Trihexyphenidyl is used along with other medications to treat the symptoms of Parkinson's disease (PD; a disorder of the nervous system that causes difficulties with movement, muscle control, and balance) and to control extrapyramidal symptoms (tremor, slurred speech) caused by certain medications. Trihexyphenidyl is in a class of medications called antimuscarinics. It works by relaxing muscles and nerve impulses that control the function of muscles.