indication

Indicated for the treatment of parkinson's disease and extrapyramidal reactions caused by drugs.

pharmacology

Trihexyphenidyl is an anticholinergic used in the symptomatic treatment of all etiologic groups of parkinsonism and drug induced extrapyramidal reactions (except tardive dyskinesia). Trihexyphenidyl possesses both anticholinergic and antihistaminic effects, although only the former has been established as therapeutically significant in the management of parkinsonism.

mechanism of action

Trihexyphenidyl is a selective M1 muscarinic acetylcholine receptor antagonist. It is able to discriminate between the M1 (cortical or neuronal) and the peripheral muscarinic subtypes (cardiac and glandular). Trihexyphenidyl partially blocks cholinergic activity in the CNS, which is responsible for the symptoms of Parkinson's disease. It is also thought to increase the availability of dopamine, a brain chemical that is critical in the initiation and smooth control of voluntary muscle movement.

toxicity

Symptoms of overdose include mydriasis, dryness of mucous membranes, red face, atonic states of bowels and bladder, and hyperthermia in high doses. Central consequences are agitation, confusion, and hallucinations. An untreated overdose may be fatal, particular in children. Premortal signs are respiratory depression and cardiac arrest.

absorption

Trihexyphenidyl is rapidly absorbed from the gastrointestinal tract.

half life

3.3-4.1 hours

drug interactions

Donepezil: Possible antagonism of action

Galantamine: Possible antagonism of action

Haloperidol: The anticholinergic increases the risk of psychosis and tardive dyskinesia

Potassium Chloride: The ulcerative effects of solid oral dosage forms of KCl may be enhanced by Trihexyphenidyl, an anticholinergic. Anticholinergics slow gastric emptying, increasing the contact time between the gastrointestinal mucosa and KCl. Prolonged exposure to KCl increases the risk of gastric and intestinal irritation and ulceration. Solid oral dosage forms of KCl should be avoided; alternatives include liquid or effervescent potassium preparations.

Pramlintide: The anticholinergic effects of Trihexyphenidyl may be enhanced by Pramlintide. Additive effects of reduced GI motility may occur. Pramlintide slows gastic emptying and should not be used with drugs that alter GI motility (e.g. anticholinergics). Consider alternative treatments or use caution during concomitant therapy.

Rivastigmine: Possible antagonism of action

Secretin: The stimulatory effect of Secretin may be reduced by anticholinergics such as Trihexyphenidyl. Concomitant use of Secretin and drugs with substantial anticholinergic effects should be avoided. If combination therapy must be used, Secretin efficacy should be closely monitored.

Tacrine: The therapeutic effects of the central acetylcholinesterase inhibitor, Tacrine, and/or the anticholinergic, Trihexyphenidyl, may be reduced due to antagonism. The interaction may be beneficial when the anticholinergic action is a side effect. Monitor for decreased efficacy of both agents.

Trimethobenzamide: Trimethobenzamide and Trihexyphenidyl, two anticholinergics, may cause additive anticholinergic effects and enhance their adverse/toxic effects. Monitor for enhanced anticholinergic effects.

Triprolidine: Triprolidine and Trihexyphenidyl, two anticholinergics, may cause additive anticholinergic effects and enhance their adverse/toxic effects. Monitor for enhanced anticholinergic effects.

Trospium: Trospium and Trihexyphenidyl, two anticholinergics, may cause additive anticholinergic effects and enhanced adverse/toxic effects. Monitor for enhanced anticholinergic effects.