Top Toxicology Presentations in SBA Exams

Topic priority at a glance

Toxicology is a reliable source of MRCEM and FRCEM questions because it tests what Emergency Medicine trainees are expected to do well: recognise patterns quickly, identify immediate threats, interpret ECGs and blood gases, and start safe first-line treatment before the exact poison is confirmed. In SBA papers, the diagnosis is often hidden inside a common ED presentation such as reduced GCS, seizure, broad-complex tachycardia, hyperthermia, unexplained metabolic acidosis, or a well-looking patient after deliberate self-poisoning. In OSCEs, candidates are expected to structure the poisoned patient approach clearly, use UK practice, and avoid dangerous interventions.

The highest-yield topics are paracetamol, salicylates, tricyclic antidepressants, opioids, benzodiazepines and flumazenil pitfalls, serotonin toxicity, sympathomimetic toxicity, carbon monoxide, lithium, and toxic alcohols. The recurring exam themes are timing, toxidromes, antidote selection, decontamination, enhanced elimination, observation periods, and what not to do.

Why These Toxicology Presentations Matter in SBA Exams

Poisoned patients are common in UK ED practice. Some are straightforward, but the dangerous cases deteriorate because of delayed recognition of respiratory depression, sodium channel blockade, hyperthermia, severe acidosis, or delayed toxicity after an apparently benign presentation.

Exam questions mirror this reality. They rarely reward obscure toxicology facts. They reward:

• ABCDE prioritisation
• Recognition of a toxidrome
• Correct interpretation of timing in paracetamol overdose
• Recognition of salicylate acid-base disturbance
• Recognition of TCA ECG toxicity
• Safe use of naloxone
• Avoidance of flumazenil in the wrong patient
• Knowing when dialysis or critical care is needed
• Using TOXBASE and NPIS-consistent UK practice

The single best answer is often the safest immediate ED step, not the most definitive later treatment.

Key Definitions

• Single acute overdose: one ingestion over a short period, with a known time. Important for paracetamol nomogram use.
• Staggered overdose: repeated ingestion over time or ingestion over more than 1 hour. The paracetamol nomogram does not apply.
• Unknown time ingestion: timing uncertain. Manage cautiously; do not use the paracetamol nomogram.
• Toxidrome: a recognisable cluster of clinical features suggesting a toxin class rather than a specific drug.
• Decontamination: measures to reduce absorption, usually activated charcoal in selected cases.
• Enhanced elimination: interventions that increase toxin removal, such as urinary alkalinisation or haemodialysis.
• Antidote: a treatment that directly reverses or limits toxicity, for example naloxone, N-acetylcysteine, atropine, or sodium bicarbonate in membrane-stabilising overdose.

Essential Pathophysiology

Most toxicology questions can be solved by understanding a few core mechanisms.

• Respiratory depression: typical of opioids and sedative-hypnotics. The immediate threat is hypoventilation, hypercapnia, and hypoxia.
• Sodium channel blockade: classically caused by TCAs, but also by some antihistamines, cocaine, and other membrane-stabilising drugs. This causes QRS widening, hypotension, ventricular arrhythmias, and seizures.
• Uncoupling and acid-base disturbance: salicylates stimulate the respiratory centre early, causing respiratory alkalosis, then produce metabolic acidosis. Severe poisoning causes mixed disturbance and rapid deterioration.
• Hyperadrenergic excess: sympathomimetics cause agitation, tachycardia, hypertension, sweating, hyperthermia, and sometimes myocardial ischaemia.
• Serotonergic excess: serotonin toxicity causes autonomic instability, agitation, hyperreflexia, and clonus.
• Antimuscarinic toxicity: dry, hot, tachycardic, delirious patient with urinary retention and reduced bowel sounds.
• Cholinergic excess: wet patient with secretions, bronchorrhoea, wheeze, bradycardia, diarrhoea, and fasciculations.
• Delayed organ toxicity: paracetamol is often clinically silent early but can later cause severe hepatotoxicity.
• High anion gap metabolic acidosis: think salicylates, toxic alcohols, severe shock, seizures, or late severe poisoning.

