woodcock, director of the Center for Drug Evaluation and Research, said in a statement released Friday.

The most recent recall occurred Jan. 18, when Prinston Pharmaceuticals announced a voluntary recall of one batch of irbesartan tablets and seven batches of irbesartan HCTZ tablets, a common blood pressure medication.

Most of the identified batches were tainted with trace amounts of NDEA or NDMA, N-nitrosodimethylamine, which the FDA has classified as a “probable human carcinogen.” These environmental contaminants are also found in water and foods, including meats, dairy products and vegetables.

The contaminated drugs were traced to factories in China that make valsartan ingredients and ship them to generic drug companies throughout the world. It's the process used to make the generic versions of the drug that has the potential to generate the carcinogenic compound, Woodcock told NBC News.

In the summer of 2018 the FDA warned the Chinese company Zhejiang Huahai Pharmaceutical Co. that some generic valsartan-containing medicines contained NDMA impurity and posed a potential safety concern. The FDA issued a warning letter to the company citing several manufacturing violations including impurity control and cross contamination from one manufacturing process line to another.

“The companies knew it had a problem, but didn’t fix it over the course of six years,” said Rosemary Gibson, a health care expert at the Hastings Center and author of “China Rx: Exposing the Risks of America’s Dependence on China for Medicine.” “It’s not a fluke, it was a known serious problem. This company was putting a dangerous product out there for human consumption.”

An FDA spokeswoman told NBC News that the agency could not predict how long the recalls would continue.

Nearly 60 million prescriptions were written for losartan in 2016 — the ninth most prescribed drug in the U.S. — and nearly 14 million for valsartan or a drug that includes it. Another 3.6 million prescriptions were written for irbesartan that year.

No adverse effects have been reported according to drug company officials, but experts believe overseas companies may be minimizing health risks.

“The main manufacturer in question had more than 200 times the acceptable level of NDMA per tablet,” Gibson told NBC News. “That was far higher than the level that the FDA found in other valsartan products.”

The FDA cited changes made to the manufacturing process and a period where the impurity was undetected as to reasons why the tainted drugs got into the hands of consumers. The FDA statement from Gottlieb and Woodcock also noted that “neither regulators nor industry fully understood how NDMA or NDEA could form during this particular manufacturing process.”

NOT ALL BATCHES RECALLED

Gibson advised patients taking valsartan to check prescriptions for the distributor and manufacturer.

“You can find that on the package or you can ask your pharmacist,” she said. “And I would call them up and ask ‘Was the product that you administered to me made with the active ingredients from this company in question?”

The FDA suggests that medications on the list be returned to the pharmacy or doctor who issued them. Because not all batches of losartan, valsartan or irbesartan drugs are involved in the recalls, patients can switch to a version made by a different company.

HOW SERIOUS IS THE CANCER RISK?

The FDA, which inspects all drug products from foreign factories that make drugs for use in the U.S., says the risk is very low. However, the FDA also notes that the levels of NDEA or NDMA, the cancer-causing agents contained in the samples, are at unacceptable levels.

“FDA scientists estimate that if 8,000 people took the highest daily valsartan dose (320 mg) that contained NDMA, for four years (the time we think the affected products had been on the U.S. market), there may be one additional case of cancer beyond the average cancer rate among those 8,000 Americans,” the FDA said in the Friday statement.

Because stopping a heart medication poses more risk than the contaminated drugs in the short term, doctors advise patients to continue taking their blood pressure medications until a replacement is prescribed, even if the drug is included in a recall.

As the FDA investigates the manufacturing process that led to the contamination, Gibson expects more blood pressure drug recalls.

“This is just the tip of the iceberg,” she said.