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# Muscarinic (cholinergic) agonists

Drugs that act like acetylcholine on muscarinic receptors: they shrink the pupil, increase saliva and sweat, and contract the bladder and bronchi.

Updated 2026-11-01

## What they are

Direct-acting parasympathomimetics. Choline esters (methacholine, bethanechol, carbachol) are charged, stay out of the brain and resist breakdown by cholinesterase more than acetylcholine does; the natural alkaloid pilocarpine and the synthetic cevimeline are tertiary amines. Uses: glaucoma (pilocarpine eye drops), dry mouth in Sjögren disease or after head and neck radiotherapy (pilocarpine, cevimeline), urinary retention without obstruction (bethanechol) and the bronchial challenge test (methacholine). See the topic «Cholinergic transmission».

## Mechanism of action

They bind the acetylcholine pocket of muscarinic receptors and activate them. M3 receptors on glands, the iris sphincter, the ciliary muscle, the bladder detrusor and airway smooth muscle drive most of the useful effects; M2 receptors in the heart slow its rate. The β-methyl group of methacholine and bethanechol reduces their action at nicotinic receptors.

Target: Muscarinic acetylcholine receptors, mainly M3

## Key points

- Classic contraindications: asthma and COPD, bowel or urinary obstruction, peptic ulcer, bradycardia or hypotension, and hyperthyroidism (risk of atrial fibrillation).
- Their side effects are the cholinergic picture: sweating, salivation, tearing, cramps, diarrhoea, urinary urgency, small pupils, bronchospasm and slow heart rate. Atropine is the antidote.
- Pilocarpine opens the trabecular meshwork by contracting the ciliary muscle; it can blur vision in dim light and rarely cause retinal detachment.
- Bethanechol treats urinary retention only when there is no mechanical obstruction.
- Methacholine inhaled in rising doses tests for airway hyperresponsiveness when asthma is suspected.

## Drugs in this class

- [**Pilocarpine** A muscarinic agonist used as eye drops to lower eye pressure in glaucoma, and as tablets to relieve dry mouth in… Muscarinic M3 receptor](https://durgical.com/en/pharmacology/pilocarpine/)
- [**Bethanechol** A choline ester that contracts the bladder to treat urinary retention when nothing is blocking the outflow. Muscarinic M3 receptor of the bladder detrusor](https://durgical.com/en/pharmacology/bethanechol/)
- [**Methacholine** An inhaled muscarinic agonist used only as a diagnostic test: people with asthma narrow their airways at much lower… Muscarinic M3 receptor of airway smooth muscle](https://durgical.com/en/pharmacology/methacholine/)
- [**Cevimeline** A muscarinic agonist taken by mouth to increase saliva in people with dry mouth from Sjögren disease. Muscarinic M1 and M3 receptors](https://durgical.com/en/pharmacology/cevimeline/)

## Others in the class

- Carbachol (eye)
- Acetylcholine (eye surgery)

## Test yourself

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

## Why is bethanechol contraindicated in urinary retention caused by an enlarged prostate?

Contracting the bladder against a mechanical obstruction can damage the bladder wall; it is used only when there is no obstruction.

## Which antidote treats an overdose of a muscarinic agonist?

Atropine.

## Which receptor subtype mediates the effects of pilocarpine on salivary glands and the eye?

Mainly the M3 muscarinic receptor.

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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); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=N07AB) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/) (World Health Organization, 2025).

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