Antimuscarinics
Blocking muscarinic receptors, so parasympathetic effects are switched off.
Also called muscarinic antagonists and muscarinic receptor antagonists.
Class card
- Drugs
- Atropine, Hyoscine butylbromide and Oxybutynin
- Used for
- Slow heart rate (symptomatic bradycardia)
- Overactive bladder
- Spasm of the gut and urinary tract
- COPD (inhaled forms such as tiotropium)
- Known for
- Dry mouth
- Difficulty passing urine or urinary retention
- Name stems
- -trop-
4 cards: the class, and each drug in it.
Mechanism of action
How do antimuscarinics work?
They occupy muscarinic acetylcholine receptors without activating them, competitively blocking the acetylcholine released by parasympathetic nerves. The heart speeds up, glands dry up, the smooth muscle of the gut, bladder and airways relaxes, and the pupil dilates and stops focusing. Tertiary amines such as atropine enter the brain; quaternary ones such as hyoscine butylbromide do not.
The drugs
Antimuscarinics with a card
- AtropineThe antidote for the muscarinic effects of organophosphate poisoning. Small doses can briefly slow the heart before the typical speeding begins.
- Hyoscine butylbromideNot the same as hyoscine hydrobromide, which is used for travel sickness and commonly causes drowsiness; the butylbromide stays out of the brain.
- OxybutyninWhen bladder antimuscarinics do not suit, mirabegron, a beta-3 agonist, relaxes the bladder in a different way.
Uses
What are antimuscarinics used for?
- Slow heart rate (symptomatic bradycardia)
- Overactive bladder
- Spasm of the gut and urinary tract
- COPD (inhaled forms such as tiotropium)
- Organophosphate and nerve agent poisoning
- Drying saliva and airway secretions
Side effects
Side effects of antimuscarinics
- Dry mouthKnown for
- Difficulty passing urine or urinary retentionKnown forSerious
- Constipation
- Blurred vision and dazzle from a dilated pupil
- Fast heart rate
- Less sweating, so the body overheats
- Flushed skin
- Confusion, agitation or delirium
- Acute angle-closure glaucomaSerious
- Anticholinergic poisoning in overdoseSerious
What the class has in common. Each drug's card has its own, and Known for marks what an exam expects you to know.
Watch for
Cautions
- Narrow-angle glaucoma: they can set off an acute attack.
- Enlarged prostate or bladder outflow obstruction: risk of complete urinary retention.
- Dementia: confusion and worse cognition; the effects add up across anticholinergic drugs.
- Myasthenia gravis and bowel obstruction or ileus: they can make both worse.
Remember them
A way to remember them
Hot as a hare, dry as a bone, red as a beet, blind as a bat, full as a flask.
The classic signs of too much antimuscarinic effect: overheating because sweating stops, a dry mouth and skin, flushing, blurred vision from a wide pupil that cannot focus, and a full bladder from urinary retention. Confusion often comes with them.
A mnemonic widely used in teaching
Name stems
-trop- means atropine-like antimuscarinic
Marks atropine relatives. Ipratropium and tiotropium add -ium: as quaternary ammonium compounds they are barely absorbed and act in the airways. Somatropin, a growth hormone, fits by chance.
- Every drug name stem
Don't mix them up
Classes easily confused with these
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Atropine sulfate injection: label (DailyMed)
- Oxybutynin chloride extended-release tablets: label (DailyMed)
- Tiotropium bromide inhalation powder: label (DailyMed)
- Antimuscarinic agent (NCI Thesaurus)
- Hyoscine butylbromide 20 mg/ml solution for injection: SmPC (emc)
- Common questions about oxybutynin (NHS)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Autonomic nervous system
Practice antimuscarinics with the other classes in this group: what each drug is, how it works, what it treats and what it's known for.
Start Practicing