The U.S. Food and Drug Administration said on Wednesday that alternatives to popular heartburn drug Zantac and its generic versions, known chemically as ranitidine, have not been found to contain the probable cancer-causing impurity that ranitidine has been linked to. U.S. retailers Walmart Inc, CVS Health Corp Walgreens Boots Alliance Inc and Rite Aid Corp have all removed Zantac off their shelves after some drugs containing its key ingredient ranitidine were found to have traces of the impurity, N-nitrosodimethylamine (NDMA). Early tests of alternatives to over-the-counter ranitidine, such as Pepcid, Tagamet, Nexium, Prevacid and Prilosec show no NDMA, the agency said. The FDA said earlier this month it found unacceptable levels of NDMA in drugs containing ranitidine. The regulator 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.