Understanding the Mechanism: How Do Anticholinergic Effects Happen?

October 10, 2025 •

3 min read

Over 600 specific medications, including many common over-the-counter drugs, exhibit unintended anticholinergic effects. This occurs due to their fundamental mechanism of blocking a key neurotransmitter, leading to a wide range of side effects that disrupt normal bodily functions.

Quick Summary

Anticholinergic effects happen when certain drugs block the neurotransmitter acetylcholine from activating its receptors in the central and peripheral nervous systems, inhibiting "rest and digest" functions.

Key Points

Core Mechanism: Anticholinergic effects happen by competitively blocking the neurotransmitter acetylcholine (ACh) from binding to its receptors.
Dominant Receptors: The majority of clinically significant anticholinergic effects occur due to the blockade of muscarinic receptors, found both centrally and peripherally.
Parasympathetic Inhibition: By blocking acetylcholine, these drugs inhibit the parasympathetic nervous system, which is responsible for "rest and digest" functions, leading to widespread effects.
System-Specific Symptoms: The side effects, such as dry mouth, blurred vision, and urinary retention, manifest from the specific organs where acetylcholine's action has been blocked.
Central vs. Peripheral: A drug's ability to cross the blood-brain barrier determines if it causes central nervous system effects like confusion and delirium, or only peripheral symptoms.
Anticholinergic Burden: The risk and severity of these effects can increase significantly with higher doses or the combined use of multiple medications with anticholinergic properties.

The Body's Chemical Messenger: Acetylcholine

To understand how anticholinergic effects happen, one must first grasp the role of acetylcholine (ACh). Acetylcholine is a primary neurotransmitter in the nervous system, a chemical messenger responsible for transmitting signals across synapses between nerve cells. It plays a crucial role in several bodily functions, particularly within the parasympathetic nervous system, which governs involuntary actions associated with the "rest and digest" state.

Among its many functions, acetylcholine is responsible for:

Smooth muscle contraction: Affecting organs like the gastrointestinal tract, bladder, and lungs.
Glandular secretions: Stimulating secretions from salivary, lacrimal, and sweat glands.
Heart rate regulation: Typically slowing the heart rate.
Eye functions: Causing pupil constriction and adjusting lens for near vision.
Central Nervous System (CNS) functions: Involved in cognition, memory, learning, and attention.

The Mechanism of Competitive Blockade

Anticholinergic effects stem from drugs interfering with acetylcholine's action. "Anti-" signifies working against, so anticholinergics counteract acetylcholine's effects. This occurs through competitive antagonism, where the drug competes with acetylcholine for receptor binding sites. The drug acts like a dummy key, fitting into the receptor lock and preventing acetylcholine from binding and initiating its action. This blockade disrupts normal parasympathetic nervous system function.

Muscarinic vs. Nicotinic Receptors

Anticholinergic drugs primarily block muscarinic receptors (M1–M5), which are found on target organs and in the CNS, causing most clinically significant anticholinergic effects. The effects depend on the location of the blocked receptors:

Peripheral muscarinic receptor blockade: Leads to side effects like dry mouth, blurred vision, constipation, and urinary retention.
Central muscarinic receptor blockade: Can cause CNS effects such as sedation, confusion, and delirium if the drug crosses the blood-brain barrier.

Some anticholinergics also block nicotinic receptors.

Effects in Different Body Systems

Blocking acetylcholine leads to varied systemic consequences depending on affected organ systems. Anticholinergic toxicity symptoms are often summarized as: "Blind as a bat, dry as a bone, red as a beet, hot as a hare, mad as a hatter, and full as a flask".

This table compares normal cholinergic function with anticholinergic effects:


Organ System	Normal Cholinergic Function	Anticholinergic Effect	Cause of Effect
Eyes	Pupil constriction, near vision adjustment	Blurred vision, dilated pupils, light sensitivity	Blockade of muscarinic receptors
Gastrointestinal Tract	Increases motility and secretions	Constipation, slowed digestion	Blockade of muscarinic receptors
Urinary System	Promotes urination	Urinary retention	Blockade of muscarinic receptors
Exocrine Glands	Increased secretions	Dry mouth, dry eyes, decreased sweating	Blockade of muscarinic receptors
Cardiovascular System	Decreased heart rate	Increased heart rate (tachycardia)	Blockade of muscarinic receptors
Central Nervous System	Involved in memory and learning	Sedation, confusion, memory impairment, delirium	Blockade of muscarinic receptors

Factors Influencing Anticholinergic Effects

Several factors influence the occurrence and severity of these effects. Older adults are particularly susceptible.

Dose: Higher doses block more receptors, increasing effects.
Drug Specificity: Some drugs have stronger affinity for muscarinic receptors or cross the blood-brain barrier more easily, leading to more CNS effects.
Polypharmacy: Taking multiple medications with anticholinergic properties increases the "anticholinergic burden" and risk of adverse effects.
Underlying Health Conditions: Conditions like BPH or narrow-angle glaucoma increase the risk of complications.

Conclusion

Anticholinergic effects happen due to the competitive blockade of acetylcholine receptors, primarily muscarinic subtypes, throughout the nervous system. This disruption of parasympathetic function leads to predictable symptoms affecting various organs. Understanding this mechanism is crucial for managing side effects and ensuring patient safety, especially in vulnerable populations. Given that many common medications have anticholinergic properties, awareness of potential effects is vital for both patients and healthcare providers. For additional information, authoritative medical sources like the Cleveland Clinic are recommended.

Frequently Asked Questions

Both are types of acetylcholine receptors. Muscarinic receptors are located on smooth muscle cells, glands, and the heart, primarily mediating involuntary functions. Nicotinic receptors are found in the central nervous system and at the neuromuscular junction, controlling muscle movement. Most clinically relevant anticholinergic effects are from muscarinic receptor blockade.

Many medications, including some over-the-counter drugs, have anticholinergic properties. Examples include older antihistamines like diphenhydramine (Benadryl), tricyclic antidepressants, some antipsychotics, and medications for urinary incontinence and Parkinson's disease.

Anticholinergics block acetylcholine's action in the eyes, which normally helps the pupils constrict and the lens focus for near vision. This leads to common side effects like blurred vision, dilated pupils (mydriasis), and sensitivity to light.

The anticholinergic burden refers to the cumulative effect of taking multiple medications with anticholinergic properties. Each drug may have a weak effect individually, but together, they can produce significant and potentially harmful anticholinergic symptoms, especially in older adults.

In cases of severe anticholinergic toxicity, which can be life-threatening, emergency medical attention is needed. Treatment often involves supportive care and may include administering physostigmine, a drug that increases acetylcholine levels to counteract the blockade.

Older adults are more sensitive to these effects due to several factors, including polypharmacy (taking multiple medications), changes in drug metabolism with age, and a higher risk of conditions that are exacerbated by anticholinergic properties, such as cognitive impairment and urinary retention.

For mild symptoms like dry mouth or constipation, simple measures can help. Chewing sugar-free gum, using saliva substitutes, increasing fluid intake, and eating high-fiber foods may provide relief. It is important to discuss persistent or worsening symptoms with a healthcare provider.