Anticholinergic drugs (more precisely, antimuscarinic drugs) competitively block acetylcholine at muscarinic receptors. They remove parasympathetic "rest and digest" tone, so the organ effects are the opposite of cholinergic drugs.

Brain entry: tertiary amines (atropine, scopolamine, benztropine, oxybutynin, diphenhydramine) cross the blood–brain barrier and can cause confusion. Quaternary amines (glycopyrrolate, ipratropium, tiotropium, trospium) cross poorly and have fewer central effects.

Many drugs from other classes are strongly anticholinergic: first-generation antihistamines (diphenhydramine), tricyclic antidepressants, some antipsychotics, and some muscle relaxants. Their effects add up — the anticholinergic burden.

Names: atropine and glycopyrrolate have the same names worldwide. Scopolamine is called hyoscine in many countries; hyoscine butylbromide is a different, quaternary antispasmodic (little CNS effect) widely sold for abdominal cramps.

Memory aid for anticholinergic effects:

• Dry as a bone — dry mouth, dry skin, no sweating
• Red as a beet — flushed skin (skin vessels dilate to lose heat)
• Hot as a hare — hyperthermia, especially in children and in hot weather
• Blind as a bat — dilated pupils, blurred vision, photophobia
• Mad as a hatter — confusion, agitation, hallucinations, delirium
• Full as a flask — urinary retention
• Plus tachycardia, constipation and ileus, decreased bowel sounds

Older adults are most vulnerable: delirium, falls, constipation and fecal impaction, urinary retention (especially men with prostate enlargement), and possibly faster cognitive decline with long-term strong anticholinergic use. The AGS Beers Criteria (2023) advise avoiding strongly anticholinergic drugs in older adults when possible and avoiding combinations of two or more.

Children are at special risk of hyperthermia and toxicity from eye drops and patches.

Contraindications

• Angle-closure (narrow-angle) glaucoma — pupil dilation blocks aqueous outflow and can cause an acute attack
• Urinary retention or bladder outlet obstruction (e.g., benign prostatic hyperplasia)
• GI obstruction, paralytic ileus, severe ulcerative colitis or toxic megacolon, gastric retention
• Myasthenia gravis — a listed contraindication for several agents (e.g., glycopyrrolate, oxybutynin); atropine may still be used to counter cholinesterase-inhibitor effects
• Tachydysrhythmias and unstable cardiovascular status (e.g., acute hemorrhage); caution in coronary disease, heart failure, and hyperthyroidism
• Hot environments and fever — risk of heat stroke

Pregnancy and lactation: atropine and glycopyrrolate are used when clearly needed (for example, in anesthesia). Routine antimuscarinics for bladder or GI symptoms are generally avoided unless the benefit is clear. Anticholinergics can reduce milk supply; choose the lowest effective dose and monitor the infant for irritability, constipation, or poor feeding.

Interactions

• Additive anticholinergic burden with diphenhydramine, other first-generation antihistamines, tricyclic antidepressants, phenothiazines, clozapine, antiparkinson drugs, and some muscle relaxants
• Cholinesterase inhibitors (donepezil) oppose anticholinergics — do not treat donepezil-related urgency by adding oxybutynin without review
• Opioids — additive constipation and urinary retention
• Slowed gastric emptying changes absorption of other drugs; solid potassium chloride tablets can cause GI ulceration
• Sedatives and alcohol add to CNS depression (scopolamine)

Listed in priority order.

