Many of these peripheral symptoms, especially considering an acute increase in anxiety with various physical complaints, as well as evidence of orthostatic hypotension and tachycardia are indicative of withdrawal, especially in people with psychiatric conditions 
Eyes: trihexyphenidyl causes mydriasis with or without photophobia. It may precipitate narrow angle glaucoma or cause blurred vision. Tolerance may develop during therapy which requires dose adjustments. Striated musculature and weight gain.Trihexyphenidyl is a pregnancy category C drug. It is advised to only use with caution if benefits outweigh risks. Overdose
Trihexyphenidyl (THP) and other antiparkinsonian drugs are known to be substances of abuse. This is true both in abusers of other substances and in chronic schizophrenics, the latter being infrequent abusers of other drugs. Trihexyphenidyl mimics an atropine intoxication with mydriasis, dryness of mucous membranes, red face, atonic states of bowels and bladder, and hyperthermia in high doses. Central consequences are agitation, confusion, and hallucinations. An untreated overdose may be fatal, particularly in children. Premortal signs are respiratory depression and cardiac arrest. A specific antagonist is physostigmine which combines a peripheral and a central action. Carbachol can be used to treat atonic bowel and bladder. The vital functions should be monitored and stabilized. It may be necessary to treat hyperthermia with cooling blankets. Clinical case reports have repeatedly shown overdose of trihexyphenidyl alongside other substances. Interactions
Other anticholinergic drugs (e.g. spasmolytics, antihistamines, TCAs) : Side effects of trihexyphenidyl may be increased. Quinidine : Increased anticholinergic action (particular on AV conduction).
Research
Equivocal preliminary results from small studies exist for:

Other dyskinesias
Huntingtons chorea
Spasmodic torticollis
Dystonia

See also
Biperiden (bicyclic ring)
Cycrimine (cyclopentanyl instead of cyclohexanyl)
Gamfexine
Procyclidine

References

 This article incorporates public domain material from Toxnet:Trihexyphenidyl. United States Department of Health and Human Services. Retrieved 8 May 2017.