Forensic Medicine and Toxicology
Pharmaceutical Toxicology
Pharmaceutical toxicology for MBBS: paracetamol, opioids, benzodiazepines, tricyclics, digoxin and insulin overdose with their antidotes, mapped to NMC code FM10.1.
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Pharmaceutical Toxicology
Pharmaceutical toxicology for MBBS: paracetamol, opioids, benzodiazepines, tricyclics, digoxin and insulin overdose with their antidotes, mapped to NMC code FM10.1.
Pharmaceutical toxicology deals with poisoning by medicines, which are among the commonest agents in overdose. This chapter covers the analgesics, opioids, sedatives and antidepressants, the cardiac drugs and plant glycosides, and insulin, linking each to its clinical picture and specific antidote.
High-yield: Pharmaceutical Toxicology
- Paracetamol overdose damages the liver through the toxic metabolite NAPQI once glutathione stores are exhausted.
- N-acetylcysteine is the antidote for paracetamol poisoning and works best when given early, ideally within 8 hours.
- Salicylate (aspirin) poisoning produces an early respiratory alkalosis followed by a high anion gap metabolic acidosis.
- Opioid overdose causes the triad of pinpoint pupils, respiratory depression and reduced consciousness.
- Naloxone is the opioid antidote; because it is short-acting it may need repeated doses or an infusion.
- Benzodiazepine overdose causes sedation and is reversed by flumazenil, which is used cautiously because it can precipitate seizures.
- Tricyclic antidepressant overdose causes anticholinergic effects, seizures and a widened QRS, which is treated with sodium bicarbonate.
- Barbiturate poisoning causes deep coma with respiratory depression and characteristic skin blisters over pressure areas.
- Digoxin toxicity causes nausea, yellow-green visual disturbance and arrhythmias, and is treated with digoxin-specific antibody fragments.
- Oleander and cardiac glycoside plants can mimic digoxin toxicity and cause fatal cardiac arrhythmias.
- Insulin overdose causes profound hypoglycaemia, treated with intravenous dextrose and glucagon.
- Iron overdose in children can cause gastrointestinal bleeding, shock and liver failure, and is treated with desferrioxamine.
- Beta-blocker overdose causes bradycardia and hypotension and may respond to glucagon and high-dose insulin therapy.
- Antihistamine and anticholinergic overdose causes a dry, flushed, delirious patient with dilated pupils and urinary retention.
- Whole-bowel irrigation is useful for sustained-release preparations and for iron, which is not adsorbed by activated charcoal.
Common drug overdoses and their antidotes
- **Paracetamol:** Delayed liver injury from NAPQI. Antidote N-acetylcysteine, most effective given early.
- **Opioids:** Pinpoint pupils, respiratory depression, coma. Antidote naloxone, often repeated.
- **Benzodiazepines:** Sedation. Reversed by flumazenil, used with caution due to seizure risk.
- **Digoxin and cardiac glycosides:** Nausea, visual change, arrhythmia. Treated with digoxin-specific antibody fragments.
NMC competencies in this chapter
- **FM10.1:** Pharmaceutical toxicology: analgesics, psychotropics, opioids, cardiac plants and insulin
Frequently Asked Questions
What is the antidote for paracetamol overdose?
N-acetylcysteine, which replenishes glutathione and neutralises the toxic metabolite NAPQI. It is most effective when started early, ideally within 8 hours of ingestion.
What is the classic triad of opioid overdose?
Pinpoint pupils, depressed respiration and reduced consciousness. Naloxone rapidly reverses it, though repeated doses may be needed as it is short-acting.
Why is sodium bicarbonate used in tricyclic antidepressant poisoning?
It narrows the widened QRS complex and counters the cardiac sodium-channel blockade that causes life-threatening arrhythmias in tricyclic overdose.
How is digoxin toxicity recognised and treated?
By nausea, yellow-green visual disturbance and arrhythmias, often with high potassium in acute cases. Digoxin-specific antibody fragments are the definitive treatment.
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