antihistamine

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A type of drug that inhibits the combination of histamine with histamine receptors. These drugs are termed either H-1 or H-2 receptor antagonists depending on which type of histamine receptor is involved. H-1 receptor antagonists are used largely for treating allergies, and H-2 receptor antagonists are used to treat peptic ulcer disease and related conditions. See also Histamine.

The primary therapeutic use of H-1 receptor antagonists is to antagonize the effects of histamine released from cells by antigen-antibody reactions; they can thus inhibit histamine-induced effects, such as bronchoconstriction, skin reactions, for example, wheals and itching, and nasal inflammation. These drugs, therefore, are quite effective in reducing allergy signs and symptoms, especially if they are administered before contact with the relevant antigen; however they are not effective in treating asthma. Their effects vary widely, both among the drugs and from individual to individual; in young children excitement may be seen. Another common set of effects caused by many of these drugs, including dry mouth, blurred vision, and urinary retention, can be ascribed to their anticholinergic actions. H-1 receptor antagonists have low toxicity. The chief adverse effect is sedation. Overdoses of H-1 receptor antagonists may be associated with excitement or depression, and although there is no pharmacologic antidote for these drugs, good supportive care should be adequate in managing cases of poisoning. See also Allergy; Antigen-antibody reaction; Asthma; Sedative.

H-2 receptor antagonists are much newer. Histamine stimulates gastric acid secretion by combining with H-2 receptors. By preventing this combination, H-2 antagonists can reduce acid secretion in the stomach, an effect that makes these drugs useful in managing various conditions, such as peptic ulcer disease. See also Ulcer.

Other conditions in which H-2 antagonists are used to lower gastric acidity include reflux esophagitis, stress ulcers, and hypersecretory states such as the Zollinger-Ellison syndrome, in which tumor cells secrete large amounts of the hormone gastrin, which stimulates gastric acid secretion. In these conditions, administration of H-2 antagonists reduces symptoms and promotes healing.

The toxicity of H-2 antagonists is quite low, and adverse effects are reported by only 1-2% of patients. The most common side effects are gastrointestinal upsets, including nausea, vomiting, and diarrhea.

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Drug that antagonizes the actions of histamine; those that block histamine H₁ receptors are used to treat allergic reactions; those that block H₂ receptors are used to treat peptic ulcers.

A drug that counteracts the effects of histamine and relieves the symptoms of some allergic conditions, such as hay fever.

(H1-receptor antagonists)

A class of drugs that antagonize the effect of histamine, a substance released by the body in large amounts during allergic reactions. Antihistamines act by blocking H₁ receptors in the skin, nose, and airways: these receptors – when stimulated by histamine – produce the symptoms of an allergic response. They relieve the sneezing, running nose, and itching associated with allergic rhinitis, including hay fever (seasonal allergic rhinitis). They are also used to treat urticaria (an itchy rash commonly occurring as an allergic reaction to eating such foods as shellfish and strawberries), other itching skin conditions, and the histamine-induced reaction to insect bites or stings. For allergic reactions antihistamines may be given as nasal sprays or inhaled; they are also taken by mouth to prevent or treat symptoms. Antihistamines may be injected intravenously as an adjunct to adrenaline and/or corticosteroids to relieve life-threatening anaphylaxis – an extreme allergic reaction. Some antihistamines (e.g. cinnarizine, cyclizine, and promethazine) are used as antiemetics or to treat vertigo or Ménière's disease. The older antihistamines (especially promethazine and alimemazine and – to a lesser extent – chlorphenamine and cyclizine) cause drowsiness and some of them are used for their hypnotic effect, for example in preparations used for treating coughs and colds and in non-prescription sleeping tablets. These antihistamines can impair the ability to drive or operate machinery; alcohol enhances this effect. The newer antihistamines, including cetirizine, desloratadine, fexofenadine, levocetirizine, loratadine, and mizolastine) are less sedating.

