(Adds Valisure response)
By Saumya Joseph
Oct 2 (Reuters) - The U.S. Food and Drug Administration said on Wednesday it found "unacceptable levels" of a cancer-causing impurity in the popular heartburn drug Zantac and its generic versions known chemically as ranitidine.
U.S. and European health regulators said last month they were reviewing the safety of ranitidine, after online pharmacy Valisure flagged the impurities to regulators. Canada's health authorities have asked makers of the drug to halt distribution as it gathers more information.
The FDA, which checked the over-the-counter drugs using a low-heat method of testing, said it found much lower levels of the impurity - N-nitrosodimethylamine (NDMA) - than was discovered with a higher temperature test employed by Valisure.
NDMA had previously been found in some blood pressure medicines from a class of drugs known as angiotensin II receptor blockers, or ARBs. The potentially cancer-causing impurities are believed to have been introduced by recent changes in the manufacturing process for the drugs.
Recalls of some ARBs since last year have led to shortages of the medicines, including valsartan, the generic of Novartis' Diovan. The FDA has since expanded its investigation of the impurities beyond that class of drugs.
The FDA said Valisure's higher temperature testing method generated very high levels of NDMA from the ranitidine drugs. But the agency said that method, which Valisure had used for testing ARBs, was not suitable for ranitidine.
Valisure said on Wednesday that it also suspects the high temperature method resulted in findings of higher NDMA levels. It said that it did not find NDMA in a body-temperature test of ranitidine it conducted.
Still, an analysis Valisure conducted in conditions similar to the human stomach generated over 300,000 nanograms of NDMA, significantly above acceptable levels set by the FDA, it said.
David Light, Valisure's chief executive, said this shows that even if the pills are not contaminated, the drug is likely to form the impurity in patients' stomachs after it is ingested.
Walmart Inc on Wednesday joined pharmacy chains CVS Health Corp, Walgreens Boots Alliance Inc and Rite Aid Corp in suspending sale of the medicines.
Last month, Novartis halted global distribution of its ranitidine drugs.
The FDA on Wednesday asked ranitidine makers to conduct their own testing to assess levels of the impurity and to send samples of their products for testing by the agency.
It will assess oral ranitidine medicines and has begun testing samples of other heartburn treatments such as proton-pump inhibitors as part of the investigation. (Reporting by Saumya Sibi Joseph in Bengaluru and Michael Erman in New York; Editing by Maju Samuel and Bill Berkrot)