Fexofenadine

H1 antihistamine · also known as Allegra, fexofenadine hydrochloride

Second-generation H1 antihistamine sold without a prescription; its label warns that fruit juice and antacids reduce absorption.

How it relates to mast cells: Acts after mast-cell mediators are released
Types of studies cited: Randomised controlled trial
Approved in the US for: hay fever and upper respiratory allergy symptoms
Known for: Sold as non-drowsy, with no general drowsiness warning on its label — unlike cetirizine and levocetirizine, which both warn that drowsiness may occur. Source
Availability: sold over the counter in the US, without a prescription

What this relationship means: Acts after mast-cell mediators are released rather than stabilising the mast cell itself. This includes treatments that block mediator receptors, degrade a mediator, or oppose the physiology of an acute reaction. It describes where the intervention acts, not its clinical importance.

Mechanism of action

Fexofenadine is a second-generation H1 receptor antagonist and the active metabolite of terfenadine, which was withdrawn over cardiac effects that fexofenadine does not share. It blocks the H1 receptor, acting after mast-cell mediators reach it.

It crosses into the central nervous system less readily than the first generation, which is the basis of the class’s lower-sedation profile. Its consumer label differs from the rest of the class in one practical respect: it directs that the product not be taken with fruit juice, and that antacids containing aluminium or magnesium be separated from it, because both reduce how much drug is absorbed.

Sources

Fexofenadine hydrochloride US over-the-counter Drug Facts label Drug label · link checked 25 Aug 2026
Cetirizine versus hydroxyzine and placebo in chronic idiopathic urticaria Peer-reviewed study 1996 PMID 8893110 · link checked 21 Aug 2026
Mast Cell Activation Syndrome and Mastocytosis: Initial Treatment Options and Long-Term Management Review article 2019 PMID 30961835 · link checked 20 Aug 2026