Histamine 2 Antagonists

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Histamine 2 antagonists


Histamine 2 antagonists are drugs that block the production of acid in the stomach.


Histamine 2 antagonists are used to treat the precancerous condition of Barrett's esophagus . They are also used to treat Zollinger-Ellison syndrome and multiple endocrine neoplasia, rare cancerous conditions in which the stomach makes too much acid and to prevent the development of gastric (stomach) and duodenal (upper part of the small intestine) ulcers.


Histamine 2 blockers are familiar to most people as the over-the counter heartburn medications Tagamet (cimetidine), Pepcid, (famotidine), and Zantac (ranitidine). Axid (nizatidine) is less well known. These drugs also come in prescription strengths. Histamine 2 blockers work by reducing the amount of acid the stomach produces.

The esophagus is a tube 10-13 inches long and about 1 inch wide that carries food from the mouth to the stomach. Normally, the esophagus is lined with cells that are similar to skin cells and look smooth and pinkish-white.

The stomach makes acid to help digest food. A different type of cell that is resistant to acid lines the stomach. These cells look red and velvety. At the place where the esophagus meets the stomach, there is a ring of muscle called a sphincter that normally keeps acid stomach juices from backflowing into the esophagus. When this sphincter is not working correctly, stomach acid enters the bottom portion of the esophagus. This backflow is called reflux or heartburn. When reflux occurs frequently over an extended period of time, it is called gastroesophageal reflux disease (GERD).

Barrett's esophagus is pre-cancerous condition in which normal cells lining the esophagus are repeatedly exposed to stomach acid and are replaced with abnormal cells that, in some people, develop into a type of cancer of the esophagus called adenocarcinoma . Histamine 2 blockers are given to reduce acid in the stomach and eliminate exposure of the esophagus cells to acid.

Histamine 2 blockers are also used to treat two rare cancerous conditions: multiple endocrine neoplasia (MEN) and Zollinger-Ellison syndrome, both of which can cause the stomach to produce too much acid. In MEN, an inherited form of cancer, tumors form in more than one gland. Depending on which glands are affected, the stomach may be stimulated to produce excess acid. In Zollinger-Ellison syndrome, a tumor in the digestive tract secretes a hormone called gastrin that stimulates the production of stomach acid. These tumors are malignant (cancerous) in 50% to 65% of people with Zollinger-Ellison syndrome.

Histamine 2 blockers are sometimes given in advance of chemotherapy to help reduce the gastrointestinal side effects of chemotherapy drugs. Cimetidine was the first histamine 2 blocker approved by the United States Food and Drug Administration (FDA) in 1976.

Recommended dosage

Recommended dose varies depending on how much stomach acid is produced. Histamine 2 blockers are available in low doses without a prescription and in higher doses with a prescription. They are available in tablet, chewable tablet, liquid, and injectable liquid form. If histamine 2 inhibitors are unsuccessful in controlling acid reflux, proton pump inhibitors (Prevacid, Prilosec) are usually given as an alternative.


People who have trouble with heartburn should stay away from acidic foods such as orange, grapefruit, and tomato juice, coffee, and carbonated drinks (sodas) because these all increase stomach acid. Although animal studies show that histamine 2 blockers appear to be safe during pregnancy, these drugs do pass into breast milk and should not be taken by nursing mothers.

Side effects

Histamine 2 blockers have few side effects. These drugs are excreted by the kidney, and may slow the excretion of other drugs excreted by the kidney. People with reduced kidney function may need a reduced dose of histamine 2 blockers.

Rare cases of irregular heart rhythms and high blood pressure have been reported when histamine 2 blockers are given intravenously (IV, injected directly into a vein). Mild diarrhea has been reported by some people taking these drugs.


Histamine 2 blockers are reported to have few interactions with other drugs. However, because they reduce the level of acid in the stomach, they may inhibit the uptake of drugs such as ketoconazole that depend on an acid environment in the stomach to work. These drugs should be administered at least two hours before histamine 2 blockers are taken. Prior to starting any over-the-counter medications, herbal medications, or new medications, patients should notify their physician and check with their pharmacists for any potential drug interactions.

Tish Davidson, A.M.



The canal between the pharynx and stomach that allows passage of food and liquids.


Pertaining to the stomach and intestine.