MRCP Toxicology: Mastering Poisoning and Overdose Management

Toxicology is one of those MRCP topics that rewards preparation disproportionately. Every Part 1 diet contains antidote-matching questions, acid-base patterns from salicylates, ECG changes from tricyclics, and decision-point questions around paracetamol levels. Part 2 written adds data interpretation and sequencing challenges. The good news? The question styles are predictable, the list of must-know facts is finite, and recent UK guidance changes (particularly the 2024 SNAP paracetamol update) have created fresh exam material that candidates using older resources will miss. This guide covers everything you need, structured the way the exam actually tests it.

Why Toxicology Is High-Yield for MRCP

• Predictable question stems: tinnitus with hyperventilation, a wide QRS on ECG, yellow vision, pinpoint pupils — the exam loves classic presentations.

• Finite fact base: antidotes, level thresholds, and management decisions can be memorised as tables.

• Cross-subject overlap: poisoning questions merge pharmacology, acid-base medicine, psychiatry (self-harm), nephrology (dialysis decisions) and ethics (capacity).

• Guideline flux: the 2024 UK SNAP guidance on paracetamol changed the nomogram, the infusion regimen and the stopping rules — new exam fodder.

Step 1: The Universal Approach to the Poisoned Patient

Whatever the toxin, the initial management answer is almost always the same, and MRCP examiners know candidates who reach for antidotes before ABCDE will fail real patients:

1. ABCDE assessment with early airway protection if GCS reduced

2. IV access, blood glucose, GCS documentation

3. Bloods: FBC, U&E, LFT, coagulation, paracetamol and salicylate levels in every deliberate self-poisoning (co-ingestion is common and silent)

4. Early 12-lead ECG — a wide QRS changes your next move immediately

5. Arterial blood gas — acid-base patterns are diagnostic fingerprints

6. Collateral history: what, how much, when, staggered or single ingestion, alcohol co-ingestion, regular medications

7. Consult TOXBASE (the UK National Poisons Information Service database) — referencing it in a Part 2 answer earns marks

Toxidromes: The Pattern Recognition Toolkit

Exam tip: Clonus plus hyperreflexia equals serotonin syndrome; lead-pipe rigidity equals NMS. The presence or absence of clonus is the discriminator examiners test.

Step 2: Decontamination and Enhanced Elimination

• Activated charcoal: most effective within 1 hour of ingestion; adult dose 50 g. Charcoal does not bind lithium, iron, alcohols, hydrocarbons or caustics.

• Multi-dose activated charcoal: know the list — theophylline, carbamazepine, quinine, dapsone (drugs with enterohepatic circulation or delayed absorption).

• Whole bowel irrigation: sustained-release preparations (lithium, theophylline) and body packers.

• Urinary alkalinisation (sodium bicarbonate, target urine pH 7.5–8.5, correct hypokalaemia first): used for salicylate poisoning — weak acids become ion-trapped in alkaline urine.

• Haemodialysis: works for drugs with low volume of distribution and water solubility — salicylate, lithium, methanol, ethylene glycol, metformin-associated lactic acidosis, phenobarbital, theophylline, valproate.

Paracetamol: The Single Most Examined Poison

Mechanism in One Sentence

Roughly 5–10% of paracetamol is metabolised by CYP2E1 (and 1A2/3A4) to the toxic metabolite NAPQI, which is detoxified by glutathione; overdose exhausts glutathione and NAPQI causes centrilobular hepatic necrosis.

High-risk patients (enzyme induction or glutathione depletion): chronic alcohol use, rifampicin, carbamazepine, phenytoin, St John's wort, malnutrition, prolonged fasting, HIV, eating disorders.

The Classic Nomogram (Traditional Exam Answer)

• Measure level at 4 hours post-ingestion minimum.

• UK treatment line: 100 mg/L at 4 hours (falling by half every 4 hours on the log-linear plot).

• High-risk patients: line halved to 50 mg/L.

