Antihistamines – Uses and Benefits – How Does it Works?

Antihistamines Information – Side Effects, Uses and Benefits

Type of Drug:

Antihistamine; allergy medications.

How the Drug Works:

Antihistamines block the effects of histamine at H1 receptors in the body .Histamine is one of the chemicals released in the body during an allergic reaction. It causes redness, itching, and irritation of respiratory mucosal tissues, and can cause watery eyes, runny nose, sneezing, itchy nose, eyes, palate, and throat, and hives.


To provide relief of symptoms associated with seasonal and year-round allergies causing runny nose, sneezing, itching of the nose or throat, or watery, itchy eyes; hives; and rash, insect bites, and stings where there is mild to moderate itching and no complications.

Certizine and fexofenadine: To treat chronic idiopathic urticaria (hives).

Diphenhydramine: To treat or prevent motion sickness, sleeplessness, and parkinsonism. Also used as a suppressant for coughs caused by colds or allergies .

Promethazine: To treat or prevent motion sickness, nausea, and vomiting associated with anesthesia and surgery, apprehension, and sleepless ­ness. Used with pain medication (eg, meperidine) to help control pain following surgery.

Unlabeled Uses: Occasionally doctors may prescribe cyproheptadine to stimulate appetite in underweight people (eg, anorexia nervosa) and to treat cluster headaches.


Pregnancy: Safety for use during pregnancy has not been established. Several possible associations with malformations have been found, but significance is unknown. Use only when, clearly needed and the potential benefits outweigh the possible hazards to the fetus. Do not use during the third trimester; newborns and premature infants may have severe reactions (eg, convulsions) to some antihistamines.

Breastfeeding: Antihistamines appear in breast milk. Breastfeeding should be discouraged while taking these medications, or antihistamines should be avoided during the breastfeeding months.

Children: Antihistamine over dosage in children may cause hallucinations, convulsions, or death. Antihistamines may decrease mental alertness or produce hyperactivity in young children. Consult your pharmacist or product label to determine the appropriate dose of nonprescription drugs to be given to children younger than 12 years of age. Do not give prescription antihistamines to children younger than 12 years of age unless advised by your doctor or approved in the package labeling.

Elderly: Elderly patients may require lower doses. Antihistamines are more likely to cause dizziness, sedation, confusion, disorientation, fainting, excitation, and decreased blood pressure.

Drug Interactions:

Tell your doctor or pharmacist if you are taking or planning to take any over­the-counter or prescription medications or dietary supplements while taking antihistamines. Doses of one or both drugs may need to be modified or a different drug may need to be prescribed. Alcohol, sedatives (sleeping pills), tranquilizers, antianxiety medications,and narcotic pain relievers all are known to interact with antihistamines. The following drug and drug classes also interact antisamines:

  • aluminum- and magnesium-containing antacids (eg, Maalox)
  • anesthesia (promethazine only)
  • barbiturates (promethazine only)
  • cimetidine (azelastine only)
  • epinephrine (eg, Adrenalin)
  • erythromycin (eg, E-Mycin) (fexofenadine only)
  • ketoconazole (eg, Nizoral)
  • MAOls (eg, phenelzine)
  • tricyclic antidepressants (eg, Elavil)

Side Effects:

Every drug is capable of producing side effects. Many antihistamine users experience no, or minor, side effects. The frequency and severity of side effects depend on many factors including dose, duration of therapy, and individual susceptibility. Possible side effects include:

Digestive Tract: Stomachache; constipation; appetite changes; nausea; vomiting; diarrhea; indigestion; decreased salivation; gas; altered sense of taste; dry mouth.

Nervous System: Drowsiness; dizziness; confusion; disorientation; nervousness; restlessness; excitation; tremor; decreased coordination; fatigue; headache; sleeplessness; sedation; irritability; exaggerated sense of well being; fainting; migraine; abnormal skin sensations; feeling of whirling motion; impaired concentration; hallucination; hysteria; amnesia; abnormal thinking; agitation; anxiety; depersonalization; depression; convulsions.

Circulatory System: Changes in blood pressure; irregular heartbeat; fast heartbeat; shock; palpitations (pounding in the chest); chest pain.

Respiratory System: Dry nose and throat; cough; sore throat; thickening of mucus in respiratory tract; chest tightness; wheezing; nasal stuffiness; hyperventilation; upper respiratory tract infection; laryngitis; bronchospasm; nasal and throat burning; nosebleed; difficulty breathing; sneezing.

