Toxicology and Drug Overdose

Paracetamol (Acetaminophen) Overdose

•    Toxic dose: >10 g (single) or >150 mg/kg, or staggered ingestion over 24 hours

•    Stages:

o    Early (0–24h): asymptomatic, nausea, vomiting

o    24–72h: ↑ ALT/AST, RUQ pain

o    72–96h: hepatic failure, encephalopathy, coagulopathy

•    Investigations: serum paracetamol level + time since ingestion (use Rumack–Matthew nomogram)

•    Antidote: N-acetylcysteine (NAC) – effective even if >8 hrs post-ingestion

•    Staggered dose or unknown time: treat immediately with NAC

Tricyclic Antidepressant (TCAs) Overdose

•    Key signs: anticholinergic (dry mouth, urinary retention), 

                                                cardiotoxicity (prolonged QRS >100 ms, arrhythmias), seizures

•    ECG findings: sinus tachycardia, widened QRS, right axis deviation

•    Antidote: IV sodium bicarbonate (even if normopH) – narrows QRS, reduces arrhythmias

•    Avoid class Ia/III antiarrhythmics

Theophylline Overdose

•    Narrow therapeutic window

•    Toxic signs: nausea, vomiting, tremor, seizures, tachyarrhythmias, hypokalaemia

•    Management: activated charcoal if within 1 hr, consider haemodialysis in severe cases

Iron Overdose

•    Toxic dose: >60 mg/kg elemental iron

•    Four phases:

1.    GI phase (0–6h): vomiting, diarrhoea, abdominal pain

2.    Latent phase (6–24h): apparent recovery

3.    Systemic toxicity (12–96h): shock, metabolic acidosis, hepatic necrosis

4.    Late phase (2–6 weeks): GI strictures

•    Investigations: serum iron (4–6h post-ingestion), metabolic panel

•    Antidote: IV deferoxamine (chelates iron) – urine turns pink ("vin rose")

Salicylate (Aspirin) Overdose

•    Toxic dose: >150 mg/kg

•    Acid-base disturbance:

o    Early: respiratory alkalosis (direct stimulation of respiratory centre)

o    Later: metabolic acidosis with increased anion gap

•    Features: tinnitus, nausea, vomiting, hyperventilation, pyrexia, confusion

•    Management:

o    Activated charcoal if <1 hr

o    IV bicarbonate (alkalinise urine)

o    Haemodialysis if pH <7.3, salicylate >700 mg/L, renal failure, or pulmonary oedema

Ethylene Glycol / Methanol Poisoning 

•    Both cause high anion gap metabolic acidosis

•    Ethylene glycol: renal failure due to calcium oxalate crystals in urine

•    Methanol: visual loss, retinal damage

•    Antidote: fomepizole (1st line) or ethanol (competitive inhibition of alcohol dehydrogenase)

•    Consider haemodialysis in severe cases

Carbon Monoxide (CO) Poisoning

•    Binds haemoglobin → carboxyhaemoglobin → tissue hypoxia

•    Signs: headache, dizziness, cherry-red lips (rare), confusion

•    Pulse oximetry is falsely normal

•    Diagnosis: elevated carboxyHb on co-oximetry

•    Treatment: high-flow O₂ (100%), consider hyperbaric O₂ if:

o    Neurological symptoms

o    CarboxyHb >25% (or >15% in pregnancy)

o    Loss of consciousness

Elimination Methods in Overdose

•    Activated charcoal:

o    Effective if given within 1 hour of ingestion

o    Not useful for metals, alcohols, or corrosives

•    Haemodialysis: for elimination of:

o    Lithium

o    Salicylates

o    Ethylene glycol

o    Methanol

o    Theophylline (in severe toxicity)