Clinical Presentation

In exams, poisoning often presents as one of the following:

• Reduced GCS or coma
• Respiratory depression
• Seizure
• Broad-complex tachycardia or unexplained arrhythmia
• Hyperthermia and agitation
• Unexplained metabolic acidosis
• Collapse after deliberate self-poisoning
• A well-looking patient after overdose where timing determines treatment

High-yield toxidrome clues are shown below.

Red Flags and High-Risk Features

• Airway compromise or loss of protective reflexes
• Respiratory depression
• Seizures
• Hyperthermia
• Shock or persistent hypotension
• Broad QRS, ventricular arrhythmia, or marked QT prolongation
• Severe acidosis
• Mixed respiratory alkalosis and metabolic acidosis suggesting salicylates
• High anion gap metabolic acidosis suggesting toxic alcohols or severe salicylate poisoning
• Paracetamol overdose with staggered ingestion, unknown timing, late presentation, or abnormal LFTs/INR
• Salicylate poisoning with altered mental state, pulmonary oedema, acidaemia, renal impairment, or rising salicylate level
• Lithium toxicity with neurological features or renal impairment
• Carbon monoxide exposure with neurological symptoms, cardiac ischaemia, pregnancy, or severe acidosis
• Any unknown overdose with reduced GCS, abnormal observations, or ECG changes

Differential Diagnosis

Not every collapsed or confused patient has taken an overdose. Examiners may include poisoning as a distractor or as the correct diagnosis among common ED differentials.

• Hypoglycaemia
• Sepsis
• Intracranial haemorrhage or stroke
• Meningitis or encephalitis
• Status epilepticus or post-ictal state
• Hypoxia or hypercapnia
• Electrolyte disturbance
• DKA or HHS
• Alcohol intoxication
• Trauma
• Heat stroke
• Thyroid storm
• Neuroleptic malignant syndrome

In practice and in exams, always check glucose early and do not anchor on poisoning before excluding immediately reversible causes.

Initial ED Assessment

Approach the poisoned patient with ABCDE. Stabilise first, then refine the diagnosis.

Immediate priorities

• Airway: assess whether the patient can maintain and protect their airway. Reduced GCS alone is not an automatic indication for intubation, but inability to protect the airway, failure of ventilation or oxygenation, or anticipated deterioration are.
• Breathing: assess respiratory rate, effort, oxygen saturation, and ventilation. Respiratory depression suggests opioid or sedative toxicity. Carbon monoxide poisoning may have normal pulse oximetry.
• Circulation: IV access, monitoring, blood pressure, temperature, and 12-lead ECG.
• Disability: GCS, pupils, capillary glucose, seizure activity, clonus, tone, and focal neurology.
• Exposure: temperature, skin moisture, needle marks, transdermal patches, signs of trauma, and evidence of contamination.

Early actions that score marks in SBAs

• Check capillary glucose in any patient with reduced GCS, seizure, or collapse
• Obtain an ECG in significant or unknown overdose
• Send a VBG or ABG early if unwell
• Give naloxone if opioid toxicity is causing respiratory depression
• Give benzodiazepines for toxin-induced seizures
• Use sodium bicarbonate for TCA-type sodium channel blockade
• Start oxygen immediately in suspected carbon monoxide poisoning
• Consult TOXBASE early in significant poisoning

Important airway caveat

Salicylate poisoning is a classic trap. These patients often maintain a compensatory high minute ventilation. Intubation can precipitate abrupt worsening if ventilation falls and acidaemia develops. If intubation is essential, it should be performed by experienced clinicians with a clear plan to maintain or exceed the patient’s pre-intubation minute ventilation.

Investigations

Targeted investigations are useful. Untargeted drug screens rarely change acute management.

Essential initial tests

• Capillary glucose
• 12-lead ECG
• VBG or ABG: pH, bicarbonate, lactate, ventilation status
• Urea, creatinine, electrolytes
• LFTs
• FBC if clinically indicated
• Pregnancy test where relevant
• Paracetamol concentration in all deliberate self-poisonings, mixed overdoses, and unknown ingestions unless there is a clear reason not to
• Salicylate concentration if suspected, or if there is unexplained tachypnoea, tinnitus, or mixed acid-base disturbance
• Carboxyhaemoglobin if carbon monoxide exposure is suspected

ECG findings that matter in toxicology

Blood gas patterns worth recognising

Management in the Emergency Department

Management is best thought of in two phases: immediate stabilisation and toxin-specific treatment.