• The nomogram is only valid for a single acute ingestion with a known time, presenting within 24 hours.

The 2024 SNAP Guidance (The New Material Examiners Will Use)

The UK's September 2024 update (MHRA Drug Safety Update and SNAP guidance published in the BMJ) changed practice significantly:

• Single treatment line of 100 mg/L is retained for acute single ingestions presenting within 24 hours.

• Levels of 50–100 mg/L no longer automatically mean treatment for standard-risk patients: repeat the paracetamol level and ALT (at least 4 hours later) and treat if the level crosses 100 mg/L or the ALT is rising.

• Staggered ingestions, unknown timing, or presentation beyond 24 hours: abandon the nomogram — start N-acetylcysteine immediately if paracetamol is detectable or ALT is abnormal, and continue based on trend.

• Routine halving of the line for high-risk patients is no longer recommended; instead, clinicians are advised to consider treatment at lower concentrations in high-risk groups. (Classic MRCP question banks may still use the halved line — be ready for both.)

• Simplified two-bag NAC regimen: 200 mg/kg over 4 hours, then 100 mg/kg over 16 hours (replacing the traditional three-bag 21-hour regimen).

• Stopping rule: continue NAC until paracetamol concentration is < 10 mg/L and ALT is normal or clearly falling.

• Anaphylactoid reactions to NAC are common, non-IgE mediated, and usually managed by pausing or slowing the infusion with an antihistamine — do not abandon the antidote.

Massive Overdose

Ingestions over ~500 mg/kg (or any large ingestion with early metabolic acidosis, rising ALT within 8 hours, hypoglycaemia, or coma), levels may be off-nomogram: start NAC immediately, monitor pH, lactate and glucose closely.

When Liver Failure Threatens: King's College Transplant Criteria

Post-overdose referral for transplant consideration if:

• Arterial pH < 7.30 after adequate fluid resuscitation (regardless of encephalopathy grade), or early lactate > 3.5 mmol/L, OR

• All of: INR > 6.5, creatinine > 300 micromol/L, and grade III/IV encephalopathy

Salicylate Poisoning: The Acid-Base Favourite

• Sources: aspirin, topical salicylates, oil of wintergreen (methyl salicylate — highly concentrated). Chronic toxicity in the elderly is insidious and easily missed.

• Clinical: tinnitus and deafness, hyperventilation, sweating, pyrexia, nausea, non-cardiogenic pulmonary oedema, confusion progressing to coma.

• The classic ABG: respiratory alkalosis (direct respiratory centre stimulation) plus high anion gap metabolic acidosis (uncoupled oxidative phosphorylation). MRCP loves this double disturbance.

• Levels: over 300 mg/L toxic; over 500 mg/L moderate-to-severe; over 700 mg/L (or lower with renal failure or severe features) — discuss haemodialysis.

• Management: repeated-dose activated charcoal; urinary alkalinisation for symptomatic poisoning with levels 500–700 mg/L; haemodialysis for severe features, refractory acidosis, fluid overload preventing alkalinisation, or renal failure.

• Danger point: avoid intubating the salicylate-poisoned patient unless unavoidable — loss of hyperventilation causes catastrophic worsening of acidosis. If ventilated, they need hyperventilation plus bicarbonate and usually early dialysis.

Tricyclic Antidepressants: The ECG Favourite

• ECG findings: sinus tachycardia with QRS prolongation (sodium channel blockade). QRS > 100 ms predicts seizures; QRS > 160 ms predicts ventricular arrhythmia. Also right-axis deviation of the terminal 40 ms and a Brugada-like pattern.

• Clinical: anticholinergic toxidrome plus seizures, coma, hypotension.

• The answer the examiner wants: intravenous 8.4% sodium bicarbonate for QRS > 100 ms, significant conduction abnormality, hypotension unresponsive to fluids, or seizures — target plasma pH 7.45–7.55, repeat boluses as needed.

• Seizures: benzodiazepines (and bicarbonate). Refractory instability: intravenous lipid emulsion.

