Antimuscarinics / AntiCholinergics (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” → Sympathetic-like effects

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

Major Clinical Uses

Bronchodilation

Overactive Bladder

GI Antispasmodic

Motion Sickness / CNS

Ophthalmic Mydriasis

Adverse Effects

All antimuscarinics share predictable toxicity:

Contraindications

Clinical Pearls

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

Common real-world uses: