Antihistamines and How They're Used

Antihistamines are a class of drugs used to relieve allergies. They do so by blocking a chemical known as histamine that is released by the immune system whenever the body is confronted by an allergy-causing substance (allergen).

By preventing histamine from attaching to cellular receptors in the skin and mucus membranes of the eyes, nose, and respiratory tract, symptoms of allergy—such as sneezing, itchy eyes, postnasal drip, and, hives—can be temporarily relieved.

This article explains the different uses of antihistamine drugs and lists some of the more common over-the-counter (OTC) and prescription formulations.

Common Uses of Antihistamines

Antihistamines are used to treat various allergic conditions and are available in oral, nasal, eyedrop, and injected formulations. Among the conditions they most commonly treat are allergic rhinitis, allergic conjunctivitis, and hives

Allergic Rhinitis (Hay Fever)

Allergic rhinitis, also known as hay fever, is an allergic response that causes sneezing, runny nose, and itchy, watery eyes. It can occur seasonally or year-round, due primarily to tree or grass pollens. Oral antihistamines can generally improve hay fever symptoms within an hour or two.

Newer oral antihistamines, such as Xyzal (levocetirizine), Zyrtec (cetirizine), Allegra (fexofenadine), and Clarinex (desloratadine), are very effective for the treatment of hay fever.

Older antihistamines, like Benadryl (diphenhydramine), Chlor-Trimeton (chlorpheniramine), and Vistaril (hydroxyzine) work well but can be sedating and cause other side effects.

There are also nasal sprays like Astelin (azelastine) and Patanase (olopatadine) that are used to treat allergic rhinitis (nasal allergy).

Allergic Conjunctivitis

Many of the same allergens that cause hay fever can also cause allergic conjunctivitis. an allergic reaction involving the lining of the eye (known as the conjunctiva). These include airborne allergens (like pollen, dust, perfume, and pet dander) but also substances placed in or around the eye (like makeup and contact lens solution).

Oral antihistamines can help treat symptoms of eye allergies. There are also antihistamine eye drops such as Optivar (azelastine), Emadine (emedastine), Visine-A (pheniramine), and Alaway (ketotifen).

Urticaria (Hives) 

Urticaria, known more as hives, is a raised, itchy rash triggered by a reaction to food, medicine, or other irritants. In some cases, the cause of an outbreak will be unknown.

Oral antihistamines are the main treatment for hives. Newer "non-drowsy" antihistamines, such as Zyrtec or Claritin, are preferred over the older antihistamines like Benadryl or Vistaril. With that said, if the itchiness of hives is keeping you up at night, older-generation antihistamines may help you sleep soundly.

Topical antihistamines, present in many over-the-counter anti-itch creams, should not be used with oral antihistamines. Certain ones, such as Benadryl cream, have been known to trigger contact dermatitis when taken with its oral version.

Other Uses

Antihistamines are used for the treatment of other allergic conditions. Some of the approved uses of antihistamines include:

• Allergic skin reactions
• Sinusitis (sinus inflammation)
• Angioedema (the generalized swelling of deeper tissues)
• Eczema (atopic dermatitis)
• Allergic drug reactions
• Bronchitis (inflammation of the bronchial tubes of the lungs)
• Motion sickness
• Nausea and vomiting
  

OTC and Prescription Antihistamines

There are both OTC and prescription antihistamines used for the treatment of allergies. The choice varies by the type and severity of allergy you have, as well as whether the condition is acute (rapidly developing) or chronic (persistent or recurrent).

Not all antihistamines work equally well for all allergic conditions.

If buying an OTC antihistamine, speak with your healthcare provider or pharmacist to determine whether it is the best option for you as well as the potential risks.

Common Side Effects

Second-generation antihistamines (like Allegra, Claritin, Tavist, Xyzal, and Zyrtec) tend to have far fewer side effects than older-generation versions. Common side effects include:

• Headache
• Stomach pain
• Fatigue
• Nervousness
• Diarrhea

First-generation antihistamines (such as Benadryl, Chlor-Trimeton, and Vistaril) also have these side effects but can additional ones as well, including:

• Drowsiness
• Constipation
• Difficulty urinating
• Dry mouth
  

Because first-generation drugs are more sedating, they are generally avoided for daytime use. They can also diminish your ability to operate motor vehicles or heavy machinery.

Certain antihistamines may also cause weight gain. According to the American Academy of Asthma, Allergy & Immunology, both older drugs like Periactan and newer ones like Allegra and Zyrtec are linked to an increased risk of obesity.

It is thought that the suppression of histamine, which is known to reduce appetite, can increase appetite in people on chronic antihistamine treatment.

Summary

Antihistamines are used to treat allergies. They work by suppressing a chemical called histamine that triggers allergy symptoms like rash, runny nose, watery eyes, and sneezing. They may also be useful in treating motion sickness, nausea, sinusitis, and eczema.

Antihistamines are available over the counter and by prescription in oral, topical, nasal, eyedrop, and injected formulations. Newer-generation antihistamines like Allegra (fexofenadine) and Claritin (loratadine) tend to cause less drowsiness than older-generation ones like Benadryl (diphenhydramine).

A Word From Verywell

As useful antihistamines are in treating allergy symptoms, they can cause problems if overused. This includes the risk of dependency if antihistamine eyedrops are overused.

If you suffer from chronic allergies and find that you need to take antihistamines every day, it may be time to ask your healthcare provider for a referral to a specialist known as an allergist.

An allergist can perform tests to see which allergens you are reacting to. They may then be able to reduce your sensitivity to these allergens with allergy shots and other forms of allergy immunotherapy.