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Antimuscarinics (Muscarinic Antagonists)

Antimuscarinics block acetylcholine at muscarinic receptors and therefore inhibit parasympathetic activity.

They produce predictable organ effects — remembering the physiology is more important than memorizing drugs.

Antimuscarinics = “Turn OFF rest-and-digest” and “Sympathetic-Like”

Muscarinic Receptors

Receptor	Location	Normal Function
M1	CNS, gastric parietal cells	Cognition, acid secretion
M2	Heart	↓ Heart rate
M3	Smooth muscle & glands	Contraction & secretion

Blocking these receptors produces opposite physiologic effects.

Core Physiologic Effects

Dry as a bone → ↓ secretions Blind as a bat → mydriasis + cycloplegia Hot as a hare → ↓ sweating Red as a beet → vasodilation Mad as a hatter → delirium

Organ System Effects

Organ	Parasympathetic Action	Antimuscarinic Effect
Eye	Constrict pupil	Mydriasis
Heart	Slow heart	Tachycardia
Lungs	Bronchoconstriction	Bronchodilation
GI	Motility & secretion	Constipation
Bladder	Detrusor contraction	Urinary retention
Glands	Salivation	Dry mouth

Drug Groups

Respiratory (Inhaled)

Bladder Antispasmodics

Oxybutynin
Tolterodine
Solifenacin

Ophthalmic

Atropine
Tropicamide

GI Antispasmodics

Dicyclomine
Hyoscyamine

CNS

Benztropine
Scopolamine

Adverse Effects

All antimuscarinics share the same predictable toxicity:

Dry mouth
Constipation
Urinary retention
Blurred vision
Tachycardia
Delirium (elderly)

Contraindications

Narrow-angle glaucoma
Urinary retention / BPH
Elderly with cognitive impairment

Clinical Pearls

If a symptom involves excessive secretion or smooth muscle contraction → antimuscarinic drugs treat it.

COPD bronchodilation
Overactive bladder
Motion sickness
Organophosphate poisoning antidote (atropine)

Related Systems

Respiratory Antimuscarinics
GI antispasmodics
Bladder antimuscarinics