Toxicology

Definitions

Antidote: acts directly on toxic drug, e.g. naloxone for opioids

Treatment: responds to toxic effects, e.g. sodium bicarbonate neutrolising acid in amitriptyline overdoses

Assessment

History & Examination

• Tablets taken: quantity, what drug, when

• Elimination: diarrhoea, vomiting

• Other symptoms

• A-E exam, noting GCS

Investigations

ECG

• Sodium channel blockers (amitriptyline) cause QRS widening

• Potassium channel blockers (sotalol) cause QT prolongation

Drug causes of acidosis 

• Paracetamol

• Aspirin

Management

Non specific management

Activated charcoal

• Give within 1hr of overdose

• Reduces GI absorption of drug

• Doesn't work on all drugs

Drug Overdose Management

Aspirin Gastric lavage > 500mg/kg within first hour

Amitriptyline NaBicarb 8.4% 100mls if QRS > 100

Benzodiazepines Flumazenil

Betablockers Glucagon

Carbamazepine NaBicarb if QRS > 100, lorazepam

Cyanide Dicobalt edetate, sodium nitrate, sodium thiosulphate

Digoxin Digibind

Ethylene glycol(antifreeze) Fomepizole

Heparin Protamine

Insulin Glucose 10-50% (C-peptide inhibited)

Lead poisoning DMSA

Lithium Hemodialysis

Methaemoglobuinaemia Methylene blue

Methotrexate Folinic Acid

Methylene blue Fomepizole 

Opioids Naloxone

Organophosphate (insecticide) Atropine

Paracetamol NAC

Sodium valproate Phenytoin

Warfarin Prothrombin complex

Specific Drugs

Paracetamol OD

Common cause of emergency liver transplants

Moa

• Paracetamol broken down into NAPQUI (toxic) 

• Glutathione mops up NAPQUI

• In paracetamol OD, there isn't enough glutathione so NAC is given which can broken down into glutathione

Paracetamol toxicity increased by

• P450 induction (rifampicin, phenytoin, carbamazepine, acute alcohol)

• Low glutathione reserves: 

  • • • Genetic variation

      • HIV +ve

      • Malnutrition

      • ALD

Opioids

Examination

• Pinpoint pupils

• Reduced GCS

Investigations

• Blood gas- respiratory acidosis

Management

Naloxone

• 200 micrograms stat + 200 micrograms

• Half life 2hrs (shorter than many opioids)

• Upgrade to infusion

Benzodiazepine

Examination

Reduced GCS

Management

Flumazenil is the antidote however it is rarely used due to the risk of inducing seizures in individuals that have chronic use. It can be used in a once off patient-naive overdose. 

Recreational Drugs

• Marjuana,

• MDMA (ectasy)

• Cocaine

• Amphetamines (speed, crystal meth)

• Opioids: prescription, fentanyl, heroin

Cocaine

• Associated with increased blood pressure and cardiovascular events: stroke & heart attacks

• Addictive due to the increasing dose from tolerance

Ectasy/ MDMA

• Stimulates serotonin activity

• Risks of hyponatraemia from polydipsia

• Mx toxicity- supportive care: IVF, active cooling, diazepam, nitrates/ CCB to control BP

Levothyroxine

• Presents late, > 24hrs and asymptomatic: no management required

Phenytoin OD

• Presentation: isolated cerebellar signs

• Commonly in MCQ, parent is epileptic

• Phenytoin is highly protein bound & renally excreted

• Phenytoin assay measures free phenytoin, can be falsely normal in phenytoin toxicity due to reduced excretion (e.g. AKI)

Methaemoglobinemia

Pathophysiology: Fe oxidases 3+ and 2+ 

Causes

• GTN/ nitrates/ party nitrates

• Prilocaine

Presentation: hypoxia, 

Diagnosis: MetHb

Treat if MetHb > 20% + symptoms or > 30%

Management: 

1. Methylene blue

2.  Ascorbic acid

Androgen Abuse/ Steroids

• Background: athletes/ body builders

• Presentation: small testicles, gynaecomastia

• Investigations: suppresion of glucorticoids/ testosterone

Methanol Toxicity

• Potatoe farmer brewing own alcohol

• Presentation: blindness, hepatitis & thrombocytopenia

Aspirin

• Alkalysation of urine facilitates excretion

• Presentation: hyperventilation, sweating, tinnitus & acidosis

• Management: sodium bicarbonate IV

• If > 500mg/kg a concerning feature

Digoxin

If severity markers, indication to give Digibind

• K+ > 5.0

• Haemodynamic instability, requiring atropine

• Large dose OD

Lithium

Management

• NaCl 0.9% 

• Level > 4.0: haemodialysis

Phenytoin

Presentation: cereballar signs

More common in hypoalbuminaemia from Liver Cirrhosis (protein bound) or CKD (Chronic Kidney Disease) 

References and Resources

1. Toxbase

Written in 2025.