PharmacologyToxicologyhigh-yield
Antidotes worth memorising cold
The poison–antidote pairs that recur in FMGE papers, grouped by mechanism so they are recalled rather than crammed.
5 min read · updated 2026-08-23
Group by mechanism, not alphabetically
Antidotes are easier to hold if you group them by how they work. There are only four mechanisms.
1. Receptor blockade — occupy the site the poison uses
| Poison | Antidote | Note |
|---|---|---|
| Opioids | Naloxone | Short half-life; the opioid usually outlasts it, so watch for re-narcotisation |
| Benzodiazepines | Flumazenil | Rarely used — it can precipitate seizures in a mixed overdose or chronic user |
| Organophosphates (muscarinic effects) | Atropine | Titrate to dry secretions, never to pupil size |
| Beta blockers | Glucagon | Bypasses the beta receptor to raise cAMP directly |
2. Enzyme reactivation or substrate repletion
| Poison | Antidote | Note |
|---|---|---|
| Organophosphates (nicotinic effects) | Pralidoxime | Must be given before ageing of the enzyme–phosphate bond |
| Paracetamol | N-acetylcysteine | Replenishes glutathione; most effective within 8 hours |
| Methanol / ethylene glycol | Fomepizole (or ethanol) | Competitively inhibits alcohol dehydrogenase, preventing toxic metabolite formation |
| Warfarin | Vitamin K (+ four-factor PCC if bleeding) | PCC gives speed; vitamin K gives durability |
| Heparin | Protamine sulfate | Does not reverse warfarin — a classic distractor pairing |
3. Chelation and binding
| Poison | Antidote |
|---|---|
| Lead | Succimer (DMSA), or calcium disodium EDTA with BAL in encephalopathy |
| Iron | Desferrioxamine |
| Copper (Wilson) | D-penicillamine, trientine |
| Arsenic, mercury, gold | Dimercaprol (BAL) |
| Digoxin | Digoxin-specific Fab fragments |
4. Displacement or bypass of the toxic effect
| Poison | Antidote | Note |
|---|---|---|
| Carbon monoxide | 100% oxygen, hyperbaric if severe | Oxygen displaces CO from haemoglobin and shortens its half-life |
| Cyanide | Hydroxocobalamin, or sodium nitrite + thiosulfate | Nitrite makes methaemoglobin, which binds cyanide away from cytochrome oxidase |
| Methaemoglobinaemia | Methylene blue | Contraindicated in G6PD deficiency — it causes haemolysis |
| Anticholinergic poisoning | Physostigmine | The mirror image of organophosphate poisoning; do not confuse the two directions |
The two directional traps
Organophosphate → atropine. Anticholinergic → physostigmine. Examiners rely on candidates reversing these.
Protamine → heparin. Vitamin K → warfarin. Never the other way round.
Anchor
If you can say what the poison does — blocks a receptor, poisons an enzyme, sits on a metal-binding site, or occupies haemoglobin — the antidote follows from the mechanism instead of a list.
Questions this note answers
- Autonomic pharmacologyA farmer is brought in with pinpoint pupils, profuse secretions, bradycardia and fasciculations after spraying insecticide. Which is the correct immediate pharmacological managemen…
- Haematological drugsA patient on warfarin presents with major gastrointestinal bleeding and an INR of 7.5. Alongside stopping warfarin, what is the most appropriate immediate treatment?
Sources
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14e
- Katzung Basic & Clinical Pharmacology, 16e
- WHO guidance on management of pesticide poisoning