The Food and Drug Administration is requesting that manufacturers pull all prescription and over-the-counter ranitidine drugs, known by the brand name Zantac, from the market immediately, the agency announced on Wednesday.The FDA noted that an ongoing investigation has determined that levels of a contaminant in the heartburn medications increase over time and when stored at higher-than-normal temperatures, pose a risk to public health.The contaminant, N-nitrosodimethylamine