What is acetaminophen and pseudoephedrine? Acetaminophen is a pain reliever and a fever reducer. It is used to treat many conditions, such as headache, muscle aches, arthritis, backache, toothaches, colds, and fevers. 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. ...