Learn More About Antihistamines

Antihistamines are used to treat many allergy symptoms

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Antihistamines are a class of medications that block the action of histamine, a substance that is released by cells in your body in response to an allergic reaction. The release of histamine is responsible for many allergy symptoms, such as sneezing, runny nose and itching. Some antihistamines are also used for their side effects, including sedation and prevention of nausea and vomiting.

Antihistamines are available by prescription and over the counter.

Examples of Antihistamines

Examples of prescription antihistamines include Allegra (fexofenadine), Astelin Nasal Spray (azelastine) and Zyrtec (cetirizine).

Examples of over-the-counter antihistamines include Claritin (loratadine), Benadryl (diphenhydramine) and Chlor-Trimeton (chlorpheniramine).

Diphenhydramine, the active ingredient in Benadryl, causes sedation and is used in many over-the-counter sleep aids.

More Information About Histamines and Antihistamines

Antihistamines counter the actions of histamine, a low molecular weight amine derived from an amino acid called histidine.

Histamine exerts various effects on the body including the following:

  • cellular growth
  • cellular proliferation
  • neurotransmitter
  • inflammation

Histamines interact with 4 receptors in the body: H1, H2, H3, and H4.

H1 receptors are widely expressed throughout the body, including on neurons, smooth muscle, endothelium, skin and so forth.

H2 receptors are also found in many locations throughout the body, including on the heart, stomach, endothelium and skin.

H3 and H4 receptors are found in fewer places throughout the body, including the neurons and bone marrow, respectively.

H1 antihistamines or blockers, such as diphenhydramine (Benadryl) and loratadine (Claritin), reversibly bind to H1 receptors.

These medications can either be sedating or low-sedating. In other words, some of these H1 blockers make you sleepier than other H1 blockers.

H1 antihistamines are used to treat various immune-mediated conditions, such as hives (urticaria), itchiness (pruritis), angioedema (skin swelling), allergies and atopic dermatitis or eczema. These medications are also used to treat symptoms of the common cold such as sneezing and sore throat. Of note, limited evidence supports the use of H1 antihistamines to treat atopic dermatitis.

Adverse Effects of Antihistamines

Drowsiness is the most prominent adverse effect of many H1 blockers. Most people become tolerant of the drowsiness associated with some H1 blockers after a few days. Please understand that if you are driving around or operating heavy machinery, you shouldn't use sedating H1 blockers. Instead, consult with your physician about a less-sedating alternative to treat allergies, itchiness, hives and so forth. For example, loratadine (Claritin) is a low-sedating histamine blocker whereas diphenhydramine (Benadryl) is sedating.

Here are some other potential adverse effects of H1 blockers:

  • dizziness
  • tinnitus (ringing in the ears)
  • irritability
  • insomnia
  • tremor
  • nausea and vomiting
  • diarrhea
  • anorexia
  • dry mucous membranes
  • hypotension
  • urinary retention
  • cardiac arrhythmias

Some of these adverse effects can become quite serious--especially in the case of antihistamine overdose--and if you suspect that you are having a bad reaction to antihistamines, inform your physician or call 911 immediately. Please note that elderly people, children, and pregnant women should proceed cautiously when taking antihistamine medications.

Like H1 blockers, H2 blockers also bind to histamine receptors--specifically, H2 receptors. Examples of H2 blockers include cimetidine (Tagamet) and ranitidine (Zantac).

Commonly, H1 blockers are combined with H2 blockers to treat hives (urticaria) and angioedema (skin swelling). Because H2 receptors exist in the stomach, H2 blockers can also be used to decrease acid secretion and treat indigestion, dyspepsia, and GERD; however, proton-pump inhibitors like pantoprazole (Protonix) are usually more effective.

H2 blockers can cause many adverse reactions similar to H1 blockers, such as confusion, drowsiness, nausea and vomiting and so forth. Furthermore, by suppressing acid secretions in the stomach, which normally kill bacteria in the mouth, some people who take H2 blockers experience oral infections and pneumonia. Increased risk of infection is particularly prevalent among immunocompromised (think HIV/AIDS), people with diabetes and elderly people.

As with H1 receptors, if you experience a bad reaction to H2 receptors, immediately contact your physician.

Selected Sources

Wood RA. Chapter 229. Antihistamines. In: Goldsmith LA, Katz SI, Gilchrest BA, Paller AS, Leffell DJ, Wolff K. eds. Fitzpatrick's Dermatology in General Medicine, 8e. New York, NY: McGraw-Hill; 2012.