How Fast Does Allegra (Fexofenadine) Work For Hives?

In our latest question and answer, the pharmacist discusses the use of Allegra (fexofenadine) for hives.

Question

I have been described Fexofenadine 180g once a day for hives, how long does it take for the medication to start working and stop the hives coming intermittentley?

Asked by Ozzy On Jul 16, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jul 17, 2018

After taking fexofenadine (Allegra), the onset of the antihistamine effect takes about 1 hour. The maximum concentration is reached after about 2 to 3 hours. Typically, the antihistamine effect will last up to 12-24 hours. The half-life is approximately 14 hours, meaning the time it takes for the drug to reach 50% of its original concentration. Read more about fexofenadine and other antihistamines below.

 

Antihistamine Effect

Antihistamines bind to histamine receptors that are found in the nose, lungs, eyes, and skin without activating the histamine response. By binding and changing the shape of the histamine receptor, antihistamines inhibit histamine from binding and creating its response. Histamine creates an immune response to essentially attempt to remove the offensive allergen.


Histamine can be triggered by a variety of things including pollen, dust, and pet dander. Some people are more sensitive to this response and may not even know what triggers their allergies. Antihistamines are useful in preventing the immune cascade so common in people suffering from allergies.


First generation antihistamines include diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), clemastine (Tavist), promethazine (Phenergan), and others. These medications cross the blood-brain barrier and have CNS effects like drowsiness and dizziness. First generation antihistamines can also cause urinary retention, GI upset, and blurry vision. The first generation antihistamines are also listed as medications to avoid in elderly adults by the Beers Criteria due to the increased risk of confusion, dry mouth, and drowsiness.


Second generation antihistamines include fexofenadine (Allegra), loratadine (Claritin), and cetirizine (Zyrtec). These are generally non-sedating, however, both loratadine and cetirizine have been shown to cause at least some drowsiness. Fexofenadine can cause headache, vomiting, upset stomach, and dizziness.


When taking fexofenadine (Allegra) it is important to avoid grapefruit, orange, and apple juice as these are known to decrease intestinal absorption. Avoid these juices at least 4 hours before and 2 hours after taking Allegra. 

About the Pharmacist

Dr. Kevin Davis Pharm.D

Dr. Kevin Davis is a licensed pharmacist with experience in retail and hospital pharmacy. He graduated from the University of Florida with a Doctor of Pharmacy degree in 2015 and a Master of Healthcare Administration degree from Adventist University in 2017. He is also a Board Certified Pharmacotherapy Specialist since 2016.

About Allegra (Fexofenadine)

Allegra (fexofenadine) is a "second-generation" non-sedating antihistamine (H1-receptor antagonist). Second-generation antihistamines like Allegra are noted for their long duration of action, up to 24 hours, and lack of sedative effects, especially when compared to "first-generation" antihistamines like Benadryl (diphenhydramine). Allegra was first FDA approved in July 1996 as a prescription only product for the treatment of seasonal allergic rhinitis (i.e. seasonal allergies). In January 2011 however, the FDA approved the over-the-counter sale of Allegra in the same formulations, strengths, and indications as available by prescription. The onset of action of Allegra is about one hour, with effects lasting around 24 hours for the 180mg dose. It may be taken with or without food but it is important to avoid grapefruit, orange, and apple juice before or after taking Allegra to avoid a potential reduction in drug absorption.

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