Drug Mnemonic

Oxybutynin

ANTICHOLINERGIC · M3 BLOCKER · OVERACTIVE BLADDER
March 30, 2026

A story built from the name oxybutynin itself - so the mechanism of action, key side effects, and the way Step 1 actually tests it stick on the first read.

RxMnemonic Anticholinergics
Oxybutynin
ANTICHOLINERGIC · M3 BLOCKER · OVERACTIVE BLADDER
OXY (3 letters) + BUTYNIN = butting in
Separate oxybutynin into oxy (sounds like OX) and butynin (sounds like BUTTING IN). Picture an OX sitting at the bottom of a water slide, completely BUTTING IN and BLOCKING the water - exactly like what oxybutynin does to urine in the bladder. A confused med student stares and mutters “Umm... M?” Our baby ox is 3 years old (they get big fast), because OXY has 3 letters. The M from “Umm... M?” + the 3 from OXY + the BUTTING IN from butynin = M3 blocker.
Oxybutynin pharmacology mnemonic illustration
M3 blocker OXY = 3 letters = M3
HOW STEP TESTS IT
1
Treats overactive bladder - blocks M3 on the detrusor muscle, relaxing it so it can't contract and no urine flows out.
2
Contraindicated in narrow-angle glaucoma - anti-cholinergic dilation of the pupil blocks aqueous drainage, spiking intraocular pressure.
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Practice Question

A patient starts oxybutynin for overactive bladder. Which adverse effect is most likely?

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Pharmacology Summary

Oxybutynin: Mechanism, Uses & High-Yield Facts for USMLE Step 1

Oxybutynin is an antimuscarinic that selectively blocks M3 receptors on the detrusor muscle, reducing bladder contractions in overactive bladder and urge incontinence. Classic anticholinergic side effects apply: dry mouth, constipation, urinary retention, and blurred vision. It is contraindicated in narrow-angle glaucoma because pupillary dilation from M3 blockade obstructs aqueous drainage and raises intraocular pressure.

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