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H2 Blockers

histamine 2 receptor antagonists


About H2 blockers

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:

Generic NameBrand Name(s)










Conditions treated

Several gastrointestinal disorders can be treated with H2 blockers. These include:

  • Peptic ulcers. H2 blockers can treat two types of peptic ulcers, duodenal ulcers and gastric ulcers. They can also prevent recurrence of duodenal ulcers. H2 blockers may be used with other medications (e.g., antibiotics) to treat peptic ulcers caused by the bacteria Helicobacter pylori and to get rid of this bacteria.
  • Gastroesophageal reflux disease (GERD). H2 blockers are typically used to treat this condition only when other lifestyle modifications, dietary changes and other medications (e.g., antacids) fail to relieve symptoms. H2 blockers provide short-term relief by raising the pH level in the stomach. Inflammation of the esophagus (esophagitis) caused by GERD may also be treated with H2 blockers.
  • Hypersecretory states (states in which a bodily secretion is produced in excess). H2 blockers can be effective in treating conditions such as Zollinger-Ellison syndrome if the more powerful acid suppressant drugs (proton-pump inhibitors) are not effective or advisable. However, higher doses of H2 blockers may be required when treating a peptic ulcer. Such higher dosage levels may increase the risk of side effects associated with H2 blockers.
  • Stress ulcers. H2 blockers are used to inhibit the formation of stress ulcers, a sudden form of gastritis with ulcers and hemorrhaging (sudden, uncontrollable bleeding) that may occur following burns, shock, trauma or critical illness.

Other conditions treated with these drugs include heartburn, acid indigestion and sour stomach. Patients usually use over-the-counter strengths of H2 blockers for these milder conditions.

Conditions of concern

Patients should inform their physician of allergic reactions to any medications, including past allergic reaction to foods or other substances.

In addition, patients with the following disorders should use H2 blockers with caution:

  • Kidney disease or liver disease. These conditions may cause an H2 blocker to build up in the bloodstream. As a result, dosage levels of H2 blockers may have to be adjusted for patients who have these diseases.
  • Phenylketonuria (PKU). This is a genetic disorder in which the body lacks the enzyme necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine. This can lead to brain damage and progressive mental retardation. Some H2 blockers contain aspartame, which is converted to phenylalanine in the body. As a result, H2 blockers must be used with caution in patients with PKU.
  • Porphyria. This rare familial disease affects the way the body digests food. Use of H2 blockers may make this condition worse.
  • Weakened immune system. In some cases, the lower stomach acid production that results from use of H2 blockers can increase the risk of certain types of infection. However, this is usually only an issue with hospitalized patients.
Side effects

Potential side effects of H2 blockers

In most cases, H2 blockers do not cause significant problems for the patients who take them. However, some patients may experience side effects such as the following:

  • Mild diarrhea
  • Neutropenia (abnormally low number of white blood cells called neutrophils)
  • Dizziness
  • Fatigue
  • Arrhythmias (heartbeat irregularities)
  • Gynecomastia (abnormal breast enlargement in males)

The H2 blockers cimetidine and ranitidine may inhibit the effects of liver enzymes. This may prevent the liver from properly metabolizing certain drugs.


Drug or other interactions

H2 blockers can slow the time it takes to eliminate certain drugs from the body. Of particular concern to individuals taking H2 blockers are:

  • Bronchodilators (drugs that expand airways in the lungs) such as aminophylline, oxtriphylline or theophylline
  • Anticoagulants (drugs that help prevent blood clots)
  • Antifungal drugs such as itraconazole and ketoconazole
  • Caffeine
  • Diazepam (anti-anxiety drug)
  • Digoxin (heart drug)
  • Metoprolol and propranolol (beta blockers)
  • Phenytoin (antiseizure drug)
  • Tricyclic antidepressants

Previous research indicated that cimetidine increased the risk of prostate cancer in men. However, recent research contradicts this finding.


Lifestyle considerations

H2 blockers may take several days to take effect. Some patients may continue to take antacids in the interim to alleviate their symptoms. However, antacids affect the stomach’s ability to absorb other medications. Therefore, antacids and H2 blockers should be taken at least an hour apart.

Patients who smoke cigarettes may find that this habit negates the beneficial effects of H2 blockers. Cigarette smoke increases the production of acid in the stomach, particularly overnight. It is best if patients refrain from smoking altogether. However, those who find they cannot do so should at least avoid smoking after their final daily dose of an H2 blocker.

Consumption of alcohol while taking an H2 blocker may result in increased levels of alcohol in the blood.

Finally, patients should inform their physician that they are taking H2 blockers before they are scheduled to have skin testing for allergies or testing to determine the level of acid their stomach produces.

How to use

Pregnancy use issues

Women who are pregnant should not take H2 blockers without first consulting a physician about whether or not the potential risks of taking such drugs outweigh the benefits. Studies on the effects of H2 blockers have not been performed on pregnant or nursing women. However, famotidine, nizatidine and ranitidine have been shown to cause birth defects and other problems in animal studies. These drugs have been shown to pass into breast milk and may cause unwanted effects in newborns, such as decreased production of stomach acid and increased excitability.

Child use issues

The safety of using H2 blockers in children has not been conclusively established, although new research indicates that premature infants who are given the drugs may have a slightly increased risk of developing necrotizing enterocolitis, a life-threatening bowel infection. However, evidence suggests that other children who take these drugs do not experience adverse events that are significantly different from those experienced by adults. Thus, the decision to use H2 blockers should be made on an individual basis, after careful consideration of risks, benefits and alternatives for that particular child.

Elderly use issues

Elderly adults may experience increased side effects associated with the central nervous system, including confusion and dizziness. In addition, dosage levels may have to be adjusted for elderly patients who have kidney disease.

Symptoms of medication overdose

Patients exhibiting any of these symptoms should contact their physician immediately:

  • Respiratory failure
  • Tachycardia (rapid heartbeat)
  • Lacrimation (excessive secretion of tears)
  • Salivation
  • Vomiting
  • Constriction of the pupils
  • Diarrhea

Questions for your doctor

Patients may wish to ask their doctor the following questions related to H2 blockers:
  1. How will H2 blockers treat my condition?
  2. Why do you recommend that I take H2 blockers instead of antacids?
  3. Why should I take H2 blockers instead of another prescription medication?
  4. Which side effects, if any, should I report to you?
  5. I smoke. Will this impact the effectiveness of my H2 blocker?
  6. Do I need to continue taking H2 blockers after my ulcer has healed?
  7. I take many other medications. Is one H2 blocker preferred to avoid interactions with those drugs?
  8. Can I still drink alcohol while taking this medication?
  9. Will dietary changes help me instead of this medication?
  10. How long will it take before I see a reduction in my symptoms?


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