Ranitidine drug, molecular model. Ranitidine is a drug used to reduce the amount of acid in the stomach. It is used to treat stomach and duodenal ulcers, reflex oesophagitis (acid in the gullet) and irritation of the stomach caused by non-steroidal anti-inflammatory drugs (NSAIDs). It is a H2-receptor antagonist that blocks the action of histamine on the parietal (acid-producing) cells in the stomach. Ranitidine's chemical formula is C13H22N4O3S. Atoms are represented as spheres and are colour-coded: carbon (C, grey), hydrogen (H, turquoise), nitrogen (N, blue), oxygen (O, red) and sulphur (S, yellow).