The Food and Drug Administration officials said Friday that some versions of the blood pressure-lowering drug valsartan contained trace amounts of a carcinogen for four years before regulators detected the impurity last summer, triggering a widespread recall of the tainted drugs.

FDA officials said the cancer risk for any person who took valsartan that had the carcinogen nitrosodimethylamine, or NDMA, is small. FDA scientists concluded there would be one more cancer case above average rates for every 8,000 people on the highest dose of valsartan for four years.

FDA Commissioner Scott Gottlieb and FDA Director Janet Woodcock said in a joint statement that investigators suspect the root cause of the contamination stemmed from a change in how the blood pressure drug ingredients were made.

"Overall, the risk to individual patients remains very small, although this doesn't diminish the significance of this episode or our concerns," the statement said.

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Drug companies recalled hundreds of lots of the front-line blood pressure and heart medications valsartan, losartan and irbesartan – alone or in combination with other drugs – after testing revealed the drugs had trace amounts of NDMA. Follow-up testing showed some batches of valsartan also contained another probable carcinogen, nitrosodiethylamine, or NDEA.

The recalled products are part of a large class of drugs called angiotensin II receptor blockers, or ARBs, which relax blood vessels. Many of these commonly prescribed drugs are not part of the recall. Consumers can check the agency's website for a full list of recalled drugs.

FDA said that patients affected by the recall should continue taking their medication until their pharmacist replaces the drugs or their doctor prescribes another treatment. Discontinuing a recalled drug could cause more immediate harm than staying on the medication.

The two identified nitrosamine impurities, NDEA and NDMA, are also found in water, dairy products and foods such as charcoal-grilled meats. Companies have recalled drugs that contained unacceptable levels of these relatively low-risk contaminants.

The FDA's investigation concluded the contaminants likely resulted from a manufacturing change adopted by factories in China and India that make the drug ingredients. The manufacturing change may generate the contaminants "when specific chemicals and reaction conditions are present" and can result of reusing materials such as solvents.

The FDA said that NDMA is difficult to detect in standard lab testing, but the federal agency has refined testing methods and shared that information with pharmaceutical companies.

Factories at risk for generating the contaminants must test both pharmaceutical ingredients and finished drugs to verify they are free of detectable impurities, the FDA said.

"While the total exposure to these impurities for most patients was small, we are deeply concerned that patients were exposed to this impurity in the first place and that the presence of nitrosamines went undetected for a period of time," Gottlieb and Woodcock said.

The FDA issued an import alert in September to block products made at the Zhejiang Huahai Pharmaceutical factory in Linhai, China, from entering the USA. Several generic drug companies purchased ingredients from the factory, but those products have been recalled.

During past inspections, the FDA found that workers at the Zhejiang Huahai factory failed to investigate testing anomalies in drug batches.

A second factory, Hetero Labs in India, also made ingredients in blood pressure drugs that were later recalled.

Peter J. Pitts, president of the Center for Medicine in the Public Interest, said the blood pressure medication recalls highlight the need to oversee the quality of drugs.

The FDA does not have the resources to inspect each drug factory every two years, so the federal agency has adopted a risk-based system that results in more frequent inspections of troubled factories.

"A product that is not acceptable FDA quality, is not a product that should be in American medicine cabinets," said Pitts, a former FDA senior communications and policy adviser.

Far fewer Americans have filled valsartan prescriptions since the recall began in July, according to data provided by IQVIA. There were more than 1.1 million valsartan prescriptions filled at U.S. pharmacies in June, a figure that dropped to fewer than 300,000 prescriptions in September.

The FDA acknowledged shortages of valsartan following the recall, and the agency said that there may be future shortages of other blood pressure medications affected by recalls.