Antihistamines are drugs used to treat the symptoms of allergies and allergic rhinitis by blocking the action of histamine, a chemical released by the immune system in allergic reactions.


Antihistamines are used to treat the sneezing, runny nose, and itchy eyes of allergies and allergic rhinitis, as well as allergic skin reactions and anaphylactic reactions to insect stings and certain foods. Antihistamines are available as prescription and over-the-counter tablets, topical preparations, nasal sprays, and eye drops.

Antihistamines work by blocking the effects of histamine, a chemical released by mast cells during an allergic response to an allergen. Histamine irritates and inflames the airways to produce sneezing and mucus production. Antihistamines attach to the areas on cells that histamines attach to, thereby blocking the allergic response.

Antihistamines are most effective when taken before exposure to an allergen. When used over time as an allergy treatment, antihistamines reduce the amount of histamine released by cells and decrease the likelihood that an allergic reaction will occur.

General use

Antihistamines are prescribed or recommended for infants, children, and adolescents with allergies and allergic rhinitis. Depending on the type of allergy, oral antihistamines may be taken regularly or seasonally to combat responses to allergens. Common allergens include dog and cat hair, dust mites, grass and tree pollen, and molds and mildew. For allergies that produce nasal symptoms, an antihistamine nasal spray may be used. For itchy eyes, antihistamine eye drops may be used.

Antihistamine tablets and topical creams, gels, sprays, or ointments are used to treat skin hives related to food allergies and itching and hives associated with allergic contact dermatitis and insect bites and stings .

In addition to treating allergies, some antihistamines have side effects that are used to treat other conditions. The strong sedating effect of some antihistamines is used to treat insomnia and difficulties in falling asleep. Some antihistamines also help inhibit nausea and vomiting and reduce motion sickness .

Commonly used antihistamines include the following:

  • diphenhydramine (Benadryl)
  • loratadine (Claritin)
  • cetirizine (Zyrtec)
  • fexofenadine (Allegra)
  • clemastine fumarate (Tavist)
  • chlorpheniramine (Chlor Trimeton)
  • brompheniramine (Dimetapp)


Some antihistamines produce drowsiness, although clinical studies have shown that children are less susceptible to antihistamine-induced drowsiness than adults. Some nonsedating antihistamines can act as stimulants in children and produce hyperactivity and sleeplessness.

Children with certain medical conditions may not be able to take antihistamines. The following are absolute or relative contraindications to use of antihistamines. The significance of the contraindication will vary with the drug and dose.

  • glaucoma
  • hyperthyroidism (overactive thyroid)
  • high blood pressure
  • heart disease
  • ulcers or other stomach problems
  • stomach or intestinal blockage
  • liver disease
  • kidney disease
  • bladder obstruction
  • diabetes

Side effects

The frequency and severity of adverse effects will vary depending on the antihistamine.

Central nervous system reactions include drowsiness, sedation, dizziness, faintness, disturbed coordination, lassitude, confusion, restlessness, excitation, tremor, seizures, headache, insomnia, euphoria, blurred vision, hallucinations, disorientation, disturbing dreams/nightmares, schizophrenic-like reactions, weakness, vertigo, nerve pain, and convulsions.

Gastrointestinal problems include increased appetite, decreased appetite, nausea, vomiting, diarrhea, and constipation .

Hematologic reactions are rare but may be severe. These include anemia, or breakdown of red blood cells; reduced platelets; reduced white cells; and bone marrow failure.

A large number of additional reactions have been reported. Not all apply to every drug, and some reactions may not be drug related. Some of the other adverse effects are chest tightness; wheezing; nasal stuffiness; dry mouth, nose, and throat; sore throat ; respiratory depression; sneezing; and a burning sensation in the nose.


Drug interactions vary with the chemical class of antihistamine. In general, antihistamines increase the effects of other sedatives, including alcohol.

Monoamine oxidase inhibitor antidepressants may prolong and increase the effects of some antihistamines.

Parental concerns

For children who resist taking pills, many antihistamines are available as flavored chewable tablets, tablets that easily dissolve on the tongue, and in flavored syrups. Because many over-the-counter allergy medicines contain multiple drugs, parents should be sure to read the prescribing and dosage information for any antihistamine their children are taking to ensure safe use.


Allergen —A foreign substance that provokes an immune reaction or allergic response in some sensitive people but not in most others.

Anaphylaxis —Also called anaphylactic shock; a severe allergic reaction characterized by airway constriction, tissue swelling, and lowered blood pressure.

Histamine —A substance released by immune system cells in response to the presence of an allergen. It stimulates widening of blood vessels and increased porousness of blood vessel walls so that fluid and protein leak out from the blood into the surrounding tissue, causing localised inflammation of the tissue.

Mast cells —A type of immune system cell that is found in the lining of the nasal passages and eyelids. It displays a type of antibody called immunoglobulin type E (IgE) on its cell surface and participates in the allergic response by releasing histamine from intracellular granules.



Simms, F. Estelle. Histamine and H1-Antihistamines in Allergic Disease. New York: Marcel Dekker Incorporated, 2002.

Taylor, R., J. Krohn, and E. M. Larson. Allergy Relief and Prevention, 3rd ed. Vancouver: Hartley and Marks, 2000.


Allergy and Asthma Network: Mothers of Asthmatics. 2751 Prosperity Ave., Suite 150, Fairfax, VA 22031. Web site:

American Academy of Allergy, Asthma, and Immunology. 611 East Wells St., Milwaukee, WI 53202. Web site:


"All about Allergies." Nemours Foundation. Available online at (accessed October 24, 2004).

Jennifer E. Sisk, MA

Also read article about Antihistamines from Wikipedia

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