More than 580,000 bottles of blood pressure medication are being recalled across the United States due to possible carcinogenic substances, according to a notice from the Food and Drug Administration (FDA).

The recall involves three separate lots of prazosin hydrochloride, a type of alpha-blocking medication, that were distributed by New Jersey-based Teva Pharmaceuticals, because a test result for N-nitroso Prazosin impurity C found that the substance’s levels are above the “acceptable intake limit” under a type of test for carcinogens.

The recall encompasses three dosages for prazosin hydrochloride capsules, according to the notice. They include 181,659 bottles of 1-milligram doses of the drug, 291,512 bottles of 2-milligram doses, and 107,673 bottles of 5-milligram doses.

It means that the medication contained higher than acceptable levels of nitrosamines, which are carcinogenic compounds that can increase the risk of cancer when taken at high doses over long periods.

The FDA classified the recall on Oct. 24 as Class II, which the agency says is a scenario where a product, drug, or food “may cause temporary or medically reversible adverse health consequences” or it includes “an outcome where the probability of serious adverse health consequences is remote.”

The medication bottles were distributed nationwide, it said. The recall, which was initiated voluntarily by Teva Pharmaceuticals, started on Oct. 7.

The notice did not say why the recalled bottles contained nitrosamine impurities above the FDA’s acceptable levels. However, a webpage that includes information about nitrosamine impurities in medications on the agency’s website says that there are several reasons that the carcinogens may be present.

It says that the “source of nitrosamines can be related to the drug’s manufacturing process or its chemical structure or even the conditions in which they are stored or packaged” and noted that “as foods and drugs are processed in the body, nitrosamines can also be formed.”

The FDA says that nitrosamines are often found in water and foods, including cured and grilled meats, dairy products, vegetables, and other items, noting that everyone is exposed to some level of nitrosamines.

But long-term exposure to high levels of the carcinogenic substances may increase the risk of cancer, the FDA says.

“Nitrosamine impurities may increase the risk of cancer if people are exposed to them above acceptable levels and over long periods of time, but a person taking a drug that contains nitrosamines at-or-below the acceptable daily intake limits every day for 70 years is not expected to have an increased risk of cancer,” according to a section on its website.

Prazosin hydrochloride, which is sometimes sold under the brand name Minipress, is primarily used to treat hypertension, or high blood pressure, but it’s also prescribed to treat post-traumatic stress disorder (PTSD) and nightmares. A report released by Scientific American found that as many as 20 percent of U.S. military veterans treated through the Department of Veterans Affairs (VA) take prazosin.

An alpha blocker is a type of medication that blocks a hormone that causes blood vessels to narrow, thereby relaxing muscles in the blood vessel walls and widening them, according to the Mayo Clinic’s website.

The FDA notice said that the drug recall is ongoing.