Antihistamines for Treating Nasal Allergies

Antihistamines are a class of drugs used to treat allergies, including nasal allergies caused by allergic rhinitis (hay fever). They work by blocking the action of histamine, a chemical released by the immune system in response to allergy-causing substances (allergens) like pollen or pet dander.

By doing so, antihistamines can provide effective, short-term relief of nasal allergy symptoms like sneezing, runny nose, and congestion.

This article takes a looks at how antihistamines work and the different types used to treat nasal allergies. It also offers tips on which antihistamines may work best if faced with sudden or persistent allergy symptoms.

What Causes Nasal Allergies?

An allergy is an inappropriate immune response to otherwise harmless substances or stimuli.

When confronted with an allergen, the body will secrete a type of immune protein, called immunoglobulin E (IgE), that searches out and binds to white blood cells known as basophils and mast cells. The binding causes the cells to degranulate (break open) and release their contents, including histamine, into the bloodstream.

Histamine is a naturally occurring compound that has normal functions in the body, most specifically the triggering of inflammation.

Inflammation is the body's way of dealing with injury, infection, or disease. It causes blood vessels to dilate (widen) so that larger immune cells can access the affected site, neutralize the threat, and start the healing process. The dilation, in turn, causes fluid to leak into adjacent tissues, leading to swelling.

However, when histamine is released inappropriately, it can trigger an inflammatory response that causes tissues to swell, leading to a rash or hives. It also commonly induces pruritus (itchiness).

When histamine affects the membranes of the sinuses, nasal passages, and eyes, it can trigger nasal allergy symptoms such as.

• Sneezing
• Runny nose
• Nasal congestion
• Postnasal drip
  
• Itchy nose, eyes, throat, or ears
• Red, watery eyes
• Cough
  

How Antihistamines Work

Antihistamines relieve allergy symptoms by binding to histamine receptors on cells throughout the body. By binding to these receptors, histamine is unable to "switch on" the inflammatory response that leads to allergies.

Not all antihistamines are used to treat allergies. There are four broad classes⁠—called H1, H2, H3, and H4 antihistamines⁠—classified by the type of histamine receptor they bind to. The type most commonly used for allergies is H1 antihistamines, available both as an oral tablet or nasal spray.

There are also different types of H1 antihistamines used for the treatment of nasal allergies. These are informally referred to as first-generation H1 antihistamines and second-generation antihistamines. Each has its place in the treatment of nasal allergies.

First-Generation H1 Antihistamines

First-generation antihistamines work by not only blocking histamine receptors but also muscarinic receptors that regulate the production of acetylcholine. Acetylcholine is a chemical that helps regulate involuntary muscle movements like heart rate, respiration, and intestinal contractions.

By inhibiting the production of acetylcholine, first-generation H1 antihistamines can cause side effects like constipation, muscle weakness, difficulty urinating, and chest congestion.

Over-the-counter (OTC) first-generation H1 antihistamines used for the treatment of nasal allergies include:

• Benadryl (diphenhydramine)
• Chlor-Trimeton (chlorpheniramine)
• Dimetapp (brompherniramine)
• Unisom (doxylamine)
  

Prescription first-generation H1 antihistamines sometimes used for the treatment of nasal allergies (generally when other allergy symptoms accompany) include:

• Atarax (hydroxyzine)
  
• Dimetane (brompherniramine)
• Palgic (carbinoxamine)
  

Each of these medications has generic versions and may be found under different brand names.

Second-Generation H2 Antihistamines

Second-generation antihistamines offer advantages over earlier generation versions in that they mainly target histamine receptors and have less effect on muscarinic receptors. They are also less likely to cross the blood-brain barrier and cause side effects like drowsiness.

Another advantage is that second-generation antihistamines have a longer drug half-life, meaning that they circulate in the body longer before being excreted. As such, their anti-allergy effects are longer lasting.

OTC second-generation H1 antihistamines used for the treatment of nasal allergies include:

• Allegra (fexofenadine)
• Astepro nasal spray (azelastine)
  
• Claritin (loratadine)
• Zyrtec (cetirizine)
  
• Xyzal (levocetirizine) 

Prescription second-generation H1 antihistamines sometimes used for the treatment of nasal allergies (generally when other allergy symptoms accompany) include:

• Astelin (azelastine)
• Clarinex (desloratadine) 
• Ilaxten (bilastine)
• Patanase nasal spray (olopatadine)
  
• Rupafin (rupatadine)
• Xyzal (levocetirizine) 

Each of these medications has generic versions and may be found under different brand names.

Which Antihistamine Works the Best?

From the broadest perspective, second-generation H1 antihistamines are the preferred option for treating nasal allergies. They last longer, tend cause less drowsiness, and have a number of effective OTC options to choose from.

As to which is "best," most research suggests that they are all more or less equal in their ability to relieve nasal allergy symptoms. Where they can vary is in their likelihood to cause drowsiness.

This doesn't mean that Allegra or Ilaxten won't cause drowsiness or other side effects that the other drugs may not. The choice is ultimately an individual one based on personal choice and experience.

This also doesn't mean that you should avoid first-generation H1 antihistamines. They have their place in the treatment of nasal allergies, especially when sneezing and coughing are keeping you up at night. Taking a dose of Benadryl is often used as a short-term, temporary solution to help you rest comfortably if you are suffering severe hay fever symptoms.

It is also important to note that the only OTC antihistamine nasal spray option, Astepro, is a second-generation H1 antihistamine. (Antihistamine nasal sprays should not be confused with decongestant nasal sprays which have an entirely different mechanism of action).

During the day, all oral first-generation antihistamines should be used with extreme caution, while some like Unisom (used mainly as a sleep aid) should be avoided. Taking these can significantly impair your ability to drive or use heavy machinery.

Summary

H1 antihistamines block the action of histamine, a chemical released by the immune system that causes allergy symptoms (including nasal allergy symptoms).

There are first-generation H1 antihistamines like Benadryl (diphenhydramine) that tend to cause more drowsiness than second-generation antihistamines like Allegre (fexofenadine). Both have their place in the treatment of nasal allergies.

A Word From Verywell

If you are using OTC antihistamines to manage chronic nasal allergy symptoms but the drugs are not working as hoped, speak with your healthcare provider. They can refer you to an allergy specialist known as an allergist who can perform tests to identify which specific allergens you are sensitive to.

By doing so, the specialist may be able to provide you with allergy shots or drops to gradually reduce your sensitivity to those allergens (a treatment referred to as allergy immunotherapy).