Antidotes for Common Poisons: A Poison-to-Antidote Master Table for NEET PG

Antidotes are easiest to learn as pairs: one poison, one antidote, one mechanism and one trap. This post puts the common pairs into grouped tables so you can revise them in one sitting, then explains the few regimens that need more than a table cell.

Doses below are typical adult regimens from standard references, and sources differ slightly on some of them. Where they do, the post says so instead of picking one silently.

Key Takeaways

• Organophosphate poisoning: atropine first (titrate to clear bronchorrhoea and bronchospasm), then pralidoxime.

• Paracetamol: N-acetylcysteine, best started within 8 hours, loading then 4-hour and 16-hour infusions.

• Opioids: naloxone. Benzodiazepines: flumazenil, which is contraindicated in seizure disorder and mixed overdose.

• Methanol and ethylene glycol: fomepizole (or ethanol) to block alcohol dehydrogenase, with haemodialysis as definitive treatment.

• Cyanide: hydroxocobalamin preferred. Iron: deferoxamine. Thallium: Prussian blue. Digoxin: digoxin-specific Fab.

• Metals: dimercaprol, CaNa2 EDTA, succimer or penicillamine depending on the metal. In severe lead poisoning, dimercaprol goes before EDTA.

• Indian snakebite: polyvalent anti-snake venom, plus a neostigmine trial for neurotoxic bites.

Once you have read the pairs below, test yourself with toxicology practice questions.

How to Use This Master Table of Antidotes

Read each table row as four questions: what blocks, binds or replaces the toxin, what is the typical dose, and what makes this antidote dangerous. The "caution" column is where most of the reasoning lives, so do not skip it.

Antidotes work in a few ways. Some compete at the receptor (naloxone, flumazenil), some block a toxic metabolite pathway (fomepizole, N-acetylcysteine), some bind the poison directly (chelators, Prussian blue, antibodies), and some restore a depleted substrate (pyridoxine, vitamin K). A useful revision habit is to group antidotes by mechanism rather than memorise a flat list.

An antidote never replaces supportive care. Airway, breathing, circulation and decontamination come first in most poisonings.

Antidotes for Pesticide and Cholinergic Poisoning

Organophosphate and carbamate poisoning is the classic antidote topic because it needs two drugs with different jobs. Atropine treats the muscarinic effects, and pralidoxime reactivates acetylcholinesterase that the organophosphate has inhibited (Merck Manual: Organophosphate and Carbamate Poisoning).

Atropine and pralidoxime in organophosphate poisoning

A 1 mg test dose that eases muscarinic signs supports the diagnosis, and the dose is then doubled every 3 to 5 minutes, sometimes reaching very large cumulative totals. Pralidoxime follows atropine, not the other way round. Benzodiazepines treat the seizures.

Do not treat pralidoxime as a sure thing. Rapid IV infusion can cause respiratory or cardiac arrest, it does not help once the enzyme has aged, and it penetrates the blood-brain barrier poorly. Systematic reviews of oxime therapy have shown no benefit or even possible harm (Chacko and Peter, Indian J Crit Care Med 2019). Dose schedules also vary: another reference gives 2 g over 20 minutes then 0.5 g/h.

A simple memory aid (our own, not a standard mnemonic): atropine dries the patient, pralidoxime frees the enzyme. If a stem asks what to titrate atropine against, the answer is secretions and bronchospasm, not the pupil.

For the receptor pharmacology behind this, see our notes on cholinergic drugs and organophosphate poisoning.

Specific Antidotes for Common Drug Overdoses

Drug overdose pairs are short to learn but each carries a contraindication. Learn the antidote and the one situation where giving it is harmful.

N-acetylcysteine for paracetamol poisoning

N-acetylcysteine is the drug in this section with the most timing detail to learn. It works best when started within 8 hours of ingestion, and one review advises starting ideally between 4 and 8 hours and not later than 14 hours. Another reference still recommends treatment beyond 24 hours in a patient with significant toxicity (Merck Manual: Acetaminophen Poisoning).

The treatment nomogram line is 150 mcg/mL at 4 hours: a level at or below this line in a patient with no symptoms makes hepatotoxicity very unlikely. It applies only to a single acute ingestion with a known time. The loading infusion runs over 15 minutes in the Merck protocol and over 60 minutes in some other references, so learn the three-bag pattern (150, 50, 100 mg/kg) rather than a single loading time.

Naloxone and flumazenil: two competitive antagonists

Both reverse their target drug quickly, and both can cause harm by doing so. Naloxone can precipitate withdrawal, and if there is no response after 10 mg, reconsider the diagnosis. A lower starting dose of 0.04 mg is used when dependence is suspected (Chacko and Peter, 2019).

