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Central Anticholinergics

Trihexyphenidyl, Biperiden

For medical students2 min readUpdated 2026-10-10

Central anticholinergics are a group of pharmacological agents that inhibit cholinergic transmission in brain structures. Their primary target is $M_1$ muscarinic receptors in the neostriatum, the blockade of which provides a moderate antiparkinsonian effect.

MechanismBlockade of M1 muscarinic receptors in the neostriatum

RepresentativesTrihexyphenidyl and biperiden

Action SpecificityRelieve tremor and rigidity, but have minimal effect on bradykinesia

RiskOverdose causes anticholinergic delirium and hallucinations

Mechanism of Action and Main Effects

The main representatives of agents that inhibit cholinergic transmission in the central nervous system are trihexyphenidyl (also known as benzhexol) and biperiden. These drugs are very similar in their pharmacological properties.

Their site of action is within brain structures, where they selectively block $M_1$ muscarinic receptors in the neostriatum. This action determines their specific pharmacological profile. Central anticholinergics produce a moderate antiparkinsonian effect that manifests unevenly:

They effectively reduce tremor.
They decrease muscle rigidity (increased muscle tone).
They have virtually no effect on bradykinesia (slowness of movement).

Clinical Application

Drugs in this group, such as biperiden, are widely used in clinical practice for various forms of extrapyramidal disorders. The main indications include:

Parkinson's disease: these drugs are most appropriately prescribed primarily in the early stages of the disease.

Regardless of the underlying etiology, they are used for various forms of parkinsonism.

Drug-induced parkinsonism: this is one of the most specific indications. Central anticholinergics are used for the prevention and rapid relief of extrapyramidal symptoms induced by neuroleptics (antipsychotic medications).

Adverse Reactions

The side effects of these drugs are directly related to their mechanism of action and are classified according to the localization of the blocked receptors:

Central effects occur due to the blockade of muscarinic receptors directly in the brain. They manifest as various cognitive impairments, such as memory loss and decreased concentration. Elderly patients are at a particularly high risk due to increased sensitivity to these central adverse reactions.

Peripheral effects are caused by the undesirable inhibition of peripheral receptors:

Gastrointestinal tract: xerostomia (dry mouth) and constipation.
Ophthalmology: accommodation disorders, up to complete cycloplegia (inability to focus on close objects).
Cardiovascular system: tachycardia (increased heart rate).
Urinary system: urinary retention may occur.

Contraindications and Overdose

The prescription of central anticholinergics is strictly limited in certain comorbid conditions due to the risk of worsening the patient's status.

Absolute Contraindications:

Glaucoma (high risk of a critical increase in intraocular pressure).
Prostatic hyperplasia (risk of acute urinary retention).
Lactation (breastfeeding).
Individual hypersensitivity to the drug components.

Exceeding the therapeutic dose leads to acute intoxication. Symptoms of overdose include marked psychomotor agitation and hallucinations, which together form the classic clinical picture of anticholinergic delirium.

Mnemonic

To remember the peripheral side effects of antimuscarinics: everything "dries out" (xerostomia, constipation), fluid is "retained" (urinary retention), the heart "speeds up" (tachycardia), and the eyes "lose focus" (cycloplegia).

Frequently asked questions

Which drug classes should not be combined with central anticholinergics due to the risk of exacerbating the anticholinergic syndrome?

Anticholinergic delirium can occur when high doses of drugs with prominent atropine-like activity are administered together. Provocative agents include:

Neuroleptics: chlorpromazine, levomepromazine, clozapine;
Tricyclic antidepressants: amitriptyline, imipramine, clomipramine, maprotiline;
Antiparkinsonian agents: trihexyphenidyl, biperiden, diphenhydramine.

For neuroleptics and tricyclic antidepressants, muscarinic receptor blockade leads to side effects such as dry mouth, tachycardia, urinary retention, constipation, mydriasis, and cycloplegia.

How does the effect on parkinsonian symptoms differ between central anticholinergics and levodopa?

The impact of these drug groups on parkinsonism differs in the magnitude of effect and spectrum of action.

Feature	Central Anticholinergics	Levodopa
Overall Efficacy	Moderate antiparkinsonian effect	"Gold standard" of treatment, provides improvement in nearly 100% of patients
Effect on Tremor and Rigidity	Predominantly reduces these symptoms	Causes general clinical improvement
Effect on Bradykinesia	Minimal effect on bradykinesia	Causes general clinical improvement

Why do central anticholinergics have little effect on slowness of movement?

Their pharmacological profile is such that they predominantly relieve tremor and muscle rigidity, but have a minimal impact on bradykinesia.

Can biperiden be prescribed to patients with glaucoma?

No, this is a strict contraindication. The blockade of peripheral muscarinic receptors carries a risk of dangerously elevating intraocular pressure.

What is the role of these drugs in neuroleptic treatment?

They are drugs of choice for the prevention and management of extrapyramidal symptoms (drug-induced parkinsonism) caused by antipsychotics.

What happens in trihexyphenidyl overdose?

A so-called anticholinergic delirium occurs, manifested primarily by pronounced psychomotor agitation and hallucinations.

Go deeper

Comparison of the pharmacodynamics of trihexyphenidyl and levodopa
Peculiarities of anticholinergic use in elderly patients
Mechanism of development of anticholinergic delirium
Differential diagnosis of parkinsonism etiologies
The role of pramipexole in the treatment of extrapyramidal disorders