What is acrivastine and pseudoephedrine? Acrivastine is an antihistamine. It works against the naturally occurring chemical histamine in your body. Acrivastine prevents sneezing; itchy, watery eyes and nose; and other symptoms of allergies and hay fever. Pseudoephedrine is a decongestant. It constricts (shrinks) blood vessels (veins and arteries). This reduces the blood flow to certain areas, which decreases swelling and allows nasal and respiratory (breathing) passages to open up. ...