• Never give flumazenil in a possible TCA co-ingestion — it precipitates seizures.

Opioids

• Triad: pinpoint pupils, respiratory depression, reduced consciousness.

• Naloxone 400 micrograms titrated every 2–3 minutes (up to 2 mg, repeat as needed). It has a half-life of 20–90 minutes — shorter than most opioids — so watch for re-sedation.

• Long-acting agents (methadone, modified-release morphine or oxycodone, fentanyl patches) need a naloxone infusion and extended observation.

Carbon Monoxide: The Oximeter Trap

• Standard pulse oximetry reads falsely normal — carboxyhaemoglobin absorbs light like oxyhaemoglobin. The exam answer for diagnosis is co-oximetry / COHb level.

• Features: headache, nausea, dizziness, syncope, confusion; check ECG and troponin for myocardial injury.

• 100% oxygen reduces the COHb half-life from ~4–6 hours on room air to ~60–90 minutes.

• Discuss hyperbaric oxygen for COHb > 25–30%, loss of consciousness, neurological deficits, ECG ischaemia or arrhythmia, severe metabolic acidosis, and pregnancy with COHb > 15–20% (fetal haemoglobin binds CO avidly).

• Warn about delayed neuropsychiatric syndrome weeks after exposure.

Digoxin: The Visual Clue

• Xanthopsia (yellow-green vision) plus GI symptoms plus confusion is the classic vignette.

• Arrhythmias: almost anything, but classic answers are paroxysmal atrial tachycardia with block and bidirectional ventricular tachycardia. Bradyarrhythmias and heart block are common.

• Acute overdose causes hyperkalaemia (Na/K ATPase poisoning — a severity marker and an indication for antidote); chronic toxicity more often presents with hypokalaemia from diuretics.

• Digoxin-specific antibody fragments (DigiFab) for life-threatening arrhythmia, cardiac arrest, potassium > 6.0 mmol/L in acute overdose, massive ingestion (traditionally > 10 mg acutely, or chronic levels > 15 micrograms/L).

• Pitfall: after Fab, total digoxin levels rise (bound drug is measured) — never re-dose based on the total level.

Toxic Alcohols: Methanol and Ethylene Glycol

• Both produce anion gap metabolic acidosis with an osmolar gap early on (osmolar gap closes as metabolism proceeds — a favourite Part 1 nuance).

• Methanol → formic acid: retinal and optic nerve toxicity → blindness, basal ganglia injury.

• Ethylene glycol → oxalic acid: calcium oxalate crystals in urine, AKI, hypocalcaemia; fluorescent urine under UV is folklore, not diagnosis.

• Antidote: fomepizole (ADH inhibitor) or IV ethanol; haemodialysis for severe acidosis, visual symptoms, renal failure or very high levels.

• Isopropyl alcohol: ketosis without acidosis.

Lithium

• Precipitants the exam tests: dehydration, AKI, thiazides, NSAIDs, ACE inhibitors, DKA.

• Features: coarse tremor (finer tremor is a side effect — coarse tremor is toxicity), ataxia, dysarthria, confusion, seizures, coma.

• Management: stop lithium, correct sodium and fluid status with normal saline; haemodialysis for severe neuro features (coma, seizures) or markedly elevated levels (broadly > 5 mmol/L acutely or > 4 chronically with severe symptoms). Charcoal is useless — lithium is not protein-bound or volume-distributed for adsorption to matter; whole bowel irrigation for sustained-release ingestion.

Iron

• Four-phase illness: GI haemorrhage → deceptive latent improvement → shock and acidosis → hepatotoxicity and later GI strictures.

• Abdominal radiograph may show tablets; desferrioxamine infusion for shock, severe acidosis, or levels > 90 micromol/L (urine turns vin rose during chelation).

Organophosphates

• Muscarinic (SLUDGE) + nicotinic (fasciculations, weakness) + CNS features, with pinpoint pupils and bradycardia.

