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The U.S. Food and Drug Administration says drug makers have recalled more than a half-million bottles of the blood pressure medication prazosin hydrochloride over concerns it may include a cancer-causing chemical.

New Jersey-based Teva Pharmaceuticals USA and drugs distributor Amerisource Health Services issued voluntary nationwide recalls earlier this month of more than 580,000 bottles of various strengths of prazosine capsules, according to the FDA.

Doctors prescribe prazosin, which relaxes blood vessels, to help lower blood pressure. It also is sometimes prescribed for nightmares and other sleep disturbances caused by post-traumatic stress disorder.

The FDA said in enforcement orders posted online that it has given the affected lots of the drug a Class II risk classification because some of the recalled medication may have nitrosamine impurities that are considered potentially cancer causing.

According to the FDA, N-nitrosamine impurities are a class of potentially cancer-causing chemicals that can form during manufacture or storage of a drug.