Sapropterin is used along with a restricted diet to control blood phenylalanine levels in people who have phenylketonuria (pku- an inborn condition in which phenylalanine may build up in the blood and causes decreased intelligence and a decreased ability to focus, remember, and organize information). sapropterin will only work for some people who have pku, and the only way to tell if sapropterin will help a particular patient is to give the medication for a period of time and see whether his or her phenylalanine level decreases. sapropterin is in a class of medications called cofactors. it works by helping the body to break down phenylalanine so it will not build up in the blood.