• Atropine titrated to drying of secretions (not to heart rate alone), plus pralidoxime to reactivate cholinesterase before it irreversibly ages.

• Know the intermediate syndrome (24–96 hours later: neck flexor and respiratory muscle weakness requiring ventilatory support).

Beta-Blocker and Calcium-Channel Blocker Overdose

• Beta-blockers: glucagon is the classic antidote answer; also atropine, high-dose insulin-euglycaemia therapy (HIET), pacing.

• Calcium-channel blockers: IV calcium chloride, HIET (the modern mainstay), pacing, lipid emulsion, ECMO in refractory cases.

• Bradycardia with hypotension, hyperglycaemia (CCB) and refractory shock are the stems to recognise.

The MRCP Antidote Table: Memorise Cold

How MRCP Part 1 Actually Asks

• Young woman, 6 hours after unknown tablets, tinnitus, respiratory rate 30, ABG shows pH 7.48 then pH 7.20 with anion gap 22 → salicylate; answer: charcoal + urinary alkalinisation.

• ECG QRS 140 ms after antidepressant ingestion → IV sodium bicarbonate.

• Elderly patient confused on methadone, pinpoint pupils, respiratory rate 6 → naloxone titration; next question: infusion because methadone outlasts naloxone.

• Confused painter, normal SpO2, headache, works in garage → co-oximetry for COHb; answer 100% oxygen.

• Patient on digoxin, yellow vision, AV block, K+ 6.7 → digoxin-specific antibodies.

• Blindness after home-distilled spirit, osmolar gap raised, anion gap acidosis → methanol; fomepizole.

• Staggered paracetamol over 12 hours, level 80 mg/L at unknown time post-dose → start NAC immediately (2024 SNAP logic).

How MRCP Part 2 Written Asks

Expect sequencing and investigation-choice questions: the single most important next investigation (paracetamol level), the definitive management step (NAC despite borderline level in staggered ingestion), and ethics crossovers — a patient who has overdosed with capacity usually retains capacity to refuse treatment; if capacity is absent, treat in their best interests under the Mental Capacity Act, and remember that treating an overdose rarely breaches confidentiality.

Does Toxicology Appear in PACES?

Rarely as a primary station, but expect it embedded in acute medicine discussions — AKI from rhabdomyolysis after overdose, capacity assessments after self-harm, and hyperkalaemia management all appear within broader scenarios. The written-exam knowledge above is assumed background.

A Practical Five-Day Toxicology Revision Block

1. Day 1: Universal approach, toxidromes, decontamination and elimination methods.

2. Day 2: Paracetamol — old nomogram, 2024 SNAP changes, NAC regimen, King's criteria. Write the differences old vs new side by side.

3. Day 3: Salicylates, TCAs, opioids, carbon monoxide — focus on ABG and ECG stems.

4. Day 4: Digoxin, lithium, toxic alcohols, iron, organophosphates, BB/CCB.

5. Day 5: Close the loop with 50+ MCQs and the antidote table under test conditions; re-drill anything missed.

Key Takeaways

• ABCDE before antidotes — always the first mark.

• Paracetamol and salicylate levels in every overdose, and an early ECG.

• Master both the classic nomogram and the 2024 SNAP guidance — the two-bag NAC regimen, the 50–100 mg/L repeat band, and immediate treatment of staggered ingestions.

• Recognise the fingerprints: double acid-base disturbance (salicylate), wide QRS (TCA), falsely normal oximetry (CO), yellow vision with hyperkalaemia (digoxin), osmolar gap with acidosis (toxic alcohols).

• The antidote table is guaranteed marks — commit it to spaced repetition until it is reflexive.

• Guidelines evolve: cross-check TOXBASE and current MHRA updates before your exam diet, as newer questions increasingly reflect the 2024 paracetamol changes.

Toxicology rewards the prepared candidate more than almost any other MRCP topic. Learn the patterns, respect the updates, and these become the fastest marks on the paper.