Levocetirizine is an antihistamine. It acts as an inverse agonist that decreases activity at histamine H1 receptors. This in turn prevents the release of other allergy chemicals and increases the blood supply to the area, providing relief from the typical symptoms of hay fever. Levocetirizine, (R)-(-)-cetirizine, is essentially a chiral switch of (±)-cetirizine. This enantiomer, the eutomer, is more selective and the (S)-counterpart, the distomer, is inactive.