Acetaminophen, found in over-the-counter products such as Excedrin and Tylenol, provides many people with relief from headaches and sore muscles. When used appropriately, it is considered mostly harmless. Over recent decades, the drug, which has been marketed since the 1950s, has become the medication most commonly used by pregnant women for fevers and pain. Now, a long-term study by UCLA, in collaboration with the University of Aarhus in Denmark, has raised concerns about the use of acetaminophen during pregnancy.