Esomeprazole drug molecule. Computer model of a molecule of the proton pump inhibitor drug esomeprazole (marketed as Nexium). Atoms are represented as spheres and are colour-coded: carbon (green), hydrogen (white), oxygen (red), nitrogen (blue) and sulphur (yellow). Esomeprazole is used to treat gastric ulcers and gastro- oesophageal reflux disease. It works by inhibiting an enzyme involved in the final stages of gastric acid production.