Antihistamines


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Uses of Antihistamines

Antihistamines are medications prescribed to treat multiple allergic conditions, including seasonal and year round nasal allergies, itching, hives, eczema, and allergic reactions.  Antihistamines work by blocking the action of histamine, a substance produced by our bodies during allergic reactions.  Histamine is the major chemical mediator responsible for many allergy symptoms including itchiness, sneezing, runny nose or eyes, and swellings.  Antihistamines commonly come in pill or liquid form but occasionally may be given during an emergency as an injection.  Some of these antihistamines are commonly combined with a decongestant such as pseudoephedrine.

Common antihistamines include:

  • chlorpheniramine (Chlor-Trimeton)
  • brompheniramine (Bromfed, Bromfenex, and Dimetane)
  • clemastine (Tavist)
  • diphenhydramine (Benadryl)
  • hydroxyzine (Atarax)
  • loratadine (Claritin, Alavert)
  • fexofenadine (Allegra)
  • cetirizine (Zyrtec)
  • levocetirizine (Xyzal)

Taking Antihistamines

Antihistamines should be taken exactly as your doctor prescribes or recommends them.  They can be used on an as needed basis since they tend to have a rapid onset of action.  For patients with more severe allergic symptoms, antihistamines may be utilized on a daily basis.

Side Effects of Antihistamines and Warnings

Possible side effects of antihistamines generally include:

  • sleepiness
  • fatigue
  • nervousness
  • agitation
  • dry mouth and eyes
  • difficulty urinating
  • thickening of mucous
  • difficulty breathing
  • upset stomach


Patients with these conditions may need to avoid antihistamines:

  • Enlarged prostate
  • Urinary tract blockage or difficult urination - Antihistamines may make urinary problems worse
  • Glaucoma
  • Intestinal obstruction
  • Liver disease
  • Kidney disease
  • Antihistamines should be used with caution if patients are taking any of these medications
  • Erythromycin
  • Ketoconazole
  • Central nervous system (CNS) depressants (medicines that cause drowsiness)
  • Monoamine oxidase (MAO) inhibitors

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your health care professional.



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