Antimuscarinic Pregnancy: Pregnancy Category C (US). Give to a pregnant woman only if clearly needed; animal reproduction studies not conducted.

Trihexyphenidyl hydrochloride

Brand names: Broflex

Trihexyphenidyl hydrochloride is an antimuscarinic (anticholinergic) agent used in Parkinson's disease, particularly for tremor, and for drug-induced extrapyramidal symptoms.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 6-10 mg daily (idiopathic parkinsonism, titrated)

Route: Oral

Frequency: Divided into 3 doses at mealtimes; high doses (>10 mg daily) may be divided into 4 (3 at mealtimes, 4th at bedtime)

Idiopathic parkinsonism: start 1 mg on day 1, then increase by 2 mg increments every 3-5 days to a total of 6-10 mg daily; some postencephalitic patients may need 12-15 mg daily. Drug-induced parkinsonism: total daily dose usually 5-15 mg (may start with a single 1 mg dose, titrate to effect). With concomitant levodopa: usually 3-6 mg daily in divided doses (doses of both may need reducing). Dosing should be individualised; start low and increase gradually, especially in patients over 60. Avoid abrupt withdrawal (risk of acute exacerbation of parkinsonism and neuroleptic malignant syndrome). US labelling — no per-kg paediatric dose stated: safety and effectiveness in paediatric patients have not been established.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to trihexyphenidyl or any of the other ingredients
Narrow angle glaucoma (blindness after long-term use due to narrow angle glaucoma has been reported)

Side effects

Dry mouth
Blurred vision
Dizziness
Mild nausea
Nervousness
Also: constipation, drowsiness, urinary hesitancy or retention, tachycardia, cognitive dysfunction (confusion, memory impairment)

Interactions

Cannabinoids, barbiturates, opiates and alcohol may have additive effects (abuse potential)
Alcohol or other CNS depressants: increased sedative effects
MAO inhibitors and tricyclic antidepressants with anticholinergic activity may intensify anticholinergic effects
Levodopa: usual dose of either drug may need to be reduced (concomitant use may increase drug-induced involuntary movements)

Clinical monograph

How it works

It blocks central muscarinic acetylcholine receptors, helping to restore the dopamine-acetylcholine balance in the basal ganglia.

Prescribing in practice

Antimuscarinic adverse effects and confusion are common, so it should be avoided or used very cautiously in older or cognitively impaired patients in whom it can precipitate delirium.
It is contraindicated or hazardous in narrow-angle glaucoma, significant urinary retention, prostatic enlargement and gastrointestinal obstruction.
It should not be withdrawn abruptly, and additive effects occur with other anticholinergic drugs.

Monitoring

Monitor cognition, mood, intraocular and urinary symptoms, and motor response during treatment.

Counselling the patient

Report dry mouth, blurred vision, difficulty passing urine, constipation or confusion.
Avoid stopping the medicine suddenly.
Be cautious with other medicines that cause dry mouth or drowsiness.

Evidence & guidelines

Antimuscarinics such as trihexyphenidyl have a long-established but limited role in Parkinson's disease, mainly for tremor, as reflected in standard guidance.

Reference: NICE NG71; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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