Skin: Rash; hives; sweating; itching; flushing; dry skin; excessively oily skin; skin inflammation; red or purple patches under the skin; acne; excessive sweating; hair loss.

Other: Weight gain; joint pain; muscle pain; sensitivity to light; frequent, abnormal, or difficult urination; urine retention; ringing in the ears; blurred or double vision; itchy, watery eyes; yellowing of skin or eyes; chills; painful menstruation (loratadine and fexofenadine only); thirst; earache; fever; leg cramps; weakness; decreased sensitivity to stimulation; eye pain or abnormality; altered tearing; twitching eyes; tight muscles; breast pain; heavy menstruation; allergic reaction; back pain; “flu-like” symptoms; general body discomfort; pain in extremities; twitching; decreased sex drive; excessive movement; inflammation of the vagina with itching or abnormal discharge; urinary discoloration; distorted sense of smell; hot flasher; anemia.

Guidelines for Use:

  • Dosage is individualized. Use exactly as prescribed.
  • If a dose is missed, take it as soon as possible. If several hours have passed or it is nearing time for the next dose, do not double the dose to catch up, unless advised by your doctor. If more than one dose is missed or it is necessary to establish a new dosage schedule, contact your doctor or pharmacist.
  • Loratadine tablets and syrup should be taken once daily on an empty stomach (1 hour before or 2 hours after a meal). Loratadine rapidly­disintegrating tablets break down quickly after being placed on the tongue; they may be given with or without water.
  • Do not crush or chew sustained-release products.
  • May cause drowsiness or dizziness. Avoid alcohol and other sedatives (eg, tranquilizers). Chlorpheniramine, loratadine, cetirizine, and fexofenadine cause the least amount of drowsiness; promethazine and diphenhydramine cause the most. Use caution while driving or performing tasks requiring alertness, coordination, or physical dexterity.
  • Promethazine – Report any involuntary muscle movements or unusual sensitivity to sunlight.
  • Use alone when sneezing and runny nose exist, but nasal congestion is slight. When nasal congestion accompanies sneezing and runny nose, an oral decongestant (eg, pseudoephedrine) may be added.
  • Avoid therapy with combination products containing antihistamines, decongestants, anticholinergics, expectorants, pain medication, cough suppressants, caffeine, and vitamins. Deal with symptoms as specifically as possible. Combination products frequently contain some ingredients in ineffective amounts. They increase risk for side effects and drug interactions, and usually cost more than products with fewer ingredients.
  • A persistent cough may be a sign of a serious condition. Contact your doctor if cough persists for more than 1 week, tends to recur, or is accompanied by a high fever, rash, or persistent headache.
  • Do not take antihistamines for persistent or chronic cough, such as occurs with smoking, asthma, or emphysema or if cough is accompanied by excessive phlegm, unless directed by a doctor. Cough is a protective reflex that helps clear the respiratory tract of mucus and other debris.
  • Do not take for 7 days before allergy skin testing.
  • Contact your doctor immediately if you experience fainting, dizziness, unusual heartbeats, stomach pain, chest pain, vomiting, or any other unusual symptoms occur,
  • If sleeplessness persists continuously for more than 2 weeks, consult your doctor.
  • If the medication loses effectiveness with continued use, make sure you are taking it according to directions. Sometimes, loss of effectiveness indicates that your condition has gotten worse. Treatment may require a different medication or dose.
  • May rarely cause photosensitivity (sensitivity to sunlight). Avoid prolonged exposure to the sun or other forms of ultraviolet (UV) light (eg, tanning beds). Use sunscreens and wear protective clothing until tolerance is determined.
  • Azelastine – Prime the nasal spray delivery system before initial use and after storage for 3 or more days. Avoid spraying in the eyes.
  • Store at room temperature (59° to 77° F) in a tightly closed container. Protect from heat, light, and moisture. Keep away from children.
  • Suppositories and some syrups – Store in the refrigerator (36° to 46°F) in a tightly closed container. See individual labels for specific storage information.

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2 Comments so far »

  1. rercvawrum said,

    Wrote on June 13, 2007 @ 2:48 pm

    Hello! Good Site! Thanks you!

  2. Justine said,

    Wrote on August 13, 2007 @ 8:20 pm

    Excellent site. Been taking antihistamines for a week and suffered terrible side effects (including severe depression) which were unknown to me. This site explains them all and has put my mind at rest!

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