Learning Objective
By the end of this section, students should be able to identify key muscarinic antagonists, their target organ systems, and their clinical applications, including high-yield USMLE mnemonics.
Muscarinic Antagonists (Antimuscarinics)
Clinical caution: These drugs can cause dry mouth, blurred vision, constipation, urinary retention, and tachycardia.
| Drug/Class | Target Organ System | Clinical Application | Mnemonic / Tip |
|---|---|---|---|
| Atropine, Homatropine, Tropicamide | Eye | Produce mydriasis (pupil dilation) and cycloplegia (paralysis of the ciliary muscle) | “Eye drops for eye exams” |
| Benztropine, Trihexyphenidyl | CNS | Parkinson’s disease, acute dystonia | “Park my Benz.” |
| Glycopyrrolate | GI, Respiratory | Parenteral: reduce airway secretions preoperatively; Oral: reduce drooling, treat peptic ulcers | Does not cross the BBB |
| Hyoscyamine, Dicyclomine | GI | Antispasmodics for irritable bowel syndrome | High-yield for GI Step 1 questions |
| Ipratropium, Tiotropium | Respiratory | COPD, asthma; note: tiotropium > ipratropium in duration of action | Inhaled therapy |
| Solifenacin, Oxybutynin, Flavoxate, Tolterodine | Genitourinary | Reduce bladder spasms, treat overactive bladder | “Make bladder SOFT” (Soft = relax) |
| Scopolamine | CNS | Motion sickness | Patch behind the ear or oral prophylaxis |
Step 1 High-Yield Pearls
- CNS-targeting antimuscarinics: Benztropine, Trihexyphenidyl, Scopolamine
- Eye-targeting antimuscarinics: Atropine, Tropicamide, Homatropine
- Respiratory antimuscarinics: Ipratropium (short-acting), Tiotropium (long-acting)
- GU antimuscarinics: Solifenacin, Oxybutynin → overactive bladder
- Mnemonic for antimuscarinic effects: Hot as a hare, dry as a bone, blind as a bat, red as a beet, mad as a hatter