1. Cardiovascular and airway
   • Atropine for bradycardia: continuous ECG, heart rate, BP, and perfusion; prepare transcutaneous pacing if the rhythm does not respond
   • Watch for excessive tachycardia and chest pain in clients with coronary disease
   • Thick secretions: anticholinergics dry secretions but can make mucus harder to clear — assess lung sounds and cough
2. Mental status and safety
   • Baseline and ongoing orientation; screen older adults for delirium; fall precautions; review the medication list for anticholinergic burden and suggest alternatives to the prescriber
3. Elimination
   • Intake and output, bladder distension and post-void residual, bowel sounds, and bowel movement pattern; report no voiding within the expected time or no stool for several days
4. Temperature and skin
   • Monitor temperature; prevent overheating (cool environment, fluids unless restricted); dry skin care
5. Eyes
   • Ask about glaucoma history before giving; report eye pain, halos, or a red eye (possible angle closure)
6. Mouth — frequent oral care, sugar-free gum or candy, saliva substitutes

• Dry mouth: sip water, use sugar-free gum or lozenges, and keep up dental checks (less saliva → more cavities)
• Constipation: increase fiber and fluids unless restricted; report no bowel movement for 3 or more days or abdominal pain
• Urination: void before each dose if possible; report difficulty starting the stream or inability to void
• Heat: avoid strenuous exercise in hot weather, saunas, and hot tubs — you may not sweat enough to cool down
• Vision: wear sunglasses; do not drive until blurred vision clears; report eye pain or seeing halos at once
• Confusion: family should report new confusion, agitation, or hallucinations, especially in older relatives
• Scopolamine patch: apply behind one ear on clean, dry, hairless skin; replace every 3 days if needed; wash hands after handling; remove before MRI; dispose of by folding sticky sides together; drowsiness — avoid alcohol and driving
• Over-the-counter caution: many sleep aids and allergy or cold products contain diphenhydramine or similar drugs; check with a pharmacist before combining

Anticholinergic toxidrome: hyperthermia, flushed dry skin, dilated pupils, tachycardia, urinary retention, absent bowel sounds, agitation, hallucinations, delirium, and in severe cases seizures and coma. Dry skin distinguishes it from sympathomimetic toxicity (which causes sweating).

Common causes: overdose of diphenhydramine or other antihistamines, tricyclic antidepressants, antipsychotics, eye drops or patches in young children, and plants such as jimsonweed (Datura, "angel's trumpet").

Management

1. Airway, breathing, circulation; continuous ECG — a wide QRS suggests a tricyclic or diphenhydramine sodium-channel effect (treated with sodium bicarbonate)
2. Cooling for hyperthermia; IV fluids
3. Benzodiazepines for agitation and seizures
4. Urinary catheter for retention
5. Activated charcoal only if early after ingestion and the airway is protected, per poison center advice
6. Physostigmine (antidote) for severe central toxicity with delirium, in a monitored setting: slow IV, with atropine at the bedside to treat excess cholinergic effects (bradycardia, bronchorrhea). Avoid when a tricyclic antidepressant is involved or the QRS is wide (risk of asystole and seizures), and in asthma or GI/urinary obstruction. Its effect is short, so symptoms can return

Medication errors to prevent: confusing atropine eye drop strength with systemic doses; leaving old scopolamine patches on the skin; double anticholinergic therapy after a new prescriber adds a second drug.

• Anticholinergic = dry, hot, red, blind, mad, full, plus tachycardia and constipation
• Contraindicated in angle-closure glaucoma, urinary retention/BPH, GI obstruction, myasthenia gravis, tachydysrhythmias
• Atropine 1 mg IV every 3–5 minutes, maximum 3 mg for symptomatic bradycardia; prepare pacing for high-grade block
• Atropine is the antidote for cholinergic (organophosphate) poisoning; physostigmine is the antidote for anticholinergic delirium
• Glycopyrrolate and ipratropium do not enter the brain much — fewer CNS effects
• Older adults: avoid strong anticholinergics (Beers) — delirium, falls, retention, constipation
• Scopolamine patch: behind the ear, 4 hours before travel, wash hands, remove before MRI
• Teach heat avoidance — anticholinergics impair sweating
• Benztropine treats acute dystonia and parkinsonism, not tardive dyskinesia
• Anticholinergic toxicity = dry skin; sympathomimetic toxicity = sweaty skin