Effective, long-lasting 24 hour relief from heartburn and acid indigestion due to gastro-oesophageal reflux with one tablet. ZANTAC DOUBLE STRENGTH works by controlling the production of stomach acid. ZANTAC DOUBLE STRENGTH 300mg ranitidine for 24 hour relief from heartburn and acid indigestion. Starts to work in as little as 30 minutes1,2,3 1. Johannessen T et al, Clin Ther. 19972. Elm M et al, Scan J Gastroenterol. 19983. Zantac Product Information Intended for patients who routinely require two 150mg ranitidine tablets for relief of symptoms. AUST R 95076 Zantac Double Strength Ranitidine 300mg Tab X 14.