Trihexyphenidyl is used to treat the stiffness, tremors, spasms, and poor muscle control of Parkinson's disease. It is also used to treat and prevent the same muscular conditions when they are caused by drugs such as chlorpromazine (Thorazine), fluphenazine (Prolixin), perphenazine (Trilafon), haloperidol (Haldol), thiothixene (Navane), and others.