trihexyphenidyl hydrochloride

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.