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Antihistamines

Medically reviewed by C. Fookes, BPharm. Last updated on Feb 5, 2019.

What are Antihistamines?

Antihistamines are a class of agents that block histamine release from histamine-1 receptors and are mostly used to treat allergies or cold and flu symptoms, although some first-generation antihistamines may also be used for other conditions.

Histamine-1 receptors are located in the airways, blood vessels and gastrointestinal tract (stomach and esophagus). Stimulation of these receptors can lead to conditions such as a skin rash or inflammation, a narrowing of the airways (bronchoconstriction), hay fever, or motion sickness. Histamine-1 receptors are also found in the brain and spinal cord, and stimulation of these receptors makes you more awake and alert. Sedating antihistamines oppose the effects of histamine on H1 receptors in your brain, which is why they cause sedation and drowsiness.

What are antihistamines used for?

Antihistamines are very good at relieving symptoms of an allergic reaction, such as:

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

This makes antihistamines very effective in the treatment of:

First generation 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
  • in people with Parkinson’s disease unable to tolerate more potent agents.

What are the differences between antihistamines?

Antihistamines can be classified into two main categories:

  • first-generation antihistamines
  • second-generation antihistamines.

First-generation 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 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 Antihistamines

Generic name Brand name Duration of action
brompheniramine Bromax, Lodrane 12-Hour 12h
carbinoxamine maleate Karbinal ER Oral Suspension 12h
chlorpheniramine Chlor-Trimeton 24h
clemastine Tavist Allergy 12h
diphenhydramine Benadryl 12h
hydroxyzine Atarax, Vistaril 24h
triprolidine Tripohist 12h

Second Generation Antihistamines

What are the side effects of antihistamines?

Side effects of first-generation antihistamines may include:

  • abdominal pain
  • 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
  • urinary problems

Side effects of second-generation antihistamines may include:

List of Antihistamines:

