Cholinergic and anticholinergic toxidromes compared — antidotes and neuromuscular reversal
Verified · Oct 2026Too much acetylcholine wets, slows and weakens; blocked muscarinic receptors dry, speed and confuse. The antidotes either block the excess at muscarinic receptors, restore or inhibit acetylcholinesterase, or — at the neuromuscular junction — remove the blocking drug itself.
One synapse, opposite poisonings
Acetylcholinesterase ends transmission by breaking acetylcholine down in the synaptic cleft. Cholinesterase inhibitors protect acetylcholine from hydrolysis and raise its level and duration: the carbamates (physostigmine, neostigmine, pyridostigmine) inhibit it reversibly, whereas organophosphate insecticides and nerve agents phosphorylate the enzyme. The excess acetylcholine acts at muscarinic and nicotinic receptors, so organophosphate poisoning has muscarinic, nicotinic and CNS effects.
Antimuscarinics competitively block acetylcholine at muscarinic receptors but cause minimal blockade at nicotinic sites, and muscarinic receptors are not found at the neuromuscular junction — so atropine treats the muscarinic effects of organophosphate poisoning but not the muscle paralysis. Pralidoxime reactivates the phosphorylated enzyme, but only before aging makes the bond irreversible. Physostigmine crosses the blood–brain barrier and reverses central anticholinergic effects; quaternary neostigmine does not. At the neuromuscular junction, sugammadex encapsulates rocuronium or vecuronium so that less blocker is available at nicotinic receptors.
Comparison
The two toxidromes side by side
| Feature | Cholinergic excess (organophosphate or cholinesterase-inhibitor poisoning) | Anticholinergic toxidrome |
|---|---|---|
| Typical causes | Organophosphate insecticides and nerve agents (irreversible); carbamates; overdose of a therapeutic cholinesterase inhibitor such as pyridostigmine (cholinergic crisis) | Belladonna plants and jimson weed (atropine, scopolamine); tricyclic antidepressants, atypical antipsychotics and first-generation antihistamines |
| Pupils and vision | Miosis (mydriasis at times, from nicotinic stimulation of sympathetic ganglia) | Mydriasis and photophobia, blurred vision |
| Secretions, sweat and skin | Salivation, lacrimation, sweating, bronchorrhea | Dry mouth and dry mucous membranes; no sweating, with flushed skin and fever |
| Heart rate | Bradycardia (tachycardia at times, from ganglionic and adrenal nicotinic stimulation) | Tachycardia |
| Airway | Bronchospasm and bronchorrhea; death mostly from respiratory failure | Reduced bronchial secretions; bronchodilation |
| Gut and bladder | Diarrhea, cramping, emesis, urinary frequency | Decreased bowel sounds, constipation or ileus; urinary retention |
| Skeletal muscle | Fasciculations, then weakness or flaccid paralysis from depolarizing block | No paralysis — muscarinic receptors are not found at the neuromuscular junction; jerking movements are possible |
| Mental status | CNS depression, coma and seizures | Delirium, hallucinations, agitation, confusion; seizures possible |
| Classic mnemonic | SLUDGEM: salivation, lacrimation, urinary frequency, diarrhea, GI cramping, emesis, miosis | “Red as a beet, dry as a bone, blind as a bat, mad as a hatter, hot as a hare, full as a flask” |
| Antidote | Atropine for the muscarinic effects; pralidoxime, before aging, for the nicotinic weakness | Supportive care; IV benzodiazepines for agitation; physostigmine for severe, pure anticholinergic poisoning with central and peripheral signs |
Antidotes and reversal agents
| Agent | Mechanism | Used for | Crosses the blood–brain barrier? | Key limit or caution |
|---|---|---|---|---|
| Atropine | Competitive (surmountable) muscarinic antagonist | Organophosphate or muscarinic mushroom poisoning; bradycardia from anticholinesterase drugs; physostigmine excess | Yes — a non-polar tertiary amine | Treats muscarinic, not nicotinic, effects — it does not relieve paralysis |
| Glycopyrrolate | Competitive muscarinic antagonist (quaternary ammonium) | Given with neostigmine to blunt its muscarinic effects; preoperative antisialagogue | Limited — fewer CNS side effects | Given simultaneously with neostigmine (similar onset); may be mixed in the same syringe |
| Pralidoxime | Reactivates organophosphate-phosphorylated cholinesterase | Organophosphate poisoning, with atropine | Acts mainly outside the CNS | Only before aging; atropine is always required with it; not for carbamate insecticides (may increase carbaryl toxicity) |
| Physostigmine | Reversible cholinesterase inhibitor (carbamate), active centrally and peripherally | Severe anticholinergic toxicity with both central and peripheral signs | Yes | In general, pure anticholinergic poisoning only; with tricyclic overdose only under continuous ECG monitoring (cardiac arrest possible); atropine ready at the bedside |
| Neostigmine (with an antimuscarinic) | Inhibits cholinesterase, so acetylcholine competes with a nondepolarizing blocker | Reversal of nondepolarizing neuromuscular blockade | No — quaternary ammonium | Give atropine or glycopyrrolate before or with it; give after some spontaneous recovery; paradoxical anticholinesterase-associated weakness |
| Sugammadex | Modified gamma cyclodextrin that encapsulates rocuronium or vecuronium | Reversal of rocuronium or vecuronium blockade, moderate or deep | — | Not for succinylcholine or benzylisoquinolinium blockers; no muscarinic effects; recurrence if other drugs displace the blocker; additional non-hormonal contraception for 7 days |
— = not stated on the source pages.
