The Food and Drug Administration will allow certain lots of the blood pressure drug losartan that contain trace amounts of a potential carcinogen to be shipped to pharmacies.

The temporary measure is designed to avert a shortage of the critical medication – the nation's ninth-most commonly prescribed drug.

Permitting losartan with tiny amounts of a nitrosamine impurity will not increase a person's cancer risk, the FDA said. If people discontinue or can't get the drug for conditions such as hypertension, heart failure or kidney disease, it could put them at risk or force them to switch to another medicine.

Drug companies have recalled hundreds of lots of the blood pressure and heart medications valsartan, losartan and irbesartan since July after testing revealed some versions had small amounts of suspected carcinogens. These medications are part of a large class called angiotensin II receptor blockers (ARBs), which lower blood pressure by widening or relaxing blood vessels.

FDA investigators suspect the root cause of the contamination stemmed from a change in how the blood pressure drug ingredients were made.

Drug companies found some batches of losartan contained small amounts of a nitrosamine impurity, N-Nitroso-N-methyl-4-aminobutyric acid, or NMBA. The FDA will allow some of those tainted drugs to be sent to pharmacies as long as the levels of the contaminant do not exceed 9.82 parts per million. The agency will decide on a case-by-case basis whether the drugs can be used.

The FDA said the stopgap measure ensures consumers will have access to the drug while manufacturers make batches of losartan that do not contain the impurity. The agency expects it will take six months to replenish the nation's supply of impurity-free losartan.

Not all versions of losartan will be allowed on the market under the new measure.

In a notice shared last week by the FDA, Torrent Pharmaceuticals said it would recall 104 lots of losartan potassium and losartan potassium/hydrochlorothiazide combination tablets that contained unacceptable levels of NMBA.

More than 49 million losartan prescriptions were filled in 2016, according to the ClinCalc DrugStats Database.

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The ARB recalls began last July when regulators discovered some drugs had small amounts of a nitrosamine impurity called NDMA, or nitrosodimethylamine – a carcinogen that FDA scientists concluded would generate one more cancer case above average rates for every 8,000 people on the highest dose of valsartan for four years.

Follow-up testing showed some batches of ARB blood pressure drugs contained another carcinogen, nitrosodiethylamine, or NDEA.

Not all ARB blood pressure drugs are under recall, and alternative treatments or medications can be prescribed in consultation with physicians, the FDA said. The FDA has a list of 40 ARB drugs that are not affected by the recall. The agency expects the number of drugs available on that list to rise as manufacturers test drug products or make manufacturing changes.

The FDA's exception on tainted blood pressure drugs applies only to losartan products that contain NMBA. No other blood pressure medication contains that impurity, but drug companies continue to test this class of drugs because of the manufacturing problems that surfaced.

Doctors said the recall has been difficult to track and manage for all facets of the health care system.

"I don’t think health care delivery systems were prepared for the burden this would entail," said Dr. Vivek Bhalla, director of the Stanford Hypertension Center, "It has fallen on physicians, but it also has fallen on pharmacies and on patients."