Trihexyphenidyl

1. Drug & class

  • Generic: Trihexyphenidyl (also known as benzhexol)
  • Brand: Heksi (2 mg, 5 mg tablets)
  • Pharmacologic class: centrally‐acting anticholinergic (antimuscarinic) agent

2. Mechanism of action

  • It blocks muscarinic acetylcholine receptors (especially M1 subtype) in the CNS, reducing cholinergic overactivity in the basal ganglia
  • This helps restore the balance between dopamine and acetylcholine in movement control pathways.

3. Indications

  • Adjunctive treatment of idiopathic and secondary parkinsonism (e.g., post‐encephalitic, arteriosclerotic)
  • Treatment of drug‐induced extrapyramidal symptoms (EPS) from antipsychotics (e.g., dystonia, rigidity)

4. Dosing & administration (clinical pharmacist key points)

  • Start low, titrate gradually, particularly in elderly due to increased risk of anticholinergic side‐effects.
  • Example: for drug‐induced parkinsonism, initial ~1 mg, increase to typical range ~5–15 mg/day depending on response.
  • Tablets 2 mg and 5 mg (as in Heksi) available.
  • Administer with or after meals to reduce GI upset.

5. Key contraindications & precautions

  • Contraindicated in narrow‐angle glaucoma — because anticholinergic effect may raise intraocular pressure.
  • Use with caution in: prostatic hypertrophy/urinary retention, GI obstruction, myasthenia gravis, elderly (risk of confusion, falls)
  • Avoid abrupt discontinuation in parkinsonism because of risk of rebound worsening.

6. Major adverse effects & monitoring

  • Common: dry mouth, blurred vision (mydriasis), constipation, urinary retention, dizziness, nausea, sedation.
  • CNS: confusion, hallucinations, memory impairment — especially in elderly.
  • Anticholinergic burden: increased risk of heat intolerance/anhidrosis, falls, cognitive decline.
  • Monitoring: urinary output/retention, intraocular pressure (in glaucoma risk), mental status.

7. Drug‐interactions & pharmacist considerations

  • Alcohol and other CNS depressants: additive sedation/dizziness.
  • Use with other anticholinergics increases anticholinergic burden; caution in polypharmacy (especially geriatrics).
  • Caution when used with tricyclics, MAO inhibitors due to increased anticholinergic/central side‐effects.

8. Practical tips for the clinical pharmacist

  • Review patient’s anticholinergic burden (other meds such as oxybutynin, TCA, antihistamines) before adding trihexyphenidyl.
  • Educate patient about side‐effects: dry mouth (recommend sugarless gum, frequent sips of water), constipation (encourage fiber, fluids), blurred vision/driving caution.
  • Check for contraindications: glaucoma, urinary retention, prostatic disease, GI obstruction.
  • In elderly, consider non‐anticholinergic alternatives if high risk of cognitive decline/falls.
  • Monitor for cognitive changes, orthostatic hypotension, risk of falls.
  • Counsel that medication should not be abruptly stopped without clinician oversight.
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