The Food and Drug Administration has announced a voluntary recall of a widely prescribed blood pressure medication made in China, reviving fears about the safety of imported drugs.Three companies that sell the generic drug, valsartan, in the United States agreed to recall it after the FDA said it might be tainted by N-nitrosodimethylamine (NDMA), considered a probable human carcinogen. The agency is still investigating, but said the contamination was believed to be related to changes in the way that valsartan was manufactured.All of the valsartan that is being recalled was made in China by the same company, Zhejiang Huahai Pharmaceutical Co. Ltd. It is distributed in the United States by three companies: Major Pharmaceuticals; Teva Pharmaceutical Industries, Ltd.; and Solco Healthcare. Solco, which is owned by Huahai Pharmaceutical, had about 45 percent of the market in 2017, according to John Brito, of Fore Pharma, the market research firm."We have carefully assessed the valsartan-containing medications sold in the United States, and we’ve found that the valsartan sold by these specific companies does not meet our safety standards," said Dr. Janet Woodcock, director of the FDA’s Center for Drug Evaluation and Research. "That is why we’ve asked these companies to take immediate action to protect patients."RELATED: Study: Lowering blood pressure helps prevent mental declineThe agency advised patients taking the drug to look at the manufacturer’s name on the label of their prescription bottle to determine if it is part of the recall. If the information is not there, patients should contact the pharmacy where they got it, the FDA said.The agency also said that patients taking valsartan, or medicines that use it as an ingredient, should continue taking it until they have a substitute. Their health care provider should be able to offer other treatment options, among them, another valsartan product that is not part of the recall. Other companies that market the drug, not subject to the recall, are Sun Pharma, Mylan, Jubiliant, Aurobindo and Hetero, according to Fore Pharma."The recall is huge, based on the volume and the large number of patients it could have impacted," said Mr. Brito, "but we believe this could get offset by other players stocking up in short term. Switch options are available for a hypertension patient."The safety of imported drugs has long been debated. The FDA said it would continue to investigate the levels of NDMA in the recalled products, determine the possible effect on patients who have been taking them, and assess what measures can be taken to reduce or eliminate the impurity from future batches.Solco declined to comment, but in a news release, acknowledged that the recall stemmed from detection of a trace amount of NDMA. A spokesman for Major Pharmaceuticals said they were recalling several lots and referred other questions to Teva, which supplies valsartan that they package and sell. Teva did not return calls for comment.The recall, which also includes valsartan-hydrochlorothiazide tablets, followed a similar action taken by the European Medicines Agency.Dr. Harry Lever, a cardiologist at the Cleveland Clinic, said he was concerned about quality control of generic medicines, like valsartan, made in China and India. He believes that manufacturers should be more transparent about where their active ingredients are made."It’s not just valsartan," Dr. Lever said. "It’s becoming very difficult for me to write prescriptions at all. There are so many drugs that are coming in from India and China and companies are buying and selling each other and you don’t know what’s what."