Ranitidine. Molecular model of the drug ranitidine (C13.H22.N4.O3.S), a histamine H2-receptor antagonist that inhibits stomach acid production and is used to treat peptic ulcer disease and gastroesophageal reflux disease (GERD). It is marketed as Zantac. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue), oxygen (red) and sulphur (yellow).