Immediate management

1. ABCDE resuscitation and monitoring.
2. Check glucose and correct hypoglycaemia.
3. Treat seizures with benzodiazepines first-line.
4. Support ventilation and oxygenation.
5. Obtain ECG and blood gas early.
6. Consider antidotes when indicated.
7. Consider decontamination only if appropriate and safe.
8. Escalate early to senior EM, critical care, and toxicology advice for severe cases.

Decontamination principles

• Activated charcoal is usually considered within 1 hour of a significant ingestion if the airway is protected and the toxin is adsorbed by charcoal.
• Later charcoal may still be appropriate for selected modified-release preparations or other specific toxins after TOXBASE advice.
• Charcoal is not useful for alcohols, lithium, metals, acids, or alkalis.
• Whole bowel irrigation is reserved for specific cases such as body packers or selected sustained-release ingestions.
• Do not delay resuscitation or antidote treatment for decontamination.

Top tested poisonings

Paracetamol overdose

Paracetamol is the highest-yield UK poisoning topic because management depends on timing and protocol rather than symptoms.

Recognition

• Often asymptomatic early
• Key question: single acute ingestion, staggered overdose, modified-release preparation, or unknown time?
• The nomogram applies only to a single acute ingestion with a known time

Investigations

• Paracetamol concentration at 4 hours or later after a single acute ingestion
• LFTs, INR, U and E, creatinine
• VBG if unwell or late presentation

Management

• Consider activated charcoal if appropriate and within the relevant timeframe
• Start N-acetylcysteine if the level is on or above the UK treatment line after a single acute overdose
• Start N-acetylcysteine without waiting for plotting if there is a clinically significant staggered overdose, unknown timing with significant ingestion, or late presentation with abnormal bloods or symptoms suggesting toxicity
• Modified-release overdose requires extra caution and TOXBASE-guided management

Exam traps

• Using a level taken before 4 hours to exclude toxicity
• Plotting a staggered overdose on the nomogram
• Ignoring paracetamol because another co-ingestant seems more obvious
• Delaying N-acetylcysteine when the history already makes treatment necessary

Salicylate poisoning

Salicylates are a classic exam topic because they combine recognisable symptoms, a characteristic blood gas, and several dangerous management traps.

Recognition

• Tinnitus, vomiting, sweating, tachypnoea, agitation, confusion
• Respiratory rate is often the clue
• Classic blood gas: respiratory alkalosis with metabolic acidosis; severe poisoning may produce marked acidaemia
• Hypoglycaemia may be absent despite neuroglycopenic CNS symptoms; brain glucose availability may be impaired even with a normal serum glucose

High-risk features

• Altered mental state
• Acidaemia
• Pulmonary oedema
• Renal impairment
• Rising salicylate concentration
• Severe hyperthermia

Management

• Resuscitate and monitor closely
• Give activated charcoal if appropriate and advised
• Correct hypokalaemia aggressively because urinary alkalinisation will fail if potassium is low
• Use IV sodium bicarbonate to alkalinise serum and urine in significant toxicity, following TOXBASE guidance
• Avoid intubation unless essential; if required, maintain very high minute ventilation
• Discuss early with critical care and renal/toxicology teams if severe

When to think haemodialysis

• Severe salicylate poisoning with acidaemia, altered mental state, pulmonary oedema, renal failure, refractory electrolyte disturbance, or clinical deterioration
• Very high salicylate concentrations in the context of symptoms or worsening physiology

Exam traps

• Missing the diagnosis in a tachypnoeic patient with vomiting and confusion
• Focusing only on the salicylate level and ignoring the blood gas
• Intubating without a plan to preserve hyperventilation
• Forgetting potassium replacement

Tricyclic antidepressant poisoning

TCA overdose is one of the most important ECG toxicology topics.

Recognition

• Reduced GCS, seizures, hypotension, anticholinergic features
• ECG: broad QRS, terminal R in aVR, ventricular arrhythmias

Management

• ABCDE and continuous cardiac monitoring
• Benzodiazepines for seizures
• IV sodium bicarbonate is first-line for sodium channel blockade causing QRS widening, ventricular arrhythmia, hypotension, or seizures
• Early critical care involvement in severe toxicity

Exam traps

• Treating broad-complex tachycardia with standard antiarrhythmics instead of sodium bicarbonate
• Missing TCA toxicity because the patient looks anticholinergic rather than cardiovascularly unstable
• Underestimating seizure risk

Opioid toxicity

Opioid toxicity is usually straightforward if you focus on ventilation rather than consciousness alone.

Recognition

• Reduced consciousness, respiratory depression, miosis
• Low respiratory rate is the key finding
• Consider heroin, morphine, methadone, fentanyl analogues, and nitazenes

Management

• Airway support and oxygen as needed
• Naloxone titrated to adequate ventilation, not full wakefulness
• Repeated boluses or infusion may be needed for long-acting opioids or potent synthetic opioids
• Observe for recurrent respiratory depression because naloxone may wear off before the opioid

Exam traps

• Giving a large naloxone dose when titration would be safer
• Assuming the patient can be discharged immediately after reversal
• Missing co-ingestion in a patient who does not behave like a pure opioid overdose

Benzodiazepine toxicity and flumazenil pitfalls

Pure benzodiazepine overdose often causes drowsiness, ataxia, and slurred speech with relatively preserved observations. The exam point is usually what not to do.

Management

• Supportive care is the mainstay
• Assess for mixed overdose, especially with alcohol, opioids, or pro-convulsant drugs
• Flumazenil is generally avoided in undifferentiated overdose because it can precipitate seizures, especially in mixed overdose or benzodiazepine dependence

Exam traps

• Using flumazenil in unknown overdose
• Using flumazenil when TCA co-ingestion is possible
• Assuming reduced GCS is due to benzodiazepines alone

Serotonin toxicity

Serotonin toxicity is a pattern-recognition diagnosis.

Recognition

• Agitation, tachycardia, hyperthermia
• Hyperreflexia and clonus are the key discriminators
• Often associated with SSRIs, SNRIs, MAOIs, tramadol, linezolid, MDMA, or combinations of serotonergic drugs

Management

• Stop serotonergic agents
• Benzodiazepines for agitation and muscle activity
• Active cooling for hyperthermia
• Critical care escalation in severe cases

Exam traps

• Confusing it with anticholinergic or sympathomimetic toxicity
• Missing clonus
• Treating hyperthermia passively when active cooling is needed

Sympathomimetic toxicity

Recognition

• Agitation, tachycardia, hypertension, mydriasis, sweating, hyperthermia
• Chest pain may occur with cocaine

Management

• Benzodiazepines are first-line for agitation, hypertension driven by catecholamine excess, and seizure control
• Active cooling for hyperthermia
• Treat complications such as ACS, arrhythmia, rhabdomyolysis, or intracranial haemorrhage

Exam traps

• Confusing with anticholinergic poisoning; sweating points towards sympathomimetic toxicity
• Under-treating hyperthermia

Cholinergic poisoning

Recognition

• Salivation, lacrimation, urination, diarrhoea, vomiting, miosis, wheeze, bronchorrhoea, bradycardia, fasciculations
• The wet patient with secretions is the classic picture

Management

• Protect staff with appropriate PPE if contamination is ongoing
• Decontaminate as advised
• Airway and breathing support
• Atropine titrated to drying of bronchial secretions and improved ventilation and oxygenation
• Specialist-guided oxime therapy where indicated

Exam traps

• Titrating atropine to pupil size rather than secretions and ventilation
• Forgetting staff safety and decontamination

Carbon monoxide poisoning

Carbon monoxide is frequently tested because the diagnosis is easy to miss.

