Cimetidine is a competitive inhibitor of histamine at the histamine H2 receptors of the parietal cells, and thus is known as a histamine antagonist. These drugs are used to treat gastric and duodenal ulcers by inhibition of gastric acid secretion stimulated by food, insulin, histamine, caffeine, and pentagastrin. Cimetidine is also helpful in treating upper gastrointestinal bleeding in critically ill patients. Side effects include skin rash, confusion, irregular heartbeat, and sore throat.