Antihistamines are drugs used to treat the symptoms of
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
as well as allergic skin reactions and anaphylactic reactions to insect
and certain foods. Antihistamines are available as prescription and
over-the-counter tablets, topical preparations, nasal sprays, and eye
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.
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
and hives associated with allergic
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
Commonly used antihistamines include the following:
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.
The frequency and severity of adverse effects will vary depending on the
Central nervous system reactions include drowsiness, sedation,
faintness, disturbed coordination, lassitude, confusion, restlessness,
excitation, tremor, seizures,
insomnia, euphoria, blurred vision, hallucinations, disorientation,
disturbing dreams/nightmares, schizophrenic-like reactions, weakness,
Gastrointestinal problems include increased appetite, decreased appetite,
nausea, vomiting, diarrhea,
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;
; 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
Monoamine oxidase inhibitor
may prolong and increase the effects of some antihistamines.
—A foreign substance that provokes an immune reaction or allergic
response in some sensitive people but not in most others.
—Also called anaphylactic shock; a severe allergic reaction
characterized by airway constriction, tissue swelling, and lowered blood
—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.
—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."
Available online at
(accessed October 24, 2004).