Trihexyphenidyl alters unusual nerve impulses and relaxes stiff muscles.
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.
Trihexyphenidyl may also be used for purposes other than those listed in this medication guide.
 

Use caution when driving, operating machinery, or performing other hazardous activities. Trihexyphenidyl may cause dizziness or blurred vision. If you experience dizziness or blurred vision, avoid these activities. Use alcohol cautiously. Alcohol may increase drowsiness and dizziness while taking trihexyphenidyl.
Avoid becoming overheated. Trihexyphenidyl may cause decreased sweating. This could lead to heat stroke in hot weather or with vigorous exercise.
Do not use trihexyphenidyl without first talking to your doctor if you have

• ever had an allergic reaction to it;
  
• narrow-angle glaucoma;
  
• an obstruction in the bowel or a complication of bowel disease known as megacolon; or
  
• myasthenia gravis.
  

Before taking this medication, tell your doctor if you have

• enlargement of the prostate or difficulty urinating;
  
• epilepsy or another seizure disorder;
  
• heart disease or an irregular heartbeat;
  
• depression or any other psychiatric illness;
  
• kidney disease; or
• liver disease.

You may require a dosage adjustment or special monitoring during treatment if you have any of the conditions listed above.