Poisoning, Antidotes, Chelators, Skin and Eye Drugs for NEET PG Pharmacology: Snakebite (MoHFW), Methylene Blue, Isotretinoin

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🎯 Exam priority: Very high. Expect: a snakebite with non-clotting blood or ptosis, the ASV dose, the antidote for iron, lead, cyanide, methanol or methaemoglobinaemia, flumazenil in a mixed overdose, isotretinoin in a young woman and timolol drops in an asthmatic.

CBME mapping: PH 9. Indian protocol: MoHFW Standard Treatment Guidelines for Snakebite (2016).

Snakebite (MoHFW)

Antidotes at a Glance

Poisons and their antidotes, from the drugs' labels. Drawn by TrackNation.

• Flumazenil reverses benzodiazepines but can provoke withdrawal seizures — risk higher after a large cyclic antidepressant overdose.

• Methylene blue treats acquired methaemoglobinaemia; boxed serotonin syndrome with serotonergic drugs; contraindicated in G6PD deficiency (haemolysis).

• Hydroxocobalamin for cyanide: 5 g IV for adults. Physostigmine for central anticholinergic syndrome.

Chelating Agents

Skin and Eye Drugs

• Isotretinoin: boxed — life-threatening birth defects; contraindicated in pregnancy; prescribed under iPLEDGE with two forms of contraception (or abstinence).

• Timolol eye drops act systemically: contraindicated in asthma, severe COPD, sinus bradycardia, AV block, overt heart failure.

• Latanoprost: increased brown iris pigmentation (likely permanent); longer, thicker, more eyelashes.

Traps the Examiner Sets

• 20WBCT is reported as clotted/not clotted, never positive/negative.

• Neostigmine test is not given for a confirmed krait bite; max ASV 20 vials in neurotoxic bites.

• Methylene blue is wrong in G6PD deficiency.

• Lead in a child: succimer (blood lead > 45) or calcium disodium EDTA.

• Timolol drops can trigger asthma.

Memory Hooks & Shortcuts

Shortcuts to lock in this article's numbers and lists. Each one only re-packages facts proven above.

• 🧠 Big Four: Cobra, Krait, Russell, Saw-scaled (CKRS) — India's main venomous snakes

• 🧠 Twenty minutes, Two answers: clotted or not — 20WBCT reporting

• 🧠 Ten, then ten, then stop (20) — neurotoxic ASV dosing

• 🧠 No AN for the krait — neostigmine test not in krait bites

• 🧠 Iron: De-FEROX-amine — iron chelator

• 🧠 Blue is bad for G6PD — methylene blue contraindication

• 🧠 Timolol: Tight chest — bronchospasm from eye drops

Rapid-Revision Cards

• Big Four

  Cobra, Russell's viper, saw-scaled viper, krait.

• 20WBCT

  Clean dry glass tube; clotted / not clotted; hourly × 3 then 6-hourly.

• ASV vasculotoxic

  10 vials; repeat if bleeding at 1 h.

• ASV neurotoxic

  10 + 10; max 20 vials.

• AN test

  Atropine 0.6 + neostigmine 1.5 mg IV; not in krait bite.

• ASV reaction

  Adrenaline 0.5 mg IM.

• Paracetamol

  N-acetylcysteine.

• Methanol

  Fomepizole.

• Benzodiazepine

  Flumazenil (seizures with TCA).

• Iron

  Deferoxamine (reddish urine).

• Lead (child)

  Succimer if > 45 mcg/dL; Ca-Na2 EDTA.

• Arsenic, mercury, gold

  Dimercaprol.

• Wilson's

  Penicillamine.

• Cyanide

  Hydroxocobalamin 5 g IV.

• Methaemoglobinaemia

  Methylene blue; not in G6PD; serotonin syndrome.

• Anticholinergic syndrome

  Physostigmine.

• Isotretinoin

  Teratogen; iPLEDGE; two contraceptives.

• Timolol drops

  Not in asthma, bradycardia, AV block, HF.

• Latanoprost

  Brown iris (permanent); eyelash growth.

Sources

Facts in this article were checked against these documents.

• Standard Treatment Guidelines: Snakebite — Directorate General of Health Services, MoHFW (with NHSRC), June 2016 (link)

• Prescribing labels: Acetadote, naloxone, Protopam, fomepizole, Lanoxin, flumazenil, deferoxamine, BAL in Oil, Chemet, edetate calcium disodium, Cuprimine, Anticholium, Cyanokit, ProvayBlue, isotretinoin, brimonidine-timolol, Xalatan — DailyMed, US National Library of Medicine, current labels, accessed November 2026 (link)

• Competency Based Undergraduate Curriculum, Pharmacology (PH 9) — National Medical Commission, 2024

Test Yourself: Practice Questions

Twenty-four questions in NEET PG style, most of them clinical vignettes.

Practice Questions (NEET PG pattern)

Original questions written in the style of the exam: +4 for a correct answer, −1 for a wrong one. Every answer was checked against the sources listed above. NEET PG papers are not published by NBEMS, so these are not past-paper questions.