High-yield
- S — Salivation
- L — Lacrimation
- U — Urinary frequency
- D — Diarrhea
- G — Gastrointestinal cramping and pain
- E — Emesis
- M — Miosis
- Red as a beet — flushing
- Dry as a bone — anhidrosis and dry mucous membranes
- Blind as a bat — mydriasis
- Mad as a hatter — altered mental status
- Hot as a hare — fever
- Full as a flask — urinary retention
Practice questions
Q1A 29-year-old woman is brought in after an intentional overdose of amitriptyline. She is agitated and confused, with flushed dry skin, dilated pupils, a heart rate of 128/min and absent bowel sounds. Her ECG shows a wide-complex tachycardia.
Which treatment addresses the ECG finding?
- A.Physostigmine
- B.Atropine
- C.Pralidoxime
- D.IV sodium bicarbonate
- E.IV benzodiazepine
Show answer
D. IV sodium bicarbonate — Tricyclic antidepressants such as amitriptyline have anticholinergic properties and can produce the anticholinergic toxidrome. If wide-complex dysrhythmias develop, IV sodium bicarbonate should be given; IV benzodiazepines are the first-line therapy for agitation, not for the rhythm. In general, physostigmine is recommended only for pure anticholinergic poisoning, and with tricyclic antidepressants acute cardiac arrest may be possible, so it is considered only under continuous ECG monitoring. Atropine is itself a muscarinic antagonist, and pralidoxime reactivates cholinesterase inhibited by organophosphates.
Q2A 17-year-old is delirious and hallucinating after eating jimson weed seeds, with hot, dry, flushed skin and dilated pupils. Physostigmine is given slowly IV. Minutes later his heart rate is 42/min and he is drooling and vomiting.
Which drug reverses these new effects?
- A.Pralidoxime
- B.Neostigmine
- C.IV sodium bicarbonate
- D.Sugammadex
- E.Atropine
Show answer
E. Atropine — Jimson weed contains the muscarinic antagonists atropine and scopolamine, and physostigmine, a cholinesterase inhibitor active in the central and peripheral nervous systems, is its antidote. Physostigmine overdose can cause bradycardia, hypersalivation and vomiting, and IV atropine is given until the symptoms normalize — which is why atropine should be ready before physostigmine is used. Neostigmine is another cholinesterase inhibitor and would add to the excess. Pralidoxime reactivates cholinesterase phosphorylated by organophosphates; its most critical effect is relieving paralysis of the respiratory muscles, and atropine is always required alongside it to block the accumulated acetylcholine. Sodium bicarbonate is for wide-complex dysrhythmias, and sugammadex binds only rocuronium and vecuronium.
Q3At the end of a laparoscopy that finished earlier than planned, a patient paralyzed with rocuronium still has a deep neuromuscular blockade with no spontaneous recovery. The team wants to reverse it now.
Which agent can reverse this block?
