H2 blockers – also known as histamine 2 receptor antagonists – are medications that decrease the amount of acid the stomach produces. These drugs are used to treat various gastrointestinal disorders. Although H2 blockers do not provide relief as quickly as antacids, they do provide longer-lasting relief than those medications.

H2 blockers work by preventing the action of histamine, a substance released by the human immune system during allergic reactions. Histamine normally interacts with  gastric H2 receptors to produce stomach acid. The H2 blockers inhibit this response and help reduce gastric acid output. Excess stomach acid can produce symptoms such as heartburn and damage tissue in the esophagus, stomach and small intestine.

H2 blockers take time to begin working. Usually, several days pass before the patient notices relief from symptoms. Physicians often allow the patient to take antacids until the H2 blocker begins to work. However, H2 blockers and antacids should not be taken within one hour of each other, as antacids can decrease the body’s ability to absorb medications such as H2 blockers.

H2 blockers are available in both prescription and nonprescription strength. They may be administered as an oral solution, tablet or injection. Patients who are taking nonprescription formulations should not take the maximum daily dosage of these drugs for more than two weeks. A physician should be consulted if symptoms persist after such a period of time.

The various types of H2 blockers include: