[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Muscarinic (cholinergic) agonists](https://durgical.com/en/pharmacology/classes/muscarinic-agonists/) › Methacholine

[Pharmacology](https://durgical.com/en/pharmacology/) · [Muscarinic (cholinergic) agonists](https://durgical.com/en/pharmacology/classes/muscarinic-agonists/)

# Methacholine

An inhaled muscarinic agonist used only as a diagnostic test: people with asthma narrow their airways at much lower doses.

Updated 2026-11-01

- Class: [Muscarinic (cholinergic) agonists](https://durgical.com/en/pharmacology/classes/muscarinic-agonists/)
- Receptor: Muscarinic M3 receptor of airway smooth muscle
- ATC code: V04CX03

◎**Muscarinic M3 receptor of airway smooth muscle**Methacholine

ⓘ No structure of methacholine bound to its receptor exists. The figure shows iperoxo, a potent research agonist, in the acetylcholine (orthosteric) pocket of the active human M3 muscarinic receptor, the subtype on glands, the eye and the bladder that methacholine stimulates.

**Target in 3D:** Muscarinic M3 receptor of airway smooth muscle. PDB structure 8E9Z (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/8E9Z) · Drag to rotate · pinch to zoom

## Mechanism of action

Methacholine is acetylcholine with an extra methyl group: it resists cholinesterase better and acts mainly on muscarinic receptors. Inhaled, it contracts airway smooth muscle. Everyone narrows the airways a little, but people with hyperresponsive airways, as in asthma, react strongly at low concentrations, which the bronchial challenge test measures with spirometry.

- Inhaled in increasing concentrations
- Activates M3 receptors of airway smooth muscle
- Airways narrow; spirometry falls
- A strong response at low doses means hyperresponsive airways

## Pharmacokinetics

**Absorption**
Inhaled from a nebulizer; acts locally in the airways

**Metabolism**
Hydrolysed by acetylcholinesterase more slowly than acetylcholine

## Indications

- Bronchial challenge test for airway hyperresponsiveness when asthma is suspected but not clinically clear [Asthma →](https://durgical.com/en/conditions/asthma/)

## Adverse effects

### Common

- Cough, chest tightness and wheeze
- Shortness of breath
- Dizziness and discomfort

### Serious (rare, important)

- Severe bronchospasm

## Contraindications

- Severe airflow limitation
- Recent heart attack or stroke
- Uncontrolled high blood pressure
- Pregnancy

## Key interactions

**Beta blockers**
Exaggerated or prolonged airway response

**Inhaled antimuscarinics (ipratropium, tiotropium)**
Blunt the response and can make the test falsely negative

## What is monitored

- Spirometry after each concentration
- Symptoms, oxygen saturation and breath sounds during and after the test

## Administration and safety

- Performed only in a pulmonary function laboratory, with oxygen, resuscitation equipment and an inhaled bronchodilator ready
- A bronchodilator is given at the end if the airways have narrowed

## Nursing care

- Confirm that pregnancy and recent heart attack or stroke have been ruled out
- Ask which inhalers and beta blockers the person uses
- Stay with the patient and watch for wheeze or distress

## Pregnancy and breastfeeding

Pregnancy is a contraindication to methacholine testing.

## Memory aid

METHAcholine = «METHod to test asthma»: it is a diagnostic, never a treatment.

## Test yourself

Think of the answer, then tap the question to see it.

## What is methacholine used for?

Only for the bronchial challenge test that detects airway hyperresponsiveness, as in asthma.

## Which medicine must be ready during the test?

An inhaled bronchodilator (beta-2 agonist), with oxygen and resuscitation equipment.

## How does methacholine differ chemically from acetylcholine?

It has an extra β-methyl group, which makes it more resistant to cholinesterase and more muscarinic.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/methacholine/)

## Related drugs

- [Bethanechol](https://durgical.com/en/pharmacology/bethanechol/)

## Related conditions

- [Asthma](https://durgical.com/en/conditions/asthma/)

[← Back to the class: Muscarinic (cholinergic) agonists](https://durgical.com/en/pharmacology/classes/muscarinic-agonists/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 11 (McGraw Hill, 2023); [Methacholine Challenge Test](https://www.lung.org/lung-health-and-diseases/lung-procedures-and-tests/methacholine-challenge-test.html) (American Lung Association, 2024); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=V04CX03) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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