View by  Brand | Generic
Drug Name Avg. Rating Reviews
Xyzal (Pro)
Generic name: levocetirizine
5.7
277 reviews
Vistaril (Pro)
Generic name: hydroxyzine
5.6
220 reviews
Zyrtec (Pro)
Generic name: cetirizine
6.1
186 reviews
Atarax (Pro)
Generic name: hydroxyzine
6.3
141 reviews
Phenergan (Pro)
Generic name: promethazine
7.5
135 reviews
Nytol
Generic name: diphenhydramine
4.2
116 reviews
Allegra (Pro)
Generic name: fexofenadine
6.8
90 reviews
Benadryl (Pro)
Generic name: diphenhydramine
7.3
67 reviews
Sominex
Generic name: diphenhydramine
6.2
60 reviews
Claritin (Pro)
Generic name: loratadine
4.5
41 reviews
Benadryl Allergy (Pro)
Generic name: diphenhydramine
7.2
30 reviews
Unisom SleepGels
Generic name: diphenhydramine
5.5
20 reviews
Simply Sleep
Generic name: diphenhydramine
5.1
19 reviews
Chlor-Trimeton (Pro)
Generic name: chlorpheniramine
9.1
15 reviews
Allegra Allergy
Generic name: fexofenadine
2.7
8 reviews
Clarinex (Pro)
Generic name: desloratadine
7.5
5 reviews
Seldane
Generic name: terfenadine
10
4 reviews
Polaramine
Generic name: dexchlorpheniramine
7.2
4 reviews
Hyzine
Generic name: hydroxyzine
6.2
3 reviews
Children's Claritin Allergy
Generic name: loratadine
8.3
3 reviews
Alavert (Pro)
Generic name: loratadine
6.8
3 reviews
Wal-itin
Generic name: loratadine
6.0
2 reviews
Palgic (Pro)
Generic name: carbinoxamine
9.0
2 reviews
Compoz Nighttime Sleep Aid
Generic name: diphenhydramine
9.0
2 reviews
Wal-finate
Generic name: chlorpheniramine
9.0
1 review
Unisom SleepMelts
Generic name: diphenhydramine
9.0
1 review
Promethegan (Pro)
Generic name: promethazine
8.4
1 review
Diphen
Generic name: diphenhydramine
10
1 review
Benadryl Children's Allergy
Generic name: diphenhydramine
9.0
1 review
Aller-Tec
Generic name: cetirizine
10
1 review
Aller-Ease
Generic name: fexofenadine
1.0
1 review
Allegra ODT
Generic name: fexofenadine
10
1 review
All Day Allergy
Generic name: cetirizine
9.0
1 review
Vanahist PD
Generic name: triprolidine
No reviews
Vanaclear PD
Generic name: triprolidine
No reviews
Valu-Dryl
Generic name: diphenhydramine
No reviews
Twilite
Generic name: diphenhydramine
No reviews
Tripohist
Generic name: triprolidine
No reviews
Topco Allergy
Generic name: chlorpheniramine
No reviews
Tavist ND
Generic name: loratadine
No reviews
Tavist Allergy
Generic name: clemastine
No reviews
Sleepinal
Generic name: diphenhydramine
5.0
No reviews
Siladryl Allergy
Generic name: diphenhydramine
No reviews
Scot-Tussin Allergy
Generic name: diphenhydramine
No reviews
Ryvent (Pro)
Generic name: carbinoxamine
No reviews
Ryclora (Pro)
Generic name: dexchlorpheniramine
No reviews
Quzyttir (Pro)
Generic name: cetirizine
No reviews
Polmon
Generic name: dexchlorpheniramine
No reviews
Phenadoz (Pro)
Generic name: promethazine
6.5
No reviews
Pediox
Generic name: carbinoxamine
No reviews
Pediatex
Generic name: carbinoxamine
No reviews
PediaTan
Generic name: chlorpheniramine
No reviews
ohm Allergy Relief
Generic name: loratadine
No reviews
M-Hist PD
Generic name: triprolidine
No reviews
Leader Allerhist
Generic name: clemastine
No reviews
Karbinal ER (Pro)
Generic name: carbinoxamine
No reviews
Histex PD
Generic name: carbinoxamine
No reviews
Histex IE
Generic name: carbinoxamine
No reviews
Histex CT
Generic name: carbinoxamine
No reviews
Histex
Generic name: triprolidine
No reviews
Ed Chlor Ped Jr.
Generic name: chlorpheniramine
No reviews
Ed ChlorPed
Generic name: chlorpheniramine
No reviews
Ed-Chlortan
Generic name: chlorpheniramine
No reviews
Dytuss
Generic name: diphenhydramine
No reviews
Diphedryl
Generic name: diphenhydramine
No reviews
Dimetapp Children's ND Non-Drowsy Allergy
Generic name: loratadine
No reviews
Dicopanol
Generic name: diphenhydramine
No reviews
Dayhist Allergy
Generic name: clemastine
No reviews
Complete Allergy Relief
Generic name: diphenhydramine
No reviews
Claritin Reditabs
Generic name: loratadine
No reviews
Clarinex Reditabs
Generic name: desloratadine
No reviews
ChlorTan
Generic name: chlorpheniramine
No reviews
Chlor-Mal
Generic name: chlorpheniramine
No reviews
Chlo-Amine
Generic name: chlorpheniramine
No reviews
Children's Allergy Relief 24 Hour
Generic name: loratadine
No reviews
Children's Allergy Relief
Generic name: cetirizine
No reviews
Carboxine
Generic name: carbinoxamine
No reviews
C.P.M.
Generic name: chlorpheniramine
No reviews
Bromax
Generic name: brompheniramine
No reviews
Banophen
Generic name: diphenhydramine
4.3
No reviews
Bactimicina Allergy
Generic name: loratadine
No reviews
Arbinoxa (Pro)
Generic name: carbinoxamine
No reviews
Antinaus 50
Generic name: promethazine
7.7
No reviews
Allermax
Generic name: diphenhydramine
No reviews
Allergy Relief Tablets
Generic name: loratadine
No reviews
Allergy Relief 24 Hour
Generic name: loratadine
No reviews
Allergy Relief
Generic name: chlorpheniramine
No reviews
Aller-Tec Children's
Generic name: cetirizine
No reviews
Aller-Chlor
Generic name: chlorpheniramine
No reviews
All Day Allergy Children's
Generic name: cetirizine
No reviews
Ala-Hist IR
Generic name: dexbrompheniramine
No reviews
12 Hour Allergy Relief
Generic name: clemastine
No reviews
For ratings, users were asked how effective they found the medicine while considering positive/adverse effects and ease of use (1 = not effective, 10 = most effective).

Further information

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