Recognition

• Headache, dizziness, nausea, confusion, collapse
• Multiple people affected in the same environment is a major clue
• Pulse oximetry may be normal and is unreliable

Investigations

• Carboxyhaemoglobin level
• Blood gas including lactate
• ECG and troponin if clinically indicated

Management

• Immediate high-concentration oxygen
• Remove from source
• Assess severity by symptoms, acidosis, neurological features, pregnancy, and cardiac involvement, not by COHb alone
• Discuss severe cases with a hyperbaric service or NPIS according to local pathways

Exam traps

• Relying on pulse oximetry
• Using COHb level alone to decide severity

Lithium toxicity

Recognition

• Tremor, ataxia, dysarthria, confusion, seizures
• Chronic toxicity often causes more neurological symptoms than acute ingestion
• Renal impairment is a major risk factor

Management

• Stop lithium
• Check lithium concentration, renal function, and ECG
• Correct dehydration and electrolyte disturbance
• Discuss severe cases early with renal and toxicology teams
• Haemodialysis may be required in severe toxicity, especially with marked neurological features or renal failure

Exam traps

• Assuming the lithium level alone determines severity
• Missing chronic toxicity in an older patient with AKI and confusion

Toxic alcohols

Recognition

• High anion gap metabolic acidosis, visual symptoms in methanol, renal injury in ethylene glycol
• History may be unclear; osmolar gap may support suspicion early

Management

• Resuscitate and correct severe acidosis
• Early toxicology advice
• Antidotal therapy such as fomepizole may be indicated according to local availability and specialist advice
• Haemodialysis is required in severe poisoning

Exam traps

• Missing the diagnosis in unexplained severe metabolic acidosis
• Waiting for definitive levels before escalating

Antidotes and high-yield emergency treatments

Disposition, Referral and Follow-Up

Disposition is frequently tested and often poorly answered.

Admit to monitored bed or higher acuity area if any of the following apply

• Reduced GCS or recurrent sedation
• Respiratory depression or naloxone requirement
• Abnormal ECG
• Seizures
• Hypotension or arrhythmia
• Hyperthermia
• Significant metabolic acidosis
• Need for antidote infusion or repeated antidote dosing
• Salicylate, TCA, lithium, toxic alcohol, or carbon monoxide poisoning with significant features

Critical care referral

• Need for airway intervention or ventilatory support
• Persistent shock
• Severe acidosis
• Refractory seizures
• Severe hyperthermia
• Ventricular arrhythmia
• Severe salicylate poisoning
• Severe TCA toxicity

Renal referral

• Poisonings where haemodialysis may be needed, especially salicylates, lithium, methanol, and ethylene glycol

Psychosocial assessment

Patients presenting after deliberate self-poisoning require appropriate mental health and psychosocial assessment once medically fit, in line with local self-harm pathways.

Special Groups

Paediatrics

• Weight-based dosing and lower physiological reserve matter
• Small ingestions can be significant depending on the toxin
• Always use paediatric toxicology guidance and TOXBASE
• Safeguarding issues may arise in accidental or unexplained ingestion

Pregnancy

• Treat the mother promptly; maternal stabilisation is the priority for fetal wellbeing
• Do not withhold indicated antidotes such as N-acetylcysteine because of pregnancy
• Carbon monoxide poisoning in pregnancy is high risk and requires early specialist discussion
• Follow current TOXBASE and obstetric guidance for dosing and escalation

Older adults

• Polypharmacy and renal impairment increase risk
• Chronic toxicity is common, especially with lithium and digoxin
• Presentation may be non-specific: confusion, falls, or delirium

Immunosuppressed patients

• Poisoning may coexist with sepsis or other pathology
• Do not anchor on overdose if the presentation is atypical

Common Pitfalls

• Failing to check glucose in reduced GCS
• Not obtaining an ECG in unknown or significant overdose
• Using a paracetamol level taken before 4 hours to exclude toxicity
• Applying the paracetamol nomogram to staggered overdose or unknown timing
• Missing salicylate poisoning in a tachypnoeic patient
• Intubating salicylate poisoning without a ventilation plan
• Treating TCA broad-complex toxicity with standard antiarrhythmics instead of sodium bicarbonate
• Giving flumazenil in unknown or mixed overdose
• Giving naloxone without planning for recurrence
• Relying on pulse oximetry in carbon monoxide poisoning
• Ordering broad drug screens instead of targeted tests
• Forgetting TOXBASE and early senior input