See also azelastine hydrochloride; clemastine; cyproheptadine hydrochloride; hydroxyzine hydrochloride.

Side effects:
the older antihistamines cause sedation or drowsiness and (less commonly) headache, antimuscarinic effects (e.g. difficulty in urinating, dry mouth, blurred vision), and stomach upsets. The newer antihistamines are less sedating and are less likely to cause antimuscarinic effects.

Precautions:
alcohol enhances the sedating effects of the older antihistamines. Antihistamines should be used with caution by people with epilepsy, an enlarged prostate gland, glaucoma, or liver disease and by women who are pregnant or breastfeeding.

Interactions with other drugs:

Antidepressants: the sedative and antimuscarinic effects of antihistamines are increased if they are taken with monoamine oxidase inhibitors or tricyclic antidepressants.
Anxiolytic and hypnotic drugs: increase the sedative effects of antihistamines.

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Definition

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.

Description

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)

Precautions

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.

Interactions

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.

Resources

Books

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.

Organizations

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

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

Web Sites

"All about Allergies." Nemours Foundation. Available online at www.kidshealth.org/parent/medical/allergies/allergy.html (accessed October 24, 2004).

[Article by: Jennifer E. Sisk, MA]

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A drug that counteracts the effects of histamine and relieves the symptoms of some allergic reactions, such as hay fever, but not others, such as asthma. Antihistamines are not on the World Anti-Doping Agency's 2005 Prohibited List. However, athletes are generally recommended to use only preparations associated with a low incidence of sedation.

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(an-tee-his-tuh-meenz, an-tee-his-tuh-muhnz)

Drugs that counteract the effects of a substance in the body called histamine. The body releases histamine, which causes many of the symptoms associated with allergies (sneezing, watery eyes, runny nose), in response to external agents such as pollen.

IN BRIEF: A medicine used to relieve stuffy nose and watery eyes.

Rachel needed an antihistamine to help treat her allergies.

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any drug or other agent that antagonizes an action of histamine on the body. Antihistamines are used in the treatment of immediate-type hypersensitivity.

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1. counteracting the pharmacological effects of histamine.
2. an antihistamine.

(an′tīhis′təmin)
n

A drug that counteracts the release of histamine such as occurs in allergic reactions; also has topical anesthetic and sedative effects, as well as a drying effect on the nasal mucosa.

For a list of words related to antihistamine, see:

• Physiology - antihistamine: chemical that blocks action of histamine
• PHARMACOLOGY - antihistamine: drug that counters effects of histamines in body, esp. in allergies

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  See crossword solutions for the clue Antihistamine.

Dansk (Danish)
n. - antihistamin

Nederlands (Dutch)
antihistaminicum

Français (French)
n. - antihistamine, antihistaminique

Deutsch (German)
n. - (chem. med.) Antihistamin

Ελληνική (Greek)
n. - (χημ.) αντιισταμίνη
adj. - (χημ.) αντιισταμινικός

Italiano (Italian)
antistaminico

Português (Portuguese)
n. - anti-histamina (m) (Med.)
adj. - anti-histamínico

Русский (Russian)
антигистамин, антигистаминный

Español (Spanish)
n. - droga antihistamínica, antihistamínico

Svenska (Swedish)
n. - antihistamin (med.)
adj. - antihistaminisk

中文（简体）(Chinese (Simplified))
抗组胺剂

中文（繁體）(Chinese (Traditional))
n. - 抗組織胺劑

한국어 (Korean)
n. - 항 히스타민제(천식.두드러기 따위의 치료제)

日本語 (Japanese)
n. - 抗ヒスタミン薬

العربيه (Arabic)
‏(الاسم) مضاد للحساسيه (صفه) دوا يستعمل لمعالجه اعراض الحساسيه مثل حساسيه الربيع‏

עברית (Hebrew)
n. - ‮מונע אלרגיה, אנטי-היסטמין‬

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