Article Author:
Talha Jilani
Article Editor:
Sandeep Sharma
10/21/2019 11:12:44 AM
PubMed Link:


Trihexyphenidyl works as an anticholinergic and is used for the treatment of tremors, spasms, stiffness, and weak muscle control seen in patients with Parkinson disease.[1] It can also be used for the prevention or treatment of similar muscular conditions which are caused by certain central nervous system (CNS) drugs such as fluphenazine, haloperidol, chlorpromazine. Although it has been pertinent in clinical trials investigating the treatment of Parkinson disease since 1949, it was approved for the management of all types of parkinsonism (idiopathic, postencephalitic, and arteriosclerotic) in June 2003 by the FDA.[2] Trihexyphenidyl is often used as an adjuvant therapy when treating the above-mentioned forms of parkinsonism with levodopa.[2]

In 2008, it was reported that Iraqi police and soldiers were using trihexyphenidyl for recreational purposes along with other prescription drugs. It was reported that the drugs were taken as they seemed to relieve combat stress. Although this could have been the primary cause for use in some, it was also seen as a substitute or stronger version of LSD by some users.

Mechanism of Action

Although the precise mechanism of action of trihexyphenidyl remains poorly understood, it is thought to act on the parasympathetic nervous system by inhibiting the efferent impulses directly. Structures innervated by the parasympathetic system such as the salivary glands, eyes, and smooth muscles (directly and indirectly) are affected, even on smaller doses. The direct central inhibition of cerebral motor centers may be seen with higher doses. The receptors thought to be affected are the dopamine and M1 muscarinic receptors.[3],[4] The drug is absorbed from the gastrointestinal tract, and the onset of action is seen 60 minutes after an oral dose with peak activity occurring after 2 to 3 hours. One dose has a duration of action of around 6 to 12 hours and is then excreted in the urine, most likely as an unchanged drug.[5]


The dosage of trihexyphenidyl HCl varies with the individual and is determined empirically. Patients are started with a low initial dose, which is increased gradually, especially in adults who are older than 60 years of age. The oral drug could be taken before or after meals. This depends on the patient as in those with excessive xerostomia (due to trihexyphenidyl’s anticholinergic effects) could take the drug before meals and those who feel nauseous or are prone to excessive salivation could take the drug after meals. Trihexyphenidyl is best tolerated if taken in 3 separate doses daily, with food. Higher doses, such as more than 10 mg per day, could be divided into 4 doses daily (one taken with each meal and one at bedtime).

Abrupt withdrawal of trihexyphenidyl should be avoided in patients undergoing treatment for parkinsonism symptoms as this could cause an acute exacerbation of existing symptoms. Neuroleptic malignant syndrome has also been reported in patients who had an abrupt withdrawal of treatment.

Idiopathic Parkinsonism

For initial therapy for symptoms of idiopathic parkinsonism, 1 mg tablet of trihexyphenidyl is given on the first day. This could then be increased by 2-mg accretions at intervals of around 4 days until a total dose of 6 to 10 mg is being administered daily to the patient. The final daily dose is determined by the physician, to what he deems the optimal level for symptom control. Most patients display significant improvement in symptoms on total daily doses of 6 to 10 mg, but some, especially those of the postencephalitic sub-group, could require daily doses of around 12 to 15 mg for symptom management.

Drug-Induced Parkinsonism

The optimal dose and dosing frequency of trihexyphenidyl required to control the extrapyramidal symptoms of commonly used CNS drugs, for example, thioxanthenes, and phenothiazines, are determined empirically by the physician. For most patients, the total daily dosage ranges from 5 to 15 mg, although there have been reported cases of symptoms being sufficiently controlled on as minimal as 1 mg daily. Therefore, it is recommended to start treatment with a single dose of 1 mg. Failure of the extrapyramidal manifestations to resolve in a few hours could prompt an increase in dosage until a satisfactory control of symptoms is observed. Adequate control of symptoms could sometimes be achieved in a shorter duration by briefly decreasing the CNS drug dosage when trihexyphenidyl is being initiated and gradually adjusting the dose of both drugs until the desired effects are observed without the onset of extrapyramidal symptoms.

