Pharmacology · Antiparkinson anticholinergics

Trihexyphenidyl !

A central anticholinergic for parkinsonian tremor and drug-induced extrapyramidal effects. Avoided in older adults and narrow-angle glaucoma.

Updated 2026-11-01

Class: Antiparkinson anticholinergics
Receptor: Muscarinic acetylcholine receptors (M1 shown)
ATC code: N04AA01

Also called: Benzhexol

Muscarinic acetylcholine receptors (M1 shown)Trihexyphenidyl

The figure shows tiotropium, another muscarinic antagonist, in the human M1 receptor; trihexyphenidyl blocks the same acetylcholine binding site.

Target in 3D: Muscarinic acetylcholine receptors (M1 shown). PDB structure 5CXV (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Trihexyphenidyl is a parasympatholytic (antimuscarinic) drug that enters the brain. In the striatum it blocks the acetylcholine released by cholinergic interneurons, which predominates when dopamine falls, easing tremor and rigidity and the extrapyramidal effects of dopamine-blocking drugs.

Dopamine loss leaves striatal acetylcholine unopposed
Trihexyphenidyl blocks central muscarinic receptors
Tremor, rigidity and drug-induced dystonia ease

Pharmacokinetics

Absorption: Given by mouth as tablets or oral solution
Distribution: Enters the brain

Indications

Adjunct in all forms of parkinsonism, often with levodopa Parkinson disease →
Extrapyramidal disorders caused by phenothiazines, thioxanthenes, butyrophenones and similar drugs Antipsychotic-induced movement disorders (dystonia, akathisia, parkinsonism, tardive dyskinesia) →

Adverse effects

Common

Dry mouth, blurred vision, dizziness
Nausea, constipation
Difficulty urinating
Nervousness; euphoria (misuse has been reported)

Contraindications

Narrow-angle glaucoma
Hypersensitivity to trihexyphenidyl
Caution: prostate enlargement, gut or urinary obstruction, older adults

Key interactions

Antipsychotics, tricyclics, antihistamines and other anticholinergics: Additive anticholinergic burden: confusion, ileus, heat intolerance
Alcohol and sedatives: More drowsiness and confusion

What is monitored

Eye examination (gonioscopy) before starting and intraocular pressure during treatment
Cognition and behaviour, especially in older adults
Urination, bowel habit and temperature in hot weather

Administration and safety

Tablets and oral solution: the solution is measured with an oral syringe or measuring device
Withdrawn gradually to avoid rebound parkinsonism
Can be given with food if it upsets the stomach

Nursing care

Ask about blurred vision or eye pain (angle closure)
Check for urinary retention and constipation
Screen older adults for confusion and fall risk
Teach to avoid overheating and keep hydrated

Pregnancy and breastfeeding

It is not known whether it harms the fetus; it is used in pregnancy only if clearly needed.

Test yourself

Think of the answer, then tap the question to see it.

Which test is recommended before starting trihexyphenidyl?

An eye examination with gonioscopy, because it can precipitate angle-closure glaucoma.

Why is trihexyphenidyl usually avoided in people over 65?

Sensitivity to antimuscarinic effects increases with age: confusion, delirium, retention and constipation.

Which Parkinson symptom responds best to anticholinergics?

Tremor (with some effect on rigidity).

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - TRIHEXYPHENIDYL HYDROCHLORIDE solution (DailyMed (U.S. National Library of Medicine), 2026); Trihexyphenidyl: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2018); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 21 (McGraw Hill, 2023).

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Representative model, for educational purposes only.