- A.Neostigmine with glycopyrrolate
- B.Atropine alone
- C.Sugammadex
- D.Dantrolene
- E.Pralidoxime
Show answer
C. Sugammadex — Sugammadex is a modified gamma cyclodextrin that encapsulates rocuronium or vecuronium, leaving less blocker available at nicotinic receptors, and it can quickly reverse both moderate and deep blockade. Neostigmine works by letting acetylcholine compete with the blocker and must be given after a relative degree of spontaneous recovery. Atropine blocks muscarinic receptors, which are not found at the neuromuscular junction. Dantrolene acts beyond the myoneural junction, on excitation–contraction coupling, and pralidoxime reactivates organophosphate-inhibited cholinesterase.
Q4A 52-year-old man is transferred several days after drinking an organophosphate insecticide. He received atropine at the first hospital; pralidoxime is now started, but his red blood cell cholinesterase activity does not recover.
What best explains why pralidoxime fails?
- A.The phosphorylated enzyme has aged into a form that can no longer be reactivated
- B.Pralidoxime acts mainly within the central nervous system
- C.Pralidoxime works by blocking muscarinic receptors rather than restoring the enzyme
- D.Atropine given first inactivated the pralidoxime
- E.Red blood cell cholinesterase does not reflect organophosphate toxicity
Show answer
A. The phosphorylated enzyme has aged into a form that can no longer be reactivated — The organophosphate–enzyme bond can undergo aging, after which acetylcholinesterase can no longer be regenerated and de novo synthesis is needed; pralidoxime must be given before aging, a time frame specific to each compound. Pralidoxime reactivates cholinesterase mainly outside the central nervous system; it is atropine that competes with acetylcholine at muscarinic receptors, and atropine must be given before pralidoxime. Red blood cell acetylcholinesterase activity correlates more strongly with organophosphate toxicity than plasma butyrylcholinesterase.
Sources
- Cholinergic Medications — StatPearls (NCBI)
- Organophosphate Toxicity — StatPearls (NCBI)
- Muscarinic Antagonists — StatPearls (NCBI)
- Anticholinergic Toxicity — StatPearls (NCBI)
- Neuromuscular Blocking Agents — StatPearls (NCBI)
- Atropine Sulfate Injection — FDA label — FDA / DailyMed
- Glycopyrrolate Injection — FDA label — FDA / DailyMed
- ANTICHOLIUM (physostigmine salicylate) — DailyMed label — FDA / DailyMed
- PROTOPAM Chloride (pralidoxime) — FDA label — FDA / DailyMed
- BRIDION (sugammadex) — FDA label — FDA / DailyMed
- Neostigmine Methylsulfate Injection — FDA label — FDA / DailyMed
- Neostigmine — quaternary ammonium, blood–brain barrier, elimination half-life — StatPearls (NCBI)
- Cholinergic Crisis — myasthenic vs cholinergic crisis, edrophonium test, nicotinic receptors — StatPearls (NCBI)
- MESTINON (pyridostigmine) — FDA label — FDA / DailyMed
- Transderm Scōp (scopolamine) — mechanism, blood–brain barrier, transdermal pharmacokinetics (FDA label) — FDA / DailyMed
- Cholinesterase Inhibitors — anionic and esteratic sites, reversible vs irreversible, pharmacokinetics — StatPearls (NCBI)
- Oxybutynin Chloride Tablets — antispasmodic vs anticholinergic activity, pharmacokinetics, CNS effects (FDA label) — FDA / DailyMed
- Phospholine Iodide (echothiophate) — long-acting inhibitor, plasma and red-cell cholinesterase, succinylcholine (FDA label) — FDA / DailyMed
- Depolarizing Neuromuscular Blocking Drugs — phase I and phase II block, pharmacokinetics — StatPearls (NCBI)
- Dantrium Intravenous (dantrolene) — excitation–contraction coupling, myoplasmic calcium in malignant hyperthermia (FDA label) — FDA / DailyMed
- Rocuronium bromide injection — competitive mechanism, hepatic elimination (FDA label) — FDA / DailyMed
- QUELICIN (succinylcholine chloride) — FDA label — FDA / DailyMed
- Cisatracurium besylate injection — Hofmann elimination, laudanosine (FDA label) — FDA / DailyMed
- Succinylcholine Chloride — receptor upregulation and hyperkalemia, bradycardia, BCHE/RYR1 — StatPearls (NCBI)
Each class page compared lists its full sources; this page reuses facts verified there.
By LearnDrugs
Educational summary for medical students. Always verify against current prescribing information and guidelines. Not medical advice.