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Antihistamines are drugs that block the action of histamine (a compound released in allergic inflammatory reactions) at the H1 receptor sites, responsible for immediate hypersensitivity reactions such as sneezing and itching. Members of this class of drugs may also be used for their side effects, including sedation and antiemesis (prevention of nausea and vomiting).


Antihistamines provide their primary action by blocking histamine H1 at the receptor site. They have no effect on rate of histamine release, nor do they inactivate histamine. By inhibiting the activity of histamine, they can reduce capillary fragility, which produces the erythema, or redness, associated with allergic reactions. They will also reduce histamineinduced secretions, including excessive tears and salivation. Additional effects vary with the individual drug used. Several of the older drugs, called first-generation antihistamines, bind non-selectively to H1 receptors in the central nervous system as well as to peripheral receptors, and can produce sedation, inhibition of nausea and vomiting, and reduction of motion sickness. The second-generation antihistamines bind only to peripheral H1 receptors, and reduce allergic response with little or no sedation.
The first-generation antihistamines may be divided into several chemical classes. The side effect profile, which also determines the uses of the drugs, will vary by chemical class. The alkylamines include brompheniramine (Dimetapp) and chlorpheniramine (Chlor-Trimeton.) These agents cause relatively little sedation, and are used primarily for treatment of allergic reactions. Promethazine (Phenergan), in contrast, is a phenothiazine, chemically related to the major tranquilizers, and while it is used for treatment of allergies, may also be used as a sedative, the relieve anxiety prior to surgery, as an anti-nauseant, and for control of motion sickness. Diphenhydramine (Benadryl) is chemically an ethanolamine, and in addition to its role in reducing allergic reactions, may be used as a nighttime sedative, for control of drug-induced Parkinsonism, and, in liquid form, for control of coughs. Consult more detailed references for further information.
Brand Name (Generic Name) Possible Common Side Effects
∗Also used in the treatment of
∗Atarax (hydroxyzine
Drowsiness, dry mouth
Benadryl (diphenhydramine
Dizziness, sleepiness, upset stomach,
decreased coordination
Hismanal (astemiozole) Drowsiness, dry mouth, fatigue, weight
PBZ-SR (tripelennamine
Dizziness, drowsiness, dry mouth and
throat, chest congestion, decreased
coordination, upset stomach
Periactin (cyproheptadine
Chest congestion, dizziness, fluttery
heartbeat, loss of appetite, hives, slee-
piness, vision problems
Phenergan (promethazine
Changes in blood pressure, dizziness,
blurred vision, nausea, rash
Polaramine (dexchlorphenira-
mine maleate)
Seldane, Seldane-D
Upset stomach, nausea, drowiness,
headache, fatigue
Tavist (clemastine fumarate) Decreased coordination, dizziness,
upset stomach
Trinalin Repetabs (azatadine
maleate, pseudoephedrine
Abdominal cramps, chest pain, dry
mouth, headache
The second generation antihistamines have no central action, and are used only for treatment of allergic reactions. These are divided into two chemical classes. Cetirizine (Zyrtec) is a piperazine derivative, and has a slight sedative effect. Loratidine (Claritin) and fexofenadine (Allegra) are members of the piperadine class and are essentially non-sedating.

Recommended dosage

Dosage varies with drug, patient and intended use. Consult more detailed references for further information.
When used for control of allergic reactions, antihistamines should be taken on a regular schedule, rather than on an as-needed basis, since they have no effect on histamine itself, nor on histamine already bound to the receptor site.
Efficacy is highly variable from patient to patient. If an antihistamine fails to provide adequate relief, switch to a drug from a different chemical class. Individual drugs may be effective in no more than 40% of patients, and provide 50% relief of allergic symptoms.

Side effects

The frequency and severity of adverse effects will vary between drugs. Not all adverse reactions will apply to every member of this class.
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, hysteria, nerve pain, and convulsions. Overdoses may cause involuntary movements. Other problems have been reported.
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.
When taking antihistamines during pregnancy, Chlorpheniramine (Chlor-Trimeton), dexchlorpheniramine (Polaramine), diphenhydramine (Benadryl), brompheniramine (Dimetapp), cetirizine (Zyrtec), cyproheptadine (Periactin), clemastine (Tavist), azatadine (Optimine), loratadine (Claritin) are all listed as category B. Azelastine (Astelin), hydroxyzine (Atarax), promethazine (Phenergan) are category C.
Regardless of chemical class of the drug, it is recommended that mothers not breast feed while taking 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
  • enlarged prostate
  • heart disease
  • ulcers or other stomach problems
  • stomach or intestinal blockage
  • liver disease
  • kidney disease
  • bladder obstruction
  • diabetes


Monoamine oxidase inhibitor antidepressants (phenelzine [Nardil], tranylcypromine [Parnate]) may prolong and increase the effects of some antihistamines. When used with promethazine (Phenergan) this may cause reduced blood pressure and involuntary movements.



Allergy and Asthma Network. 3554 Chain Bridge Road, Suite 200. (800) 878-4403.
American Academy of Allergy, Asthma, and Immunology. 611 East Wells St, Milwaukee, WI 53202. (800) 822-2762.
Asthma and Allergy Foundation of America. 1125 15th Street NW, Suite 502, Washington, DC 20005. (800)727-8462.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


Drugs having an action antagonistic to that of histamine on either H1 or H2 receptors.
Farlex Partner Medical Dictionary © Farlex 2012


Drugs with an action antagonistic to that of histamine; used to treat allergic symptoms.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


Drugshaving an action antagonistic to that of histamine on either H1 or H2 receptors.
Medical Dictionary for the Dental Professions © Farlex 2012
References in periodicals archive ?
A potential drug interaction, resulting in stillbirth, occurred when a mother took 50 mg of the antihistamine for itching and then, 1.5 hours later, a 30-mg dose of temazepam for sleep.
Antihistamines are very effective treatments for allergic rhinitis and have very good effects on rhinorrhoea, sneezing and itching.
It may take a week or so before a reasonable tolerance to allergens is achieved, thus the potential need for eye washes, decongestants or topical ocular antihistamines to initially manage these conditions.
Your favorite antihistamine can't achieve these levels.
If so, would taking an antihistamine have an anti-cancer effect?
"For instance, oral antihistamines, because of the time required for absorption, are not good options necessarily if you're going to be taking the medication just prior to exposure," he said.
The often-cited distinction between first- and second-generation antihistamines is mostly marketing hype.
Some antihistamines only start to work any where from one to three hours.
Since many children with acute ear infections have cold symptoms caused by a virus, these same doctors recommend an antihistamine and/or a decongestant to relieve symptoms.
To control for confounding by indication, exposure to other nonsedating or sedating antihistamines during the same period also was assessed.
Decongestants can help open stuffed nasal passages; antihistamines may dry a runny nose.
Nonsedating prescription antihistamines are on Anthem's third tier, which costs $30, or 50% of the drug's cost, for Anthem's two most popular plans.