Muscarinic Antagonists (Anticholinergics): Atropine & Friends
If cholinergic agonists switch rest-and-digest ON, the anticholinergics switch it OFF — and that single move can restart a dangerously slow heart, open a wheezing chest, calm an overactive bladder, or stop motion sickness. But push too far and you get a patient who is 'blind, mad, red, hot, and dry.' One class of drugs, a dozen uses, and one unforgettable toxic picture.
A patient in the emergency department has a heart crawling along at 35 beats a minute; he's pale, dizzy, and on the edge of collapse. The team pushes a dose of atropine into his vein, and within a minute the monitor climbs — 50, 60, 75 — as his blood pressure recovers and the color returns to his face. Atropine didn't stimulate his heart. It did the opposite of stimulation: it BLOCKED the parasympathetic brake (the vagus nerve's M2 signal), and with the brake released, the heart's own pacemaker sped back up. That single idea — block the brake — runs through every anticholinergic drug.
The mirror image of SLUDGE
Anticholinergics block muscarinic receptors. By occupying the muscarinic receptor without activating it, these drugs shut down parasympathetic tone and let the sympathetic side show through. So their effects are the exact opposite of the cholinergic (SLUDGE) picture. In the EYE: the pupil dilates (mydriasis) and near-focus is lost (cycloplegia). HEART: rate rises. LUNGS: airways widen and secretions dry up. GUT & BLADDER: motility slows and the bladder relaxes (risking retention). GLANDS: less saliva, tears, and sweat, so the skin is dry and, without sweating, can overheat. And in the BRAIN: confusion, restlessness, even hallucinations. Learn this anti-SLUDGE list once and every drug and toxicity below follows from it.
The drugs, organized by where they act
Anticholinergics are best learned by their target organ. Heart: atropine raises the rate in bradycardia and heart block. Lungs: inhaled ipratropium and tiotropium are bronchodilators for COPD and asthma, drying and widening the airways with little body-wide effect. Bladder: oxybutynin, tolterodine, and solifenacin calm an overactive bladder by relaxing the detrusor muscle. Eye: tropicamide and cyclopentolate dilate the pupil for an eye exam. Gut: dicyclomine and hyoscyamine ease intestinal cramps (antispasmodics). Brain/motion: scopolamine (a patch) prevents motion sickness and nausea; benztropine and trihexyphenidyl treat Parkinsonian tremor and drug-induced movement effects. Secretions: glycopyrrolate dries secretions before surgery without entering the brain.
In COPD, overactive parasympathetic tone keeps the airways tight and mucus-filled. Inhaled ipratropium blocks the M3 receptors right there in the airway, relaxing the bronchial muscle and reducing secretions — a bronchodilator that works by REMOVING a constricting signal. Delivering it by inhaler keeps it in the lungs, avoiding the dry mouth and racing heart that a whole-body dose would cause.
- Anticholinergics block muscarinic receptors → the opposite of SLUDGE (anti-SLUDGE).
- Atropine → bradycardia & organophosphate poisoning; ipratropium/tiotropium → COPD/asthma.
- Oxybutynin/tolterodine → overactive bladder; scopolamine → motion sickness.
- Effects: mydriasis, cycloplegia, tachycardia, bronchodilation, dry mouth, urinary retention, confusion.
The anticholinergic toxidrome
Push muscarinic blockade too far — an overdose, too many anticholinergic drugs stacked together, or eating a plant like jimsonweed or deadly nightshade (belladonna) — and you get a classic, examinable toxidrome summed up in a rhyme: 'Blind as a bat' (dilated pupils, blurred near vision), 'Mad as a hatter' (agitation, delirium, hallucinations), 'Red as a beet' (flushed skin), 'Hot as a hare' (fever from no sweating), 'Dry as a bone' (dry mouth and skin), and 'Full as a flask' (urinary retention) — all with a racing heart. In the elderly especially, even ordinary doses of everyday anticholinergic drugs can quietly cause confusion and falls, a problem called anticholinergic burden.
The antidote to severe anticholinergic poisoning is physostigmine — the one cholinesterase inhibitor that crosses into the brain. By raising acetylcholine, it out-competes the blockade and reverses both the peripheral and the central (delirium) signs. Notice the beautiful symmetry of this whole section: atropine reverses cholinergic poisoning, and physostigmine reverses anticholinergic poisoning. Each class is the other's antidote.
- Giving an anticholinergic in narrow-angle glaucoma. Pupil dilation can trigger acute glaucoma.
- Using one in prostate enlargement (BPH). It worsens urinary retention.
- Overlooking anticholinergic burden in the elderly — confusion and falls from stacked drugs.
- Forgetting the skin is DRY (not sweaty) — a key clue separating it from other hyperthermias.
A patient presents with dilated pupils, dry hot skin, urinary retention, a racing heart, and delirium. This toxidrome is:
- Anticholinergics block muscarinic receptors, producing the opposite of SLUDGE.
- Uses: bradycardia (atropine), COPD (ipratropium), overactive bladder, motion sickness, eye exams.
- Toxidrome: blind, mad, red, hot, dry + tachycardia; antidote is physostigmine.
- Avoid in narrow-angle glaucoma, BPH, and mind anticholinergic burden in the elderly.
- Katzung BG. Basic & Clinical Pharmacology — Cholinoceptor-blocking drugs (muscarinic antagonists).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Muscarinic receptor antagonists.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Muscarinic antagonists.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Antimuscarinic (anticholinergic) drugs.
- Clinical toxicology references — Anticholinergic toxidrome & physostigmine.