Q1. A farmer bitten by a snake has abnormal bleeding; blood in a clean dry glass tube is still liquid at 20 minutes. Per MoHFW, the result is reported as

• (a) 'positive'

• (b) 'borderline'

• (c) 'negative'

• (d) 'not clotted' — vasculotoxic envenomation

Q2. In that vasculotoxic bite, after the first 10 vials of ASV bleeding continues at 1 hour. MoHFW advises

• (a) a repeat dose of ASV

• (b) heparin

• (c) vitamin K only

• (d) stopping ASV

Q3. A boy with ptosis and difficulty breathing after a cobra bite needs ASV. The MoHFW regimen is

• (a) 50 vials at once

• (b) 10 vials over 30 min, then 10 more if no improvement in 1 hour (max 20)

• (c) no ASV — neostigmine alone

• (d) 2 vials only

Q4. In neurotoxic envenoming, the 'AN' test uses

• (a) atropine alone

• (b) adrenaline and naloxone

• (c) neostigmine alone without atropine

• (d) atropine 0.6 mg then neostigmine 1.5 mg IV, neostigmine 0.5 mg with atropine every 30 min × 5

Q5. The AN (neostigmine) test should not be given when

• (a) a cobra bite is suspected

• (b) a krait bite is confirmed

• (c) the patient is a child

• (d) the patient has ptosis

Q6. During ASV infusion a patient develops an allergic reaction. MoHFW's treatment is

• (a) stop ASV permanently and give nothing

• (b) IV calcium

• (c) adrenaline 0.5 mg IM (children 0.01 mg/kg)

• (d) oral cetirizine only

Q7. India's 'Big Four' venomous snakes are

• (a) common cobra, Russell's viper, saw-scaled viper, common krait

• (b) only vipers

• (c) king cobra, pit viper, sea snake, python

• (d) rat snake, cobra, krait, mamba

Q8. A toddler swallows many of her mother's iron tablets. The specific chelator is

• (a) deferoxamine

• (b) succimer

• (c) dimercaprol

• (d) penicillamine

Q9. A 3-year-old has a blood lead of 52 mcg/dL without encephalopathy. A labelled oral chelator is

• (a) penicillamine

• (b) deferoxamine

• (c) methylene blue

• (d) succimer

Q10. A worker with arsenic poisoning needs chelation. The labelled drug is

• (a) flumazenil

• (b) dimercaprol (BAL)

• (c) deferoxamine

• (d) fomepizole

Q11. A teenager with Wilson's disease is started on a chelator. The classic one is

• (a) penicillamine

• (b) dimercaprol

• (c) deferoxamine

• (d) succimer

Q12. After smoke inhalation in a house fire, cyanide poisoning is suspected. Give

• (a) deferoxamine

• (b) flumazenil

• (c) hydroxocobalamin 5 g IV

• (d) methylene blue

Q13. A patient with acquired methaemoglobinaemia is known to have G6PD deficiency. Methylene blue is

• (a) contraindicated (risk of haemolytic anaemia)

• (b) given orally only

• (c) the drug of choice

• (d) safe at double dose

Q14. A patient on an SSRI is given IV methylene blue and develops agitation and hyperthermia. The boxed warning is

• (a) QT prolongation

• (b) serotonin syndrome with serotonergic drugs

• (c) hepatotoxicity

• (d) anaphylaxis

Q15. An unconscious patient has taken unknown pills, possibly benzodiazepines and a tricyclic. Flumazenil is risky because

• (a) it has no effect

• (b) it can provoke seizures, especially after a large cyclic antidepressant overdose

• (c) it is an opioid

• (d) it causes respiratory depression

Q16. A child has central anticholinergic syndrome after a plant poisoning. A labelled antidote is

• (a) neostigmine

• (b) pralidoxime

• (c) atropine

• (d) physostigmine

Q17. A man drinks illicit liquor and develops visual blurring and acidosis. The labelled antidote is

• (a) N-acetylcysteine

• (b) flumazenil

• (c) fomepizole

• (d) naloxone

Q18. Pralidoxime works in organophosphate poisoning by

• (a) blocking muscarinic receptors

• (b) stimulating nicotinic receptors

• (c) reactivating phosphorylated cholinesterase

• (d) binding the organophosphate in the gut

Q19. A 20-year-old woman starting isotretinoin for nodular acne must

• (a) take it only in winter

• (b) use two forms of contraception (or abstain) under iPLEDGE — it causes severe birth defects

• (c) only avoid sun

• (d) take folic acid only

Q20. An asthmatic with glaucoma is prescribed timolol eye drops and develops severe wheeze. The label

• (a) contraindicates timolol in bronchial asthma

• (b) says eye drops have no systemic effect

• (c) blames the glaucoma

• (d) recommends a higher dose

Q21. A patient using latanoprost in one eye notices that eye's iris has turned browner. The label says this change is

• (a) likely permanent

• (b) a sign of infection

• (c) due to cataract

• (d) always reversible

Q22. Specific antivenom for which bites is not available in India?

• (a) krait

• (b) sea snake and pit viper

• (c) cobra

• (d) Russell's viper

Q23. Dimercaprol is used for acute lead poisoning together with

• (a) penicillamine

• (b) methylene blue

• (c) deferoxamine

• (d) calcium disodium EDTA

Q24. N-acetylcysteine (Acetadote) is indicated to

• (a) treat iron overdose

• (b) reverse opioid overdose

• (c) chelate lead

• (d) prevent or lessen liver injury after a hepatotoxic paracetamol dose

Active Recall

From memory: outline MoHFW snakebite management (Big Four, 20WBCT, ASV doses for vasculotoxic and neurotoxic bites, the AN test, treating ASV reactions); match common poisons to their antidotes and chelators; and give the key warnings for methylene blue, flumazenil, isotretinoin, timolol and latanoprost.