|  | Conditions & Treatments | Poisoning

Acetaminophen (Tylenol) Overdose: Liver Toxicity Thresholds, Risk Factors & Emergency Management

Acetaminophen (paracetamol) overdose is the leading cause of acute liver failure in the US and UK. 10,000 mg (10 g) is a potentially lethal dose—2.5× the maximum daily limit—but hepatotoxicity is not guaranteed; it depends on risk factors and timing of intervention.

Dosing Limits

  • Therapeutic max: 4,000 mg/24h (some guidelines now 3,000 mg for chronic use/frail).
  • Single dose: ≤1,000 mg every 6 hours.
  • Pediatric: 10–15 mg/kg/dose q4–6h; max 75 mg/kg/day (≤4,000 mg).

Toxic Thresholds

  • Acute ingestion: >150 mg/kg or >7.5 g (whichever lower) in 24h → risk of hepatotoxicity.
  • Repeated supratherapeutic ingestion (RSTI): >4 g/day × multiple days + risk factors.
  • 10 g: High probability of toxicity without treatment; fatality possible >20–30 g.

Risk Factors for Hepatotoxicity at Lower Doses

  • Chronic alcohol use (induces CYP2E1 → more NAPQI; depletes glutathione).
  • Fasting/malnutrition (depleted glutathione reserves).
  • Isoniazid, carbamazepine, phenytoin, valproate (enzyme induction).
  • Pre-existing liver disease (viral hepatitis, NAFLD, cirrhosis).
  • Age extremes, sepsis, shock (impaired perfusion).

Pathophysiology

  • 90% glucuronidation/sulfation (safe).
  • 5–10% CYP2E1/1A2/3A4 → NAPQI (toxic metabolite).
  • NAPQI detoxified by glutathione (GSH).
  • Overdose → GSH depleted → NAPQI binds hepatocyte proteins → necrosis (centrilobular, zone 3).

Clinical Stages

PhaseTimeFeatures
I0–24hNausea, vomiting, diaphoresis, malaise; often asymptomatic
II24–72hRUQ pain, hepatomegaly, ↑AST/ALT (peak 72–96h), ↑INR, ↑bilirubin
III72–96hPeak liver injury: coagulopathy, encephalopathy, renal failure, lactic acidosis, death
IV4–14 daysRecovery (if survive) or fulminant failure → transplant/death

Emergency Management

  1. Activated charcoal: If <4h post-ingestion (or later for extended-release).
  2. N-acetylcysteine (NAC) – ANTIDOTE: Replenishes GSH; scavenges NAPQI; improves microcirculation.
    • IV protocol (US): 150 mg/kg over 1h → 50 mg/kg over 4h → 100 mg/kg over 16h (21h total).
    • Oral: 140 mg/kg loading → 70 mg/kg q4h ×17 doses (72h total).
    • Start ASAP; benefit up to 24h+ post-ingestion if hepatotoxicity evolving.
  3. Monitor: Serial AST/ALT, INR, bilirubin, creatinine, glucose, pH, acetaminophen level (Rumack-Matthew nomogram for acute single ingestion 4–24h).
  4. Transplant criteria (King's College): pH <7.30 OR (INR >6.5 + creatinine >3.4 + grade III/IV encephalopathy).

Key Message

Any suspected overdose → emergency department immediately. NAC is highly effective if started early. Do not wait for symptoms.

Poisoning - Related Articles