If you experience any of the following serious side effects, stop taking trihexyphenidyl and seek emergency medical attention or contact your doctor immediately:
an allergic reaction (difficulty breathing; closing of the throat; swelling of the lips, tongue, or face; or hives);
fever;
fast or irregular heartbeats;
anxiety, hallucinations, confusion, agitation, hyperactivity, or loss of consciousness;
seizures;
eye pain; or
a rash.
Other, less serious side effects may be more likely to occur. Continue to take trihexyphenidyl and talk to your doctor if you experience
dryness of the mouth;
large pupils or blurred vision;
drowsiness or dizziness;
difficulty urinating or constipation;
nervousness or anxiety;
upset stomach; or
decreased sweating.