A pharmacy warned the Food and Drug Administration that it found a chemical believed to cause cancer in a widely used blood pressure medication, according to a filing from the federal agency.

Valisure, an online pharmacy company licensed in 37 states, told the FDA last week that high levels of dimethylformamide were found in valsartan, a drug produced by Swiss drugmaker Novartis and other pharmaceutical companies. The drug is used to treat hypertension in adults. The World Health Organization classifies dimethylformamide, or DMF, as a probable human carcinogen.

Valisure asked that the medication be recalled and requested that the FDA review and significantly lower the acceptable intake of DMF from its current level of 8,800,000 nanograms to less than 1,000 nanograms. The online pharmacy said it found the cancer-causing chemical in valsartan produced by five companies.

The FDA will evaluate Valisure's findings and will respond directly to the online pharmacy firm, FDA spokesman Jeremy Kahn said in a statement to CNBC. Patients should continue to take their blood pressure medication even if it is recalled until their doctor provides a replacement or alternative treatment, he added. Abruptly discontinuing a medication is risky, he said.

In a statement to CNBC, a spokesperson for Novartis said in general the company's manufacturing process does not use DMF but it cannot currently fully "exclude the possibility that traces of DMF (within applicable limits) may have been present in materials of other Drug Substances suppliers."

"The quality and safety of all our products is of the utmost importance to Novartis," the spokesperson added.

Several blood pressure drugs have already been recalled due to concerns about other cancer-causing chemicals. Earlier this month, Israel-based Teva Pharmaceuticals said it would expand a recall of its heart medication, losartan potassium, after a carcinogen known asN-Nitrosodimethylamine, or NMBA was detected. Torrent Pharmaceuticals in April said it would also recall losartan and Camber Pharmaceutical told the FDA in February it would recall the drug.

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