Flumazenil can provoke seizures when benzodiazepines are masking a tricyclic antidepressant overdose, and in dependent patients it can cause withdrawal (Merck Manual: Symptoms and Treatment of Specific Poisons). Resedation is possible as it wears off.

Antidotes for Toxic Alcohols and Cyanide

Methanol, ethylene glycol and cyanide share one theme: the parent compound is not the main danger, so the antidote either blocks conversion or mops up the poison. Early treatment matters in all three.

Fomepizole schedules differ between references, with a longer schedule that steps the dose up after four doses and continues until ethanol falls below 20 mg/dL (Chacko and Peter, 2019). The principle is the same: block the enzyme early and keep blocking it until the toxin is cleared.

The cyanide kit sequence

The older kit works in a fixed order. Amyl nitrite is inhaled first, then 3% sodium nitrite 10 mL IV (given slowly), then 25% sodium thiosulfate 25 to 50 mL. Thiosulfate is slow and penetrates cells poorly, so it is usually considered when hypoxia persists despite maximum hydroxocobalamin.

Chelating Antidotes for Heavy Metal and Iron Poisoning

Chelators bind the metal and let the kidney or gut clear it. The choice depends on the metal, so learn the metal-to-chelator mapping first and doses second (Merck Manual: Chelation Therapy for Heavy Metal Poisoning).

Why dimercaprol goes before EDTA

In severe lead poisoning, dimercaprol should precede EDTA to avoid redistribution of lead to the brain (Chacko and Peter, 2019). Dimercaprol dosing differs between references, so the table gives the Merck schedule as a typical regimen.

Deferoxamine for iron overdose

Deferoxamine chelates free serum iron. IV therapy is reserved for severe poisoning, such as acidosis, shock, severe gastroenteritis or a serum iron above 500 mcg/dL (Merck Manual: Iron Poisoning).

The Prussian blue dose also differs: another review lists 20 g/day, so treat the figure above as one reference's regimen. Prussian blue works because it traps thallium in the gut, where about 99% is then excreted in faeces.

Antidotes for Anticoagulant, Digoxin and Other Reversals

These are reversal agents, so the useful skill is matching each agent to its drug. Match the drug to its reversal and note the one limitation.

Five more pairs are best learned as straight poison-to-antidote matches (Merck Manual: Common Specific Antidotes).

Digoxin Fab is not for asymptomatic patients, and digoxin levels read falsely high for at least 3 weeks after Fab therapy. That is a useful reasoning point if a stem shows a high level after treatment.

Snakebite: Anti-Snake Venom and the Neostigmine Trial

In India, the antidote for venomous snakebite is polyvalent anti-snake venom (ASV), and neurotoxic bites also get a neostigmine trial. The figures below come from one ICU series that cites the National Snakebite Management Protocol, so treat them as that series' regimen (Saini et al., IJCCM 2014).

Neostigmine likely does not work in presynaptic envenoming (krait, Russell's viper) but can work in postsynaptic envenoming (cobra). Antivenom has not been shown to reverse established presynaptic paralysis (Toxins 2023 review). Anaphylaxis can follow any amount of ASV, and routine premedication remains controversial (Lancet Reg Health SE Asia 2025).

The atropine given before neostigmine blocks the muscarinic side effects of neostigmine. Remember that sequence as "atropine first, then neostigmine".

Common Mistakes When Revising Antidotes

Most mistakes in this topic come from mixing near-neighbours. These are the ones to guard against.

• Pupils as the atropine endpoint. The endpoint is clearing of bronchorrhoea and bronchospasm.
• Flumazenil in any benzodiazepine overdose. It is unsafe in seizure disorder, mixed overdose and tolerance.
• Giving EDTA before dimercaprol in severe lead poisoning.
• Penicillamine for thallium. Prussian blue is the antidote, and penicillamine can move thallium into the CNS.
• Calling pralidoxime curative. Its benefit in trials is mixed.
• Forgetting that neostigmine depends on the venom type (postsynaptic versus presynaptic).

A good revision method is to cover the antidote column and recall it from the poison, then cover the caution column and recall it from the antidote. Repeat until the pairs come without prompting.

You can then check your recall with pharmacology toxicology and antidotes practice.

Conclusion

Antidotes become manageable once you sort them by mechanism: receptor antagonists, enzyme blockers, chelators, binders and reversal agents. For each pair, hold on to the one caution that changes management, such as the atropine endpoint, the flumazenil contraindications, dimercaprol before EDTA and the presynaptic limits of neostigmine. Practise the pairs with toxicology questions after each pass through the tables.