Anticholinergics (Atropine)

Anticholinergics do the opposite of cholinergic agonists: they competitively block muscarinic receptors, so acetylcholine can’t deliver its "rest-and-digest" message. With the parasympathetic brakes off, the sympathetic effects dominate: the heart speeds up, pupils dilate, secretions dry up, and smooth muscle in the gut and bladder relaxes.

That predictable set of effects is turned into a range of uses. Atropine speeds a dangerously slow heart (symptomatic bradycardia) and dries secretions before surgery; oxybutynin/tolterodine calm an overactive bladder; scopolamine prevents motion sickness; benztropine eases Parkinsonian tremor and drug-induced EPS; and ipratropium (on the Respiratory page) dilates airways.

Atropine is a two-way antidote worth memorizing. Because it blocks muscarinic receptors, it reverses cholinergic excess — it is the antidote for organophosphate/nerve-agent poisoning and for cholinergic-crisis overdose (paired with pralidoxime for organophosphates). And by removing vagal tone, it raises a slow heart rate in symptomatic bradycardia.

Given too much anticholinergic, you get the classic antimuscarinic toxidrome, remembered as a rhyme: "blind as a bat" (dilated pupils, blurred vision), "dry as a bone" (no sweat/saliva), "red as a beet" (flushed), "hot as a hare" (hyperthermia — can’t sweat), "mad as a hatter" (confusion, delirium), plus "can’t see, can’t pee, can’t spit, can’t poop" (urinary retention, dry mouth, constipation) and tachycardia. The antidote is physostigmine.

The side effects define the contraindications. Blocking the pupil’s drainage worsens narrow-angle glaucoma. Relaxing the bladder worsens urinary retention in benign prostatic hyperplasia (BPH). And because older adults are especially sensitive to the confusion, falls, and retention, anticholinergics are prominent on the Beers list of drugs to avoid in the elderly. Heat illness is a real risk too — without sweating, the body can’t cool itself.