The U.S. Food and Drug Administration said on Wednesday that it found "unacceptable levels" of a cancer-causing impurity in limited testing using low-heat method of samples of heartburn medicines such as Zantac containing the ingredient ranitidine. However, the agency said its testing method found much lower levels of the impurity, N-nitrosodimethylamine (NDMA), than a higher temperature method used by a third-party laboratory. The FDA recommends the low-heat testing method for the drugs. NDMA is one of the impurities that has been found in blood pressure and heart failure medicines called Angiotensin II Receptor Blockers (ARBs). Those impurities are believed to have been introduced by recent changes in the manufacturing processes for the drugs. Regulators have been recalling those drugs since last year, leading to shortages, and the FDA expanded its investigation of the impurities beyond that class of drugs. The FDA found that the method used by the third-party laboratory used higher temperatures and generated very high levels of NDMA from the ranitidine drugs that were being tested. The high-temperature method was for testing impurity levels in ARBs and not suitable for ranitidine, the agency said.