drowsiness, vertigo, headache, and dizziness are frequent. With high doses nervousness, agitation, anxiety, delirium, and confusion are noted. Trihexyphenidyl may be abused due to a short acting mood-elevating and euphoric effect. The normal sleep architecture may be altered (REM sleep depression). Trihexyphenidyl may lower the seizure-threshold. Peripheral side effects: dry mouth, impaired sweating, abdominal discomfort, nausea, and constipation are frequent. Tachycardia or heart palpitations (fast heart rate) may be noted. Allergic reactions are rare, but may occur. 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). Antipsychotics : Long term use of trihexyphenidyl may mask or increase the risk of tardive dyskinesia. Pethidine (meperidine) : Central effects and side effects of pethidine may be increased. Metoclopramide : Action of metoclopramide is decreased. Alcohol : Risk of serious intoxication. Pharmacology The exact mechanism of action in parkinsonian syndromes is not precisely understood, but it is known that trihexyphenidyl blocks efferent impulses in parasympathetically innervated structures like smooth muscles (spasmolytic activity), salivary glands, and