Pharmacology Cheat Sheet
Toxicology Recognition and Antidotes
Toxicology for NEET-PG: organophosphate antidotes and ageing, chelation for heavy metals, cyanide hydroxocobalamin, prazosin for scorpion sting.
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Clinically reviewed by Dr Shameer Deen, MBBS, MS, MRCS
Toxicology Recognition and Antidotes
Toxicology for NEET-PG: organophosphate antidotes and ageing, chelation for heavy metals, cyanide hydroxocobalamin, prazosin for scorpion sting.
Toxicology and environmental pharmacology covering organophosphates, heavy metals, common poisons, envenomation, and their specific antidotes.
High-yield lines
- Organophosphates irreversibly inhibit acetylcholinesterase, and the enzyme-inhibitor bond undergoes ageing within 24 to 48 hours, so pralidoxime must be given early.
- Atropine treats the muscarinic features of organophosphate poisoning, while pralidoxime reactivates the enzyme before ageing.
- The intermediate syndrome appears 24 to 96 hours after organophosphate poisoning with proximal and respiratory muscle weakness after apparent recovery.
- Aluminium phosphide (Celphos) poisoning has no antidote and carries 60 to 80% mortality, managed with IV magnesium and haemodynamic support.
- Prazosin is the drug of choice for scorpion sting in India, reversing the autonomic storm and catecholamine-induced pulmonary oedema.
- In lead encephalopathy, BAL is given first and EDTA started about 4 hours later to avoid redistributing lead into the brain.
- BAL is contraindicated in organic mercury poisoning because it redistributes mercury to the brain; DMSA is used instead.
- The paracetamol antidote is N-acetylcysteine, guided by the Rumack-Matthew nomogram.
- Hydroxocobalamin is the first-line cyanide antidote and is safe in smoke-inhalation victims, with nitrites plus thiosulfate as alternatives.
- Naloxone reverses opioids, flumazenil reverses benzodiazepines, and fomepizole or ethanol treats methanol and ethylene glycol.
- Activated charcoal binds poorly to heavy metals, iron, lithium, alcohols, and corrosives, summarised by the PHAILS mnemonic.
- Desferrioxamine is the antidote for iron poisoning, and a pink-orange vin rose urine confirms free iron is being chelated.
- Aldrich-Mees transverse white nail bands are a classic sign of arsenic and thallium poisoning.
- Neostigmine reverses cobra postsynaptic neurotoxicity but is less effective for krait presynaptic venom, and blood failing to clot in 20 minutes indicates vasculotoxic envenomation needing antivenom.
Mapped competency codes
- PH1.51
- PH1.52
- PH1.53
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This summary maps to PH13-toxicology-and-environmental-pharmacology.
Frequently Asked Questions
What is the drug of choice for scorpion sting in India?
Prazosin, an alpha-1 blocker that reverses the autonomic storm and catecholamine-induced pulmonary oedema.
Why give BAL before EDTA in lead encephalopathy?
EDTA alone can redistribute lead into the brain; BAL crosses into the CNS and pre-chelates intracellular lead, so it is given first.
What is the first-line cyanide antidote?
Hydroxocobalamin, which is safe in smoke-inhalation victims and turns urine and skin red; nitrites plus thiosulfate are alternatives.
Why is aluminium phosphide poisoning so lethal?
It has no specific antidote and carries 60 to 80% mortality from refractory cardiogenic shock and metabolic acidosis.
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