Trihexyphenidyl hydrochloride is used as a synthetic antispasmodic drug that is prescribed as an adjunct in the treatment of all forms of parkinsonism. The drug allows greater muscle control and reduces stiffness, allowing more normal movements for afflicted patients. In some instances, this drug is also used to control severe reactions to other drugs such as reserpine or phenothiazines. Side effects include headache, loss of coordination, slurred speech, and vision changes.