Understanding How Allegra Works: What Does Fexofenadine Do?

October 11, 2025 •

3 min read

Fexofenadine, commonly known by the brand name Allegra, was first FDA-approved in 1996. So, what does fexofenadine do? It provides fast, non-drowsy relief from symptoms of seasonal allergies and hives by blocking the body's natural histamine response.

Quick Summary

Fexofenadine is a second-generation antihistamine that selectively blocks H1 receptors to alleviate symptoms of seasonal allergies and chronic hives, including sneezing, itching, and watery eyes. It is known for its non-drowsy properties.

Key Points

Mechanism: Fexofenadine works by blocking H1-receptors to prevent the action of histamine, the chemical responsible for allergy symptoms.
Non-Drowsy: As a second-generation antihistamine, fexofenadine minimally crosses the blood-brain barrier, resulting in little to no drowsiness.
Fast Relief: It provides rapid symptom relief for allergies and hives, with effects often felt within one hour.
Common Uses: Fexofenadine is used to treat both seasonal allergic rhinitis (hay fever) and chronic idiopathic urticaria (hives).
Food/Drink Interactions: It should not be taken with fruit juices or certain antacids, as these can reduce its effectiveness.
Adults and Children: Appropriate formulations and dosages are available for both adults and children, but always follow specific age guidelines.
Extended Duration: One daily dose can provide 24-hour relief, depending on the product formulation.

The Science Behind Fexofenadine: Blocking Histamine

When your body encounters an allergen—such as pollen, pet dander, or dust mites—it releases a chemical called histamine. Histamine is a key player in the body's inflammatory response, triggering the common allergy symptoms that can make life miserable. Histamine binds to specific receptors throughout the body to initiate this reaction.

Fexofenadine is a second-generation antihistamine that belongs to a class of drugs known as selective peripheral H1-receptor antagonists. Its mechanism of action involves selectively blocking the effects of histamine at the peripheral H1 receptors, which are located outside the central nervous system. By blocking these receptors, fexofenadine prevents histamine from causing the inflammation and irritation that lead to symptoms. Since fexofenadine does not readily cross the blood-brain barrier, it avoids interacting with H1 receptors in the brain, which is the main reason it is far less likely to cause drowsiness than older, first-generation antihistamines.

Medical Uses for Fexofenadine

Fexofenadine is prescribed or available over-the-counter for the treatment of various allergic conditions in adults and children. The primary uses include:

Seasonal Allergic Rhinitis (Hay Fever): Fexofenadine effectively relieves symptoms caused by allergens found indoors and outdoors, including pollen and dust mites.
Chronic Idiopathic Urticaria (Hives): It is used to relieve the itching and reduce the number of wheals associated with chronic hives in adults and children over 6.

Common Allergy Symptoms Relieved by Fexofenadine

Sneezing
Runny nose
Itchy, watery eyes
Itching of the nose or throat
Skin itching and hives

Fexofenadine vs. Other Common Antihistamines

When choosing an antihistamine, it's important to understand the differences in their effects, especially regarding drowsiness. Fexofenadine is a second-generation antihistamine, offering different properties compared to older, first-generation drugs like diphenhydramine (Benadryl®). It also differs slightly from other second-generation options like loratadine (Claritin®) and cetirizine (Zyrtec®).


Feature	Fexofenadine (Allegra)	Diphenhydramine (Benadryl)	Cetirizine (Zyrtec)	Loratadine (Claritin)
Generation	Second-generation	First-generation	Second-generation	Second-generation
Drowsiness	Non-drowsy	Significantly drowsy	May cause some drowsiness	Generally non-drowsy
Speed of Action	Starts working within 1 hour	Rapid onset	Starts within 1 hour	Delayed onset (1-3 hours)
Duration	12 or 24 hours	4-6 hours	24 hours	24 hours
Side Effects	Headache, nausea, dizziness	Drowsiness, dry mouth, dizziness	Drowsiness, headache, dry mouth	Headache, dry mouth, drowsiness
Interactions	Avoid fruit juices and antacids	Interacts with many medications	Less interaction risk	Less interaction risk

Important Considerations and Safety

While fexofenadine is considered a safe and effective medication for many people, there are important precautions to observe.

Kidney Disease: Patients with kidney disease should consult a doctor, as dosage adjustments may be necessary.
Fruit Juice Interaction: Do not take fexofenadine with fruit juices such as orange, grapefruit, or apple, as these can significantly reduce its effectiveness. Take it with water instead.
Antacid Interaction: Antacids containing aluminum or magnesium should be taken at least two hours before or after fexofenadine to avoid interfering with absorption.
Pregnancy and Breastfeeding: Always consult a healthcare provider before using fexofenadine if you are pregnant or breastfeeding.

Potential Side Effects

Though generally well-tolerated, some individuals may experience side effects, including:

Headache
Nausea or stomach discomfort
Drowsiness or tiredness (though much less common than with first-generation antihistamines)
Dizziness
Indigestion

How to Take Fexofenadine Correctly

To get the most out of your fexofenadine medication, it's crucial to follow the directions carefully.

Dosage: Fexofenadine is typically taken once or twice daily, depending on the dosage and product. Always follow the instructions on the package or your doctor's prescription.
Administration: Tablets and gelcaps should be taken with water, not fruit juice. Orally disintegrating tablets should be placed on the tongue to dissolve and then swallowed with saliva.
Storage: Store the medication at room temperature away from excess heat and moisture, and keep it out of reach of children.

Conclusion: A Modern Approach to Allergy Relief

In summary, fexofenadine is a safe and effective second-generation antihistamine that works by blocking histamine at peripheral H1 receptors. This mechanism provides relief from the sneezing, runny nose, and itching associated with seasonal allergies and chronic hives without causing the significant drowsiness common to older antihistamines. By understanding what fexofenadine does and following proper usage instructions, individuals can achieve effective and consistent symptom control, allowing for a more comfortable daily life. As with any medication, consulting a healthcare provider or pharmacist is the best way to ensure it is the right option for your specific needs.

For more detailed professional information, refer to the Fexofenadine monograph on Drugs.com.

Frequently Asked Questions

Fexofenadine starts working relatively quickly, with many people experiencing relief from allergy symptoms within one hour of taking a dose.

Yes, Allegra is a common brand name for the generic drug fexofenadine. Many generic versions are also available.

Yes, fexofenadine can be taken daily to manage symptoms of chronic allergies like hay fever, as long as you follow the dosage directions on the package or from your doctor.

Taking fexofenadine with fruit juices, including orange, grapefruit, or apple, can significantly decrease the amount of the medication your body absorbs, making it less effective.

Yes, fexofenadine is available in specific formulations and dosages for children. Always use pediatric products designed for the correct age group and follow instructions carefully.

Common side effects may include headache, nausea, and dizziness. Serious side effects are rare, but contact a doctor if you experience hives, swelling, or breathing issues.

Depending on the formulation, fexofenadine can provide relief for 12 or 24 hours.

Fexofenadine is an antihistamine for allergies and will not treat a viral infection like the common cold, though it may alleviate some associated symptoms like a runny nose.