Trihexyphenidyl

How it works

It blocks central muscarinic acetylcholine receptors, helping to redress the relative cholinergic excess that results from dopamine deficiency or dopamine-receptor blockade in the basal ganglia.

Prescribing in practice

• Antimuscarinic effects make it hazardous in the elderly and it should be avoided where there is cognitive impairment, narrow-angle glaucoma, urinary retention or significant prostatic enlargement.
• Confusion, hallucinations, dry mouth, blurred vision and constipation are common, especially in older patients.
• It should be withdrawn gradually rather than stopped abruptly.

Monitoring

Monitor cognition, intraocular pressure where relevant, bowel and bladder function and overall antimuscarinic burden.

Counselling the patient

• Report confusion, hallucinations, eye pain or difficulty passing urine.
• Dry mouth and blurred vision are common; take care with tasks needing clear vision.

Evidence & guidelines

Its role in Parkinsonian tremor and drug-induced extrapyramidal symptoms is long established in clinical practice and standard references.