The FDA is investigating an impurity in certain heartburn drugs previously found in blood pressure drugs. Low levels of NDMA (N-nitrosodimethylamine), a probable carcinogen, were found in Zantac (ranitidine) and other heartburn medications, the agency said.

NDMA and other nitrosamines were first found in some “sartan” blood pressure and heart failure drugs last year. Investigations into those products are ongoing.

The FDA is “working with international regulators and industry partners to determine the source of this impurity,” said CDER Director Janet Woodcock. The agency is “evaluating any possible risk to patients,” but said OTC users may want to consider using a different product.