Regulators in the U.S. and Europe are investigating why traces of a possible carcinogen were found in certain heartburn drugs, including Zantac, more than a year after the same impurity was found in some blood pressure pills, a revelation that has raised questions about the safety of the pharmaceutical supply chain.

At issue is NDMA, an organic chemical that was once used to make rocket fuel and is an unintended by-product of certain chemical reactions. Last year, the substance was detected in a class of medicines known as angiotensin II receptor blockers, notably, valsartan, prompting an increasing spate of product recalls even as regulators have attempted to reassure consumers that cancer risks are low.

On Friday, both the Food and Drug Administration and the European Medicines Agency released statements disclosing that NDMA was found in certain ranitidine medicines, which are histamine-2 blockers that decrease the amount of acid created by the stomach. The medication is sold both over-the-counter and by prescription.

“The agency is examining levels of NDMA in ranitidine and evaluating any possible risk to patients,” the FDA said in its statement. The agency is not telling consumers to stop taking ranitidine medications, “at this time,” but did say consumers “could consider using other OTC medicines approved for their condition. There are multiple drugs on the market that are approved for the same or similar uses as ranitidine.”Related: 

The EMA, by the way, separately noted that traces of the impurity were also found in a diabetes medicine but were found to be within acceptably safe limits.

A spokesperson for Sanofi (SNY), which manufactures Zantac, said the company “takes patient safety seriously, and we are committed to working with the FDA,” and that Zantac “has been around for over a decade and meets all the specified safety requirements for use in the [over-the-counter] market.”

The presence of NDMA in the heartburn medicines, though, is certain to heighten anxiety.

Over the past year, the number of companies recalling the blood pressure pills has expanded as evidence indicated, in some cases, that supplies dated back several years. Meanwhile, potentially tainted drugs have been sold to millions of people in more than two dozen countries.

On a larger scale, the episode has raised questions about regulatory oversight, especially since some of the suppliers are located in India and China, where quality control problems are a regular — and growing — problem. The FDA, in particular, has been on the defensive after reports that earlier inspections found troubling laboratory procedures at Chinese and Indian companies that supplied a key ingredient.

Earlier this year, the House Committee on Energy and Commerce wrote the FDA to express concern about the ability to oversee foreign suppliers. They cited a 2016 Government Accountability Office report that found the FDA significantly increased the number of inspections of overseas facilities, but failed to assess the extent to which its efforts were improving the quality of drugs entering the country