FRCEM and MRCEM Exam Tips

• Start with the immediate threat: airway, breathing, circulation, seizure, arrhythmia, hyperthermia.
• Use toxidrome clues before trying to name the exact poison.
• If timing is given, it is usually crucial.
• If an ECG is shown, ask whether this is sodium channel blockade or QT prolongation.
• If a blood gas is given, interpret it properly before reading the options.
• The best answer is often the safest next step, not the final diagnosis.
• In UK exams, follow TOXBASE-style logic.
• Be suspicious of attractive but unsafe options such as flumazenil, premature discharge after naloxone, or standard antiarrhythmics in TCA toxicity.

How This Appears in SBA Questions

Typical question stems

• “A 24-year-old is brought to the ED drowsy after an overdose. RR 6, pupils pinpoint. What is the next best step?”
• “A 19-year-old presents 2 hours after taking 30 paracetamol tablets. A paracetamol level is sent immediately. What should you do next?”
• “A patient is agitated, tachypnoeic, vomiting, and complains of tinnitus. Blood gas shows mixed respiratory alkalosis and metabolic acidosis. What is the most appropriate management?”
• “An overdose patient has hypotension, seizure, and a broad QRS with terminal R in aVR. Which treatment is indicated?”
• “Which intervention is contraindicated in this patient with reduced GCS after unknown overdose?”
• “Which poisoned patient is most likely to require haemodialysis?”

Key discriminator clues

• Pinpoint pupils plus hypoventilation: opioid toxicity
• Dry skin and urinary retention: anticholinergic poisoning
• Sweating and agitation: sympathomimetic poisoning
• Clonus and hyperreflexia: serotonin toxicity
• Tinnitus and tachypnoea: salicylates
• Broad QRS and hypotension: TCA or similar sodium channel blockade
• Normal observations do not exclude dangerous paracetamol poisoning
• Normal pulse oximetry does not exclude carbon monoxide poisoning

Common wrong answer traps

• Flumazenil for unknown overdose
• Paracetamol nomogram use in staggered overdose
• Waiting for a 4-hour level when the history already mandates N-acetylcysteine
• Amiodarone or other antiarrhythmics instead of sodium bicarbonate in TCA toxicity
• Immediate discharge after naloxone reversal
• Intubation as a reflex in salicylate poisoning without appreciating the risk

Key Takeaways

• Toxicology SBAs usually test recognition of an ED pattern, not obscure poison facts.
• ABCDE, glucose, ECG, and blood gas solve many toxicology questions.
• Paracetamol management depends on timing; the nomogram is only for single acute ingestion with a known time.
• Salicylate poisoning classically causes respiratory alkalosis with metabolic acidosis and can deteriorate rapidly.
• TCA toxicity is an ECG diagnosis; broad QRS and hypotension should trigger IV sodium bicarbonate.
• In opioid toxicity, titrate naloxone to ventilation and watch for recurrence.
• Avoid flumazenil in undifferentiated or mixed overdose.
• Clonus suggests serotonin toxicity; sweating suggests sympathomimetic toxicity; dryness suggests anticholinergic toxicity.
• Pulse oximetry is unreliable in carbon monoxide poisoning.
• Use TOXBASE early and escalate severe cases to critical care, renal, or toxicology services.

Further Reading

• TOXBASE, National Poisons Information Service
• NICE guidance on self-harm assessment, management and preventing recurrence
• RCEM guidance relevant to poisoning, self-harm and emergency care pathways
• Resuscitation Council UK adult advanced life support guidance
• British Thoracic Society guidance relevant to oxygen use and carbon monoxide exposure pathways
• SIGN guidance on the management of deliberate self-harm where locally applicable

Related on EM Final Exams

• Top 20 Most Examined Topics in FRCEM
• Top 50 Emergency Drugs for FRCEM
• RSI Drugs Explained for FRCEM Exams
• Common Cognitive Errors in Exam Questions

Authoritative Sources

• NICE guidance
• Royal College of Emergency Medicine (RCEM)

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