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.