Levocetirizine
H1 antihistamine · also known as Xyzal, levocetirizine dihydrochloride
Second-generation H1 antihistamine; the active enantiomer of cetirizine, sold without a prescription.
- 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
- Xyzal is widely believed to be the less-drowsy version of cetirizine. Its label does not say that. It carries the same plain warning that cetirizine's does — drowsiness may occur — in the same words. 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
Mechanism of action
Levocetirizine is the active enantiomer of cetirizine — the half of the racemic mixture that binds the H1 receptor. Cetirizine sold as a racemate contains it already. Like the rest of the class it blocks the H1 receptor, acting after mast-cell mediators have been released rather than preventing their release.
Its separate existence is a regulatory and commercial fact more than a mechanistic one. It is listed here because a reader looking for Xyzal should find something, and because its own label — not cetirizine’s — is what governs how it is sold.
Sources
- Levocetirizine dihydrochloride 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