Concomitant Use with Levodopa

The usual dose of both levodopa and trihexyphenidyl may need to be reduced when both of these drugs are being used concomitantly. Any adjustment in dosage needs to be made carefully, depending on the level of symptom control and subsequent side effects. An adequate dosage for symptom control with minimal side effects is observed to be around 3 to 6 mg daily, given in divided doses.[6][7][8]

Adverse Effects

Adverse effects of trihexyphenidyl are frequently seen in a dose-dependent manner but usually decrease over time as tolerance develops and the body adapts to the drug. Even with all of the adverse effects considered, trihexyphenidyl demonstrated dramatic and consistent improvement of neurologic defects in people of ages between 16 to 86 years, over a 5-year course. Confusion and delirium were frequently noted in patients who were older than 86 years or had a psychiatric condition.

Adverse effects of trihexyphenidyl include but are not limited to:

Ocular effects: Mydriasis has been seen in patients with and without photophobia. This can lead to blurred vision or precipitate narrow-angle glaucoma by angle closure which increases intraocular pressure.

CNS effects: Frequently reported side effects include a headache, dizziness, drowsiness, and vertigo. Anxiety, nervousness, confusion, agitation, and confusion were seen in patients who were on higher doses. Trihexyphenidyl also produces a short-acting euphoric and mood-elevating effect, and that is why it is used as a drug of abuse. There have been cases of the disruption of the normal sleep architecture (REM sleep depression). It could also potentially lower the seizure threshold and should be used with caution in people with epilepsy or other seizure disorders.

Peripheral side effects: As with other anticholinergics, impaired sweating, dry mouth, abdominal discomfort, nausea, urinary retention, and constipation[9] are frequently seen (patients should be monitored for long-term use). Tachycardia has also been seen in patients. Although allergic reactions are very rare, they could occur with the use of trihexyphenidyl. Fatal hyperthermia and severe anhidrosis have been reported, and that is why caution is advised when using the drug during exercise or in extremely hot weather.

Patients taking trihexyphenidyl has also reported weight gain.

Tolerance could develop with the prolonged use of the drug and dosing adjustments have to be made.

Trihexyphenidyl is classified as "pregnancy risk factor class C.”[10]


Trihexyphenidyl is contraindicated in those with hypersensitivity to the drug (trihexyphenidyl HCl) or any of its active ingredients.

It is also contraindicated in patients with narrow-angle glaucoma. This is because it has anticholinergic activity that could cause mydriasis, further narrowing the angle of the lens, increasing the IOP, and worsening the condition.


Trihexyphenidyl is not strictly contraindicated for patients who have liver, kidney, or cardiac disorders, but close monitoring of these patients when using the drug is recommended. Patients with hypertension should also be monitored for the duration of therapy.

Some patients may require the indefinite use of trihexyphenidyl, and since it has properties similar to atropine, constant and long-term supervision should be implemented to prevent allergic and other unwanted reactions. Because of the parasympathetic activity of trihexyphenidyl, it should be used precautiously and monitored closely in patients with obstructive genitourinary or gastrointestinal diseases, glaucoma, and in older males with prostatic hypertrophy. Geriatric patients, especially those who are older than 60, commonly develop an increased sensitivity to drugs of these type and require strict regulation of their dosage.


As with some other antiparkinsonian medications, trihexyphenidyl is known to be a drug of abuse. Abuse has been reported primarily in both chronic schizophrenics and those who abuse other substances, the former being less frequent abusers of other substances. Trihexyphenidyl toxicity resembles atropine intoxication (antimuscarinic effects) with xerostomia, anhidrosis, mydriasis, nausea/vomiting, tachycardia, hyperpyrexia, decreased bowel and bladder movements, rash and hyperthermia which is usually seen in excessive doses. CNS symptoms observed with overdose include confusion, restlessness, agitation, incoordination, paranoid and psychotic reactions, delirium and hallucinations. CNS depression leading to coma, respiratory and circulatory failure and death have been reported in cases of severe overdose.[11] If the overdose is not treated quickly, it could be fatal, especially in smaller children. [4]

Management of overdoses is always supportive. Adequate airway patency should immediately be established. Physostigmine is a specific antagonist which acts centrally and peripherally to counter the antimuscarinic effects. Convulsions and hyperactivity should be managed with diazepam, but with caution as the risk of CNS depression could be exacerbated. Acidosis and hypoxia should be managed appropriately. Dysrhythmias should not be managed with antiarrhythmic drugs. Atonic bladder and bowel can be managed with carbachol.

Enhancing Healthcare Team Outcomes

Due to the significant toxicity of trihexyphenidyl, its use is best managed by an interprofessional team consisting of the nurse, physician, and pharmacist monitoring the patient for complications and communicating any concerns to the healthcare team. [Level V]


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