Toxicology · Short notes

Antidotes: Short Notes & Cheat Sheet

Specific antidotes for common poisonings, how antidotes work, and important cautions in their use.

Short notes

Read these before you attempt the topic's MCQs.

Knowing the mechanism explains the timing: enzyme reactivators and metabolic blockers work only if given early.

Receptor antagonism: naloxone (opioids), flumazenil (benzodiazepines), atropine (muscarinic effects of organophosphates).
Enzyme reactivation: pralidoxime reactivates acetylcholinesterase if given before the enzyme 'ages'.
Blocking toxic metabolism: fomepizole or ethanol blocks alcohol dehydrogenase in methanol and ethylene glycol poisoning.
Replacing a depleted defence: N-acetylcysteine restores glutathione in paracetamol poisoning.
Chelation: desferrioxamine (iron), dimercaprol, succimer and calcium disodium EDTA (lead, arsenic, mercury).
Binding or neutralising the poison: digoxin-specific Fab, antivenom, protamine for heparin.
Physiological opposition: glucagon for β-blocker overdose, calcium and high-dose insulin for calcium channel blocker overdose.

Naloxone is shorter-acting than most opioids, so re-sedation can occur; repeat doses or an infusion may be needed.
Flumazenil can trigger seizures in mixed overdoses with tricyclics and in long-term benzodiazepine users, so it is rarely used.
Methylene blue for methaemoglobinaemia must not be given in G6PD deficiency (it can cause haemolysis).
Dimercaprol (BAL) is given IM in peanut oil and should not be used for iron or cadmium, because it forms toxic complexes.
Activated charcoal is not an antidote for iron, lithium, alcohols, acids, alkalis or hydrocarbons, because it does not bind them.

High-yield facts in one place. Cover the right-hand column and test yourself.

Poison	Antidote
Paracetamol	N-acetylcysteine
Opioids	Naloxone
Benzodiazepines	Flumazenil (rarely)
Organophosphates	Atropine + pralidoxime
Anticholinergics	Physostigmine
Digoxin	Digoxin-specific Fab
Warfarin	Vitamin K ± prothrombin complex
Heparin	Protamine sulfate
Methanol, ethylene glycol	Fomepizole or ethanol + dialysis
Cyanide	Hydroxocobalamin; nitrites + thiosulfate
Carbon monoxide	100% oxygen (hyperbaric)
Iron	Desferrioxamine
Lead	Succimer, Ca-EDTA, dimercaprol
β-blockers	Glucagon
Calcium channel blockers	Calcium, high-dose insulin
Tricyclic antidepressants	Sodium bicarbonate
Isoniazid	Pyridoxine
Methaemoglobinaemia	Methylene blue
Local anaesthetic toxicity	20% lipid emulsion
Sulfonylurea hypoglycaemia	Dextrose + octreotide
Methotrexate	Folinic acid (leucovorin)
Snake bite	Polyvalent antivenom

Na-Na-Naloxone for Narcotics: Naloxone reverses opioids; watch for re-sedation because it wears off first.

Physostigmine, not neostigmine, is used for anticholinergic poisoning because it enters the brain.
Pralidoxime works only before the phosphorylated enzyme ages; give it early.
Ethanol is an antidote for methanol because both compete for alcohol dehydrogenase.
Methylene blue is contraindicated in G6PD deficiency.

Attempt the Antidotes MCQs while the notes are fresh. Anything you get wrong is saved to your incorrect list for the next revision round.