Medical Specialties: Allergy/immunology, Family practice, Internal medicine

Clinical Definition

Antihistamines inhibit the action of histamine, an important chemical in the body.  During seasonal or perennial allergic rhinitis, histamines are part of a cascade of events that leads to nasal blockage, discharge, sneezing, itch and postnasal drip.


Antihistamines have an antagonistic action at the H1 or H2 receptor sites. The medication may cause side effects including sedation and an antiemetic effect. In some instances, antihistamines are administered therapeutically for these side effects. 

In Our Own Words

During an allergic reaction, histamines are released into the bloodstream and cause typical allergy symptoms, such as sneezing, runny nose and watery eyes. Antihistamines work toward preventing these symptoms from occurring.


First generation (older) antihistamines often cause side effects such as drowsiness and may also decrease nausea. Thus, antihistamines are sometimes given to achieve these side effects, rather than simply fight allergies.


Nonsedating antihistamines are available both over the counter and by prescription, in the form of oral medications or nasal sprays. 

Relevant Conditions
Common Types
  • Loratadine
  • Diphenhydramine
  • Dimenhydrinate
Side Effects
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  • The Ohio State University Wexner Medical Center. “Allergy Medications.” Accessed February 2014.
  • The University of Texas Medical School at Houston. “Antihistamines and Decongestants. Accessed February 2014.
  • American Academy of Otolaryngology – Head and Neck Surgery. “Antihistamines, Decongestants, and Cold Remedies.” Accessed February 2014.
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