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.
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. Step pairs this with any anticholinergic.
PRACTICE QUESTION
A man with GERD starts an H2 blocker and later develops gynecomastia. Which drug is most likely responsible?
A) Famotidine
B) Omeprazole
C) Cimetidine ✓
D) Sucralfate
Answer: C – Cimetidine. Unlike other H2 blockers, cimetidine has antiandrogenic effects and can cause gynecomastia.
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