The US Food and Drug Administration has warned of a looming shortage of drugs used by millions in the United States to treat high blood pressure.

Valsartan Products In Short Supply

On Friday, the agency said there is now a national shortage of the ARBs, or angiotensin II receptor blockers that include valsartan, irbesartan, and losartan, which doctors widely prescribe to treat high blood pressure and heart failure.

The FDA also warned of further shortage of blood pressure drugs as other medicines could also become short in supply because of ongoing recalls.

"Valsartan products are in shortage, and we know that other types of products may fall into shortage soon," FDA Commissioner Scott Gottlieb and FDA director of drug evaluation and research Janet Woodcock said in a statement.

Blood Pressure Drugs Contaminated With Cancer-Causing NDMA

Over the last six months, the FDA recalled dozens of batches of blood pressure medicines over concerns these may have been contaminated with a cancer-causing substance.

The drugs contain N-Nitrosodimethylamine (NDMA) and N-Nitrosodiethylamine (NDEA), which are both probable human carcinogen. These substances are an environmental contaminant and can be found in water and food products such as vegetables, dairy products, and meat. The amount of these substances found in affected drug products, however, were unacceptable.

The long-term risk posed by these substances in blood pressure medications is not yet clear, but Erin Michos, from Johns Hopkins Medicine, said the amount consumed would play a role in the risks.

"The dose makes the poison," Michos told CNN. "It is a toxin that can affect the liver and cause liver fibrosis or scarring and liver tumors in rats. So it is assumed to be toxic to humans, as well."

The presence of the impurities in the drugs is likely due to manufacturing processes that reuse materials such as solvent. Reusing solvent is an accepted practice in the industry, albeit manufacturers are expected to ensure the reused materials meet safety standards.