Antihistamines

What are Antihistamines?

Antihistamines block histamine release from histamine-1 receptors and are mostly used to treat allergies or cold and flu symptoms, although some first-generation antihistamines (also called sedating antihistamines) may be used for other conditions, such as nausea or motion sickness.

Histamine-1 receptors are located in your airways (breathing tubes), blood vessels, stomach, and esophagus (throat area). Allergens, such as pollen or pet fur can stimulate these receptors, causing the release of histamine, which results in symptoms such as a rash, sneezing, a runny nose, or a narrowing of the airways (also called bronchoconstriction). Histamine-1 receptors are also found in the brain and spinal cord and first-generation antihistamines (sedating antihistamines) can cross through the blood/brain barrier into the brain and act on these receptors, causing sleepiness (sedation), and drowsiness.

What are antihistamines used for?

Antihistamines can help relieve the symptoms of an allergic reaction, such as:

  • a rash
  • itchiness
  • redness (inflammation)
  • swelling (edema)
  • red and watery eyes
  • a runny nose
  • sneezing.

This makes antihistamines very effective for the treatment of:

  • allergic rhinitis
  • colds
  • flu
  • food allergies
  • hay fever
  • hives (urticaria)
  • hypersensitivity reactions to drugs
  • insect bites
  • stings.

Sedating antihistamines (see explanation below) also act in the brain and spinal cord, and on other receptors. This makes some of them also useful for:

  • inducing sleep
  • preventing or treating motion sickness
  • reducing anxiety
  • some symptoms of Parkinson’s disease in people unable to tolerate traditional medications for Parkinson's disease.

What are the differences between antihistamines?

Antihistamines can be classified into two main categories:

  • first-generation antihistamines (also known as sedating antihistamines because they all cause drowsiness)
  • second-generation antihistamines (also known as non-sedating antihistamines because they don't tend to cause drowsiness, although they may do so in some people).

First-generation (sedating) antihistamines were developed more than seventy years ago and are still in widespread use today. They act on histamine receptors in the brain and spinal cord and in the rest of the body (called the periphery). They also act on muscarinic, alpha-adrenergic, and serotonin receptors. This means that first-generation antihistamines are more likely to cause side effects such as sedation, dry mouth, dizziness, low blood pressure, and a rapid heart-beat. They are also more likely than second-generation antihistamines to impair a person’s ability to drive or operate machinery. Interactions with other drugs are more common with first-generation antihistamines compared with second-generation antihistamines.

Second generation (non-sedating) antihistamines were developed in the 1980s and are much less sedating than first-generation antihistamines. They act on histamine-1 receptors in the periphery and are unlikely to penetrate the brain, so are less likely to cause side effects or interact with drugs. Most second-generation antihistamines do not cause drowsiness, although some (such as cetirizine and fexofenadine), may be more likely to do so at higher dosages.

Common antihistamines available in the U.S.

First Generation (Sedating) Antihistamines

Generic name Brand name examples Duration of action
brompheniramine Generic 12h
carbinoxamine maleate Generic 12h
chlorpheniramine Aller-Chlor 24h
clemastine Generic 12h
diphenhydramine Benadryl 12h
hydroxyzine Vistaril 24h
triprolidine Discontinued 12h

Second Generation (Non-sedating) Antihistamines

Generic name Brand name Duration of action
azelastine nasal spray Astelin 12h
cetirizine Zyrtec Allergy, Children’s Zyrtec 24h+
desloratadine Clarinex, Clarinex Reditabs 24h+
fexofenadine Allegra Allergy, Children's Allegra Allergy 12-24h
levocetirizine Xyzal 24h+
loratadine Alavert, Claritin 24h
olopatadine nasal spray Patanase 12h

What are the side effects of antihistamines?

Side effects of first-generation (sedating) antihistamines may include:

  • blurred or double vision
  • constipation
  • dry eyes
  • a dry mouth
  • drowsiness or sedation
  • a headache
  • low blood pressure
  • mucus thickening in the airways
  • a rapid heartbeat
  • stomach pain
  • urinary problems

Side effects of second-generation (non-sedating) antihistamines may include:

  • coughing
  • drowsiness (this is uncommon)
  • fatigue 
  • a headache
  • nausea
  • a sore throat
  • stomach pain.

List drug with Antihistamines

Drug Name
Carbihist
Generic name: Carbinoxamine
Carbinoxamine
Generic name: Carbinoxamine
Carboxine
Generic name: Carbinoxamine
Cetirizine
Generic name: Cetirizine
Cetirizine (Intravenous)
Generic name: Cetirizine
Children's Allegra Allergy
Generic name: Fexofenadine
Children's Allegra ODT
Generic name: Fexofenadine
Children's Claritin
Generic name: Loratadine
Children's Clear-Atadine
Generic name: Loratadine
Children's Dimetapp ND Allergy
Generic name: Loratadine
Chlo-Amine
Generic name: Chlorpheniramine
Chlor-Mal
Generic name: Chlorpheniramine
Chlor-Trimeton
Generic name: Chlorpheniramine
Chlorpheniramine
Generic name: Chlorpheniramine
ChlorTan
Generic name: Chlorpheniramine
Clarinex
Generic name: Desloratadine
Clarinex Reditabs
Generic name: Desloratadine
Desloratadine
Generic name: Desloratadine
Dexbrompheniramine
Generic name: Dexbrompheniramine
Dexchlorpheniramine
Generic name: Dexchlorpheniramine
Ed ChlorPed
Generic name: Chlorpheniramine
Ed-Chlor Ped Jr
Generic name: Chlorpheniramine
Ed-Chlortan
Generic name: Chlorpheniramine
Fexofenadine
Generic name: Fexofenadine
Histex CT
Generic name: Carbinoxamine
Histex I/E
Generic name: Carbinoxamine
Histex PD
Generic name: Carbinoxamine
Histex PD Drops
Generic name: Triprolidine
Histex Syrup
Generic name: Triprolidine
Histuss PD
Generic name: Carbinoxamine
Karbinal ER
Generic name: Carbinoxamine
Levocetirizine
Generic name: Levocetirizine
Loradamed
Generic name: Loratadine
Loratadine
Generic name: Loratadine
M-Hist PD
Generic name: Triprolidine
Mintex PD
Generic name: Carbinoxamine
Ohm Allergy Relief
Generic name: Loratadine
Palgic
Generic name: Carbinoxamine
PediaClear PD
Generic name: Triprolidine
PediaTan
Generic name: Chlorpheniramine
Pediatex
Generic name: Carbinoxamine
PediaVent
Generic name: Dexbrompheniramine
Pediox
Generic name: Carbinoxamine
Polaramine
Generic name: Dexchlorpheniramine
Quzyttir
Generic name: Cetirizine
RyClora
Generic name: Dexchlorpheniramine
Ryvent
Generic name: Carbinoxamine
Seldane
Generic name: Terfenadine
Tavist Allergy
Generic name: Clemastine
Tavist ND
Generic name: Loratadine
Triaminic Allerchews
Generic name: Loratadine
Triaminic Allergy
Generic name: Chlorpheniramine
Triprolidine
Generic name: Triprolidine
Vanaclear PD
Generic name: Triprolidine
Vanahist PD
Generic name: Triprolidine
Wal-finate
Generic name: Chlorpheniramine
Wal-itin
Generic name: Loratadine
Xyzal
Generic name: Levocetirizine
Zyrtec
Generic name: Cetirizine

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