trihexyphenidyl 5 mg tablet

Uses

Trihexyphenidyl hydrochloride is used for the adjunctive treatment of all forms of parkinsonian syndrome including the postencephalitic, arteriosclerotic, and idiopathic types. Trihexyphenidyl is also used for the relief of parkinsonian signs and symptoms of antipsychotic agent-induced (e.g., butyrophenones, phenothiazines, thioxanthenes) extrapyramidal effects.

Trihexyphenidyl may also be effective in diminishing the frequency and duration of oculogyric crises, in decreasing salivation, in reducing spastic contractions and involuntary movements characteristic of dyskinesia, and in relieving mental inertia and depression characteristic of all forms of parkinsonian syndrome. As with other antiparkinsonian drugs, tolerance to trihexyphenidyl may develop during prolonged use. The maximum therapeutic response attainable with trihexyphenidyl is in the range of 20-30% symptomatic improvement in 50-75% of patients. Frequently, the maximum response requires empiric combination of trihexyphenidyl with other antimuscarinic drugs or with antihistaminic or dopaminergic agents. Some clinicians believe trihexyphenidyl to be of little value, but the majority have found it a useful adjunct in the multidimensional therapeutic approach to parkinsonian syndrome. Trihexyphenidyl is effective as adjunctive therapy for parkinsonian syndrome in patients receiving levodopa.

Clinical results from preliminary trials with trihexyphenidyl in the treatment of other dyskinesias, Huntington's chorea, spasmodic torticollis, and associated disorders have been equivocal. The drug generally is not helpful in spastic states such as cerebral palsy and hemiplegia.

Dosage and Administration

Administration

Trihexyphenidyl hydrochloride is administered orally. Trihexyphenidyl elixir or tablets may be given 3 times daily with meals; if a fourth dose is necessary, it should be given at bedtime. If trihexyphenidyl produces excessive xerostomia, the drug should be given preferably before meals, unless nausea is a problem. When trihexyphenidyl is taken after meals, xerostomia may be relieved by mint candies, chewing gum, or water. Xerostomia may also be alleviated by using a saliva substitute (e.g., Xero-lube).

Dosage

Dosage of trihexyphenidyl hydrochloride must be carefully adjusted according to individual requirements and response, especially in patients older than 60 years of age.

Parkinsonian Syndrome

For the symptomatic relief of parkinsonian syndrome, the usual initial dosage of trihexyphenidyl is 1 mg on the first day. Subsequent dosage may be increased by 2-mg increments at 3- to 5-day intervals until a total dose of 6-10 mg daily is achieved. In patients with postencephalitic parkinsonian syndrome, up to 12-15 mg daily may be necessary.

When trihexyphenidyl and levodopa are used concomitantly, it may be necessary to use reduced dosages of each drug. Dosage of each drug should be adjusted individually according to the patient's combined response and tolerance.

When trihexyphenidyl is replacing, in whole or in part, therapy with another antimuscarinic, the dosage of trihexyphenidyl is gradually increased while the dosage of the other drug is gradually decreased.

Drug-Induced Extrapyramidal Reactions

The usual dosage of trihexyphenidyl for the relief of antipsychotic agent-induced extrapyramidal disorders may range from 5-15 mg daily. An initial dose of 1 mg may control some reactions; however, if extrapyramidal manifestations are not controlled within a few hours, subsequent doses may be progressively increased until adequate response is achieved. Alternatively, a more rapid control may be achieved by reducing the dosage of the drug causing the reaction, then adjusting the dosage of both drugs to attain the desired drug effect without extrapyramidal symptoms. Once control of extrapyramidal reactions has been maintained for several days, the dosage of trihexyphenidyl may be reduced or discontinued.

Cautions

Adverse Effects

Adverse reactions to trihexyphenidyl are mainly extensions of its anticholinergic effects. Adverse effects of trihexyphenidyl, which are experienced by 30-50% of patients receiving the drug, may include dryness of the mouth, dizziness, blurred vision, nausea, and nervousness. Other adverse effects typical of those produced by antimuscarinic drugs include constipation, tachycardia, mydriasis, urinary hesitancy or retention, drowsiness, increased intraocular tension, weakness, vomiting, and headache. CNS stimulation, usually manifested by restlessness, agitation, confusion, delirium, and hallucination or euphoria may occur with high dosage, or in persons with a history of hypersensitivity to other drugs, or in patients with arteriosclerosis. Isolated instances of rashes, dilatation of the colon, paralytic ileus, and suppurative parotitis secondary to dryness of the mouth have been reported. Angle-closure glaucoma has reportedly occurred in patients receiving prolonged therapy with trihexyphenidyl. Rarely, psychiatric disturbances such as delusion, amnesia, depersonalization, a sense of unreality, and one possible case of paranoia have been reported with trihexyphenidyl. The incidence and severity of adverse effects are generally dose related and adverse effects may occasionally be obviated by reduction in dosage. If a severe reaction occurs, the drug should be discontinued for several days and then readministered at a lower dosage.

Precautions and Contraindications

Trihexyphenidyl should be used with caution or may be contraindicated in patients with conditions in which anticholinergic effects are undesirable. The usual precautions and contraindications associated with antimuscarinics should be observed with trihexyphenidyl. For a complete discussion of the precautions and contraindications associated with antimuscarinics, see .

Patients should receive a gonioscopic examination prior to initiation of trihexyphenidyl therapy. Intraocular pressure should be monitored at regular intervals during prolonged therapy with the drug.

Pharmacokinetics

Trihexyphenidyl is rapidly absorbed from the GI tract. Following oral administration of trihexyphenidyl hydrochloride tablets, the onset of action occurs within 1 hour, peak effects last 2-3 hours, and the duration of action is 6-12 hours. The metabolic fate of trihexyphenidyl has not been determined; the drug is excreted in the urine, probably as unchanged drug.