trihexyphenidyl hydrochloride
Pharmacologic class: Anticholinergic
Therapeutic class: Antidyskinetic
Pregnancy risk category C
Action
Inhibits parasympathetic nervous system, relaxing smooth muscles and decreasing involuntary movements
Availability
Syrup: 2 mg/5 ml
Tablets: 2 mg, 5 mg
Indications and dosages
➣ Adjunct in idiopathic, postencephalitic, or arteriosclerotic parkinsonism
Adults: 1 mg P.O. on first day; may increase in 2-mg increments q 3 to 5 days, up to a maximum of 6 to 10 mg/day. In postencephalitic parkinsonism, 12 to 15 mg P.O. daily.
➣ Drug-induced extrapyramidal symptoms
Adults: Initially, 1 mg P.O. daily, increased progressively if extrapyramidal symptoms aren't controlled within several hours. Usual dosage range is 5 to 15 mg/day P.O. in divided doses.
Dosage adjustment
• Concurrent use of levodopa or other parasympathetic inhibitor
• Elderly patients
Off-label uses
• Dystonia
Contraindications
• Hypersensitivity to drug or its components
• Angle-closure glaucoma
• Pyloric or duodenal obstruction
• Stenosing peptic ulcer
• Megacolon
• Prostatic hypertrophy or bladder-neck obstruction
• Achalasia
• Myasthenia gravis
Precautions
Use cautiously in:
• chronic renal, hepatic, pulmonary, or cardiac disease; hypertension; tachycardia secondary to cardiac insufficiency; hyperthyroidism
• elderly patients
• pregnant or breastfeeding patients
• children (safety not established).
Administration
• Give with meals. However, if drug causes severe dry mouth, give before meals.
• Administer last dose at bedtime.
Adverse reactions
CNS: dizziness, nervousness, drowsiness, asthenia, headache
CV: orthostatic hypotension, tachycardia
EENT: blurred vision, mydriasis, increased intraocular pressure (IOP), angle-closure glaucoma (with long-term use)
GI: nausea, vomiting, constipation, dry mouth
GU: urinary hesitancy or retention
Interactions
Drug-drug. Amantadine, other anticholinergics (including disopyramide, phenothiazines, quinidine, tricyclic antidepressants): additive anticholinergic effects
Other CNS depressants (such as antihistamines, opioids, sedative-hypnotics): additive CNS depression
Phenothiazines: decreased phenothiazine effects
Drug-herbs. Angel's trumpet, jimsonweed, scopolia: increased anticholinergic effects
Drug-behaviors. Alcohol use: additive CNS depression
Patient monitoring
• With prolonged use, monitor vision and IOP regularly.
• Assess drug efficacy to help guide dosage titration.
• Monitor vital signs. Watch for orthostatic hypotension.
• Closely monitor fluid intake and output. Stay alert for urinary retention.
Patient teaching
• Instruct patient to take with meals or, if severe dry mouth occurs, before meals.
• Tell patient drug has a bitter taste, which may be followed by numbness and tingling in mouth.
• Stress importance of follow-up eye exams.
• Instruct patient to consult prescriber before taking over-the-counter preparations or herbs.
• Advise patient to avoid alcohol and hazardous activities during drug therapy.
• Tell patient to move slowly when sitting up or standing, to avoid dizziness from sudden blood pressure decrease.
• As appropriate, review all other significant adverse reactions and interactions, especially those related to the drugs, herbs, and behaviors mentioned above.