How Do Antihistamines Work To Treat Allergy Symptoms?

Itch, scratch. Itch, scratch. Itch, scratch. Repeat. That's what allergies feel like, except that scratching makes no difference — and some itches are in places you can't scratch, like your eyeballs or inside your nose.

Using antihistamines for allergies is a convenient over-the-counter (OTC) solution to bring some much needed relief. But what are antihistamines (and histamines, for that matter)? How do antihistamines work? And how do you decide which one to take?

How Do Histamines Cause Allergic Reactions?

Your immune system aims to protect you from harmful invaders like disease-causing germs. However, it sometimes mistakes harmless substances such as foods, medications, dust, pollen and pet dander for threats, causing uncomfortable (and sometimes dangerous) allergic reactions.

When the immune system identifies a threat — be it bacteria, a virus or environmental pollutant — it develops antibodies to that substance. Antibodies are proteins that bind to unwanted substances to remove them from the body. This process helps the immune system remember the threat so it can recognize it the next time it enters the body. When the body is again exposed to that threat, the immune system signals mast cells to release stored histamines.

Histamines cause common allergy symptoms such as itchy, watery eyes, sneezing, and a stuffy or runny nose. These secretions are meant to trap and rid the body of your allergen. Histamines can also cause hives, or itchy raised bumps across the body. In severe cases, histamines can lead to anaphylaxis, a life-threatening allergic reaction that can cause facial swelling, airway obstruction and, if not treated right away, unconsciousness or death.

How Do Antihistamines Work?

Antihistamines block histamine receptor sites in the body, preventing histamine from binding to receptors and triggering allergic symptoms.

First-Generation Antihistamines

First-generation antihistamines offer quick, effective relief of allergy symptoms, but they cross the blood-brain barrier and, therefore, can have a side effect of sleepiness. They are not recommended for daytime use or regular treatment of allergy symptoms.

• Benadryl (diphenhydramine) is a first-generation antihistamine used to treat allergies. It is one of the oldest and most widely used allergy medications.

• Other OTC first-generation antihistamines include brompheniramine (found in Dimetapp Children's Cold & Allergy) and doxylamine (an active ingredient in Vicks NyQuil Cold & Flu Relief).

Second-Generation Antihistamines

Newer second-generation antihistamines do not cross the blood-brain barrier as easily as first-generation antihistamines, making them less likely to cause drowsiness and more suitable for daily use.

• Allegra (fexofenadine) is rapid-acting and lasts 12-24 hours with minimal side effects. Allegra can reduce symptoms associated with hay fever and allergies like runny nose, sneezing, itchy and watery eyes, itchy nose or throat, as well as hives. Do not drink fruit juice with Allegra, as it can interfere with absorption; it's best to take this product with water.

• Zyrtec (cetirizine) provides 24-hour relief from seasonal allergies and other upper respiratory allergy symptoms, including itchy eyes, sneezing, runny nose, itchy nose or throat. Zyrtec has been found to be slightly longer-lasting than Allegra; however, it is more likely to cause drowsiness than other second-generation antihistamines, especially in those 60 and older.

• Claritin (loratadine) does not work as quickly as Allegra or Zyrtec but is safe for long-term use and effective against a wide range of allergens.

Side Effects and Other Things You Should Know

Side effects of antihistamines are more prevalent in first-generation than in second-generation versions but may include the following:

• Drowsiness

• Dry mouth

• Dry eyes and blurred vision

• Headache and dizziness

• Low blood pressure

• Rapid heart rate

• Decreased urine output

• Constipation

If you have glaucoma or an enlarged prostate, consult a healthcare provider before taking antihistamines, as these medications may worsen those conditions. Individuals with severe allergic reactions, including anaphylaxis, should consult an allergy specialist to discuss their condition.

Antihistamines can interact with other medications, so make sure your healthcare providers are aware of all the medications you take. Rite Aid pharmacists are available to answer any questions about antihistamines and check for interactions with any medications or supplements you routinely take.

Finding Allergy Relief

Antihistamines are a safe and effective way to manage mild to moderate allergy symptoms. You can confidently choose the best remedy for your needs by understanding your options. Whether you choose a first-generation or second-generation antihistamine is up to you, but rest assured that you can find relief from the agony of the itch.

Clinically reviewed on March 11, 2024