Drug Toxicity & Antidotes for OPRA: High-Yield Toxicology, Toxidromes and MCQs

Recognising toxidromes

A toxidrome is a recognisable cluster of signs and symptoms that points to a class of poison before you know exactly what was taken. OPRA stems often describe the toxidrome rather than naming the drug directly, so pattern recognition is the first skill being tested — the antidote decision comes second, once the toxidrome (or specific agent) is identified.

Matching poison to antidote

Only a minority of overdoses have a specific antidote — most poisonings are managed with supportive care (airway, breathing, circulation) and, where relevant, decontamination. The antidotes below are the ones OPRA tests repeatedly precisely because they're specific, high-stakes, and easy to mismatch under exam pressure.

Gastrointestinal decontamination

• Activated charcoal is most effective within about 1 hour of ingestion — its value falls away quickly after that window.
• It only binds selected substances — it isn't useful for every poisoning (notably ineffective for iron, lithium, alcohols and most metals).
• Avoided in caustic/corrosive ingestions — charcoal obscures the endoscopic view needed to assess injury and doesn't bind these substances anyway.
• Avoided in hydrocarbon ingestions — aspiration risk is the concern, not lack of binding.
• Contraindicated with an unprotected airway — aspiration of charcoal itself is a serious risk in a drowsy or vomiting patient.

Practise this topic

The toxidrome-then-antidote pattern above is straightforward to read but takes real practice to apply fast under exam conditions. Practise more Pharmacology and Toxicology OPRA questions with ClinicalStem's OPRA question bank.

Paracetamol overdose — timing decides the answer

N-acetylcysteine (NAC) is most effective when started early, and the decision to treat is guided by a paracetamol level plotted against time since ingestion (not taken before 4 hours post-ingestion, since earlier levels don't reliably reflect peak absorption). If the ingestion time is unknown, staggered, delayed, or the treatment threshold is exceeded while still awaiting results, NAC is generally started without waiting for liver injury to appear — by the time transaminases rise, treatment is already delayed.

Why flumazenil is used cautiously, not routinely

Flumazenil reverses benzodiazepine sedation, but it can precipitate seizures — particularly in patients who are benzodiazepine-tolerant (long-term use) or who have co-ingested a pro-convulsant drug (e.g. a tricyclic antidepressant). For that reason it isn't given routinely for every suspected benzodiazepine overdose; supportive airway management is often the safer default, with flumazenil reserved for specific circumstances.

Naloxone's duration mismatch

• Naloxone's duration of action is usually shorter than that of many opioids — heroin itself is relatively short-acting, but methadone, modified-release oxycodone and fentanyl patches last considerably longer than a single naloxone dose.
• A patient can be resedated once the naloxone wears off even though the opioid is still active — this is why repeat dosing or an infusion, not a single injection, is often required.
• Ongoing observation after reversal is part of correct management, not an optional extra.

Tricyclic antidepressant overdose — sodium bicarbonate

TCA overdose is a classic OPRA scenario: widened QRS, ventricular arrhythmias and hypotension point to sodium channel blockade, and sodium bicarbonate is the specific treatment — it works by increasing extracellular sodium and alkalinising the serum, both of which counter the sodium-channel effect. Recognising the ECG/haemodynamic picture is the key skill; bicarbonate is the answer once TCA toxicity is identified.

Serotonin syndrome

Serotonin syndrome presents with a classic triad of hyperreflexia/clonus, autonomic instability (hyperthermia, tachycardia, diaphoresis) and altered mental status, typically from combining serotonergic drugs (e.g. an SSRI with tramadol, linezolid or a triptan). Management is primarily supportive — stopping the causative agents and benzodiazepines for agitation — with cyproheptadine reserved as a specific antidote in more severe cases.

Salicylate toxicity

Salicylate (aspirin) toxicity classically presents with tinnitus and hyperventilation, producing a mixed picture of respiratory alkalosis (from direct respiratory centre stimulation) and metabolic acidosis (from the salicylate itself) — a combination that's a recognisable exam signature on its own. Management includes urinary alkalinisation (to enhance renal elimination) and, in severe toxicity, haemodialysis.

Toxic alcohols

Methanol and ethylene glycol toxicity both produce toxic metabolites via alcohol dehydrogenase, and both are treated with fomepizole, which blocks that enzyme and prevents further toxic metabolite formation. These are less commonly tested than the agents above but occasionally appear, so the agent-to-antidote link (both → fomepizole) is worth keeping straight.

Recognising digoxin toxicity

Digoxin toxicity is easy to miss because its features are non-specific in isolation: nausea, vomiting, visual disturbance (classically yellow-tinged vision), bradycardia and hyperkalaemia. In an OPRA stem, that combination — especially in a patient established on digoxin — should prompt suspicion of toxicity rather than being read as an unrelated GI or visual complaint.

Does every overdose have a specific antidote?

No — most poisonings are managed with supportive care (and sometimes decontamination) alone. Specific antidotes exist for a relatively small number of agents, which is exactly why OPRA tests them repeatedly — they're the exception worth knowing precisely, not the rule.

Why is flumazenil not given for every benzodiazepine overdose?

Because it can precipitate seizures in patients who are benzodiazepine-tolerant or who have co-ingested a pro-convulsant drug. Supportive airway management is often preferred, with flumazenil reserved for more specific circumstances rather than used as a routine reversal agent.

Why might naloxone need to be given more than once?

Naloxone's duration of action is shorter than that of many opioids, so a patient can become resedated as it wears off while the opioid is still active — repeat dosing or an infusion, with ongoing observation, is standard rather than exceptional.