Zantac 75 tablets are used to treat & prevent indigestion & heartburn, lasting for up to 12 hours. Your stomach produces acid which in turn helps to digest food & on some occasions your stomach produces too much acid causing indigestion, heartburn & the discomfort associated with these. The active ingredient in Zantac is Ranitidine which is a medicine called an H2 blocker, this works by preventing your stomach producing too much acid.