fexofenadine
Meaning of fexofenadine: Built to stay out of the brain, it blunts allergy symptoms without the sedation that older antihistamines are known for.
Definition of fexofenadine
Fexofenadine is a second generation antihistamine taken by mouth for hay fever and long lasting hives. It blocks the H1 receptor, the docking point through which histamine produces sneezing, itching, a streaming nose and watering eyes during an allergic reaction. Because it barely enters the brain, it relieves those symptoms with little of the drowsiness older antihistamines cause.
Why does it cause so little drowsiness?
Histamine is not only an allergy signal. In the brain it is one of the chemicals that maintains wakefulness, so a drug blocking it there brings on sleepiness. Older antihistamines cross into the brain readily. Fexofenadine barely does, partly through its chemistry and partly because a transporter in the vessel walls pumps back out what does get in. It is also selective, leaving alone the other receptors whose blockade gives the older drugs a dry mouth, blurred vision and difficulty passing urine. Sedation is not abolished in every person, but it is uncommon at usual doses.
How does it compare with an older antihistamine?
| Feature | Fexofenadine | Diphenhydramine |
|---|---|---|
| Entry into the brain | Minimal | Ready |
| Drowsiness | Uncommon | Common and often marked |
| Other receptors blocked | Few | Several, causing dry mouth and blurred vision |
| Dosing | Once or twice daily | Every few hours |
Both control the itching, sneezing and streaming of an allergic response about equally well. The difference lies in everything else they do, which is why the older drugs now survive mainly for other purposes, such as motion sickness and short term sedation, rather than for allergy.
What is it used for, and what interferes with it?
Seasonal hay fever, year round allergic rhinitis and long lasting hives are the usual targets, and it is sold without prescription in many places for exactly that reason. Unlike most drugs of its class it is hardly broken down by the liver and leaves the body largely unchanged, which keeps its interactions few. Two exceptions are well documented: fruit juices cut how much is absorbed, by blocking the transporter that carries the drug from the gut into the body, and antacids containing aluminum and magnesium do much the same. The drug exists because of a safety problem in its ancestor. Fexofenadine is the active breakdown product of terfenadine, an early nonsedating antihistamine withdrawn after it was found to disturb heart rhythm when its own breakdown was blocked, an effect fexofenadine does not carry.
Also known as
Used in a sentence
Fexofenadine was recommended for the patient's seasonal allergy symptoms.
Good to know
Fexofenadine belongs to the second-generation class of antihistamines, which cross into the brain far less than older first-generation antihistamines and therefore cause comparatively little drowsiness.