Trihexyphenidyl

Trihexyphenidyl is an oral anticholinergic used for parkinsonism, drug-induced extrapyramidal symptoms and, under specialist care, dystonia. It shares the toxicity of benztropine, and it has misuse potential because of its euphoriant effect.

Mechanism. Blocks central muscarinic receptors in the striatum, and also has a mild direct antispasmodic effect.

Pharmacokinetics. Rapidly absorbed with onset in about 1 hour and effect lasting 6 to 12 hours. Half-life is 3.7 hours, longer in older adults. It is metabolised in the liver and excreted in urine.

Alternative use. Parkinson disease tremor and rigidity, and drug-induced parkinsonism.

• Oral trihexyphenidyl: 1 mg on day 1, then increase by 2 mg every 3 to 5 days. Usual 6 to 10 mg daily in three divided doses with meals. Maximum 15 mg daily; some practice uses up to 20 mg daily under supervision.

• Drug-induced parkinsonism: 1 mg daily initially, increased to a usual 5 to 15 mg daily in divided doses.

• Dystonia in children or adults uses higher, slowly titrated doses under specialist care only.

Kidney, liver, weight and age.

• Kidney and liver: use caution and lower doses.

• Older adults: start 0.5 to 1 mg daily; avoid in dementia.

Contraindications. Narrow-angle glaucoma, myasthenia gravis, obstructive gastrointestinal disease, urinary retention, tardive dyskinesia in some labels, and hypersensitivity.

Serious adverse effects. Anticholinergic toxicity, delirium and psychosis, acute angle-closure glaucoma, ileus, urinary retention, heat stroke, and dependence or misuse.

Common adverse effects. Dry mouth, blurred vision, constipation, urinary hesitancy, tachycardia, dizziness, nausea and confusion.

Interactions. Other anticholinergics (tricyclics, antipsychotics, antihistamines) add toxicity. It opposes cholinesterase inhibitors. Alcohol and sedatives add drowsiness.

Common prescribing errors. Use in older adults with memory problems. Routine long-term prescribing with antipsychotics. Doses taken in a single large amount.

Special groups.

• Pregnancy: avoid unless needed.

• Breastfeeding: may suppress lactation; avoid.

• Regional: diversion and misuse are recognised in some settings, so limit supply and review use.

Formulations. Tablets 2 and 5 mg. Syrup 5 mg/5 mL in some markets. Take with food to limit nausea and dry mouth.

Monitoring.

• Cognition, bowel and bladder function, vision, temperature in hot weather and signs of misuse.

• Review the need for ongoing use at least every 3 months.

Stopping. Taper over 1 to 2 weeks to avoid rebound.

Referral and escalation. Refer for persistent movement disorder, cognitive decline, or suspected misuse.