Fexofenadine

The pharmacologically active metabolite of terfenadine; it selectively blocks peripheral H1 receptors and, even at high concentrations, does not block the heart's delayed-rectifier potassium channel or prolong QTc. It is barely metabolized and leaves mostly unchanged in bile. As a P-glycoprotein and OATP substrate, its absorption shifts with some drugs and foods.

Like other second-generation antihistamines: Second-generation antihistamines selectively block peripheral H1 receptors, so histamine can no longer make small vessels leak and swell or trigger itching, sneezing and a runny nose. They pass poorly into the brain and have little affinity for other receptors, so at usual doses they cause little drowsiness and almost no anticholinergic effect, and one dose lasts about a day.

• Fruit juice reduces its absorption, so it is taken with water.
• Aluminum and magnesium antacids bind it in the gut; doses are kept about 2 hours apart.
• P-glycoprotein inhibitors such as erythromycin and ketoconazole raise its levels two to threefold, without QT changes.
• Data are limited in older people and in kidney or liver impairment.

Good to know: Because it barely enters the brain and is hardly metabolized, it does not sedate at usual doses and has no liver-enzyme interactions.

Easily mixed up with first-generation antihistamines, H2 receptor antagonists and leukotriene receptor antagonists.