What is the toxic level of Tylenol (acetaminophen)?

Toxic Level of Acetaminophen

The toxic level of acetaminophen (Tylenol) is generally considered to be above 150 mg/kg in adults or above 200 mg/kg in children, with a maximum safe daily dose of 4000 mg for adults. 1

Understanding Acetaminophen Toxicity

Acetaminophen toxicity is assessed using several key parameters:

• Acute single ingestion thresholds:

  • Adults: >150 mg/kg (125 mg/kg if risk factors present)
  • Children: >200 mg/kg 2

• Maximum daily dose: 4000 mg per day for adults 3

• Risk assessment tool: The Rumack-Matthew nomogram is the standard tool used to determine risk of hepatotoxicity after acute ingestion 3, 1

The Rumack-Matthew Nomogram

The Rumack-Matthew nomogram is used to predict hepatotoxicity risk by plotting serum acetaminophen concentration against time since ingestion:

• When to use: Only applicable for single acute ingestions with known time of ingestion within 24 hours 3
• How to interpret:
  • Concentrations above the "200 mg/L at 4 hours" line indicate probable risk for hepatotoxicity
  • Concentrations between the "200 mg/L at 4 hours" line and "100 mg/L at 4 hours" line indicate possible risk
  • Concentrations below the "100 mg/L at 4 hours" line indicate no risk 3, 1

Risk Factors for Increased Toxicity

Certain factors lower the threshold for toxicity:

• Enzymatic induction (certain medications)
• Malnutrition
• Chronic alcoholism
• Pre-existing liver disease 2

In patients with these risk factors, the lower treatment threshold line on the nomogram ("100 mg/L at 4 hours") should be used 2.

Clinical Course of Toxicity

Acetaminophen toxicity typically follows a predictable pattern:

1. 0-24 hours: Nausea, vomiting, diaphoresis 4
2. 24-72 hours: Patient may feel better clinically while liver enzymes begin to rise
3. 48-96 hours: Peak serum transaminase activities occur 5
4. 4-7 days: Recovery in most cases without long-term sequelae 4

Treatment Approach

When toxic ingestion is suspected:

• Early intervention: Administer N-acetylcysteine (NAC) ideally within 8-10 hours of ingestion 3, 1
• Activated charcoal: Consider if presenting within 1-2 hours of ingestion 2
• NAC administration:
  • Oral: Loading dose of 140 mg/kg followed by 70 mg/kg every 4 hours for 17 doses
  • IV: Loading dose of 150 mg/kg in 5% dextrose over 15 minutes, followed by maintenance doses 1

Common Pitfalls to Avoid

1. Relying solely on patient history: Histories concerning time and amount of ingestion are often unreliable 5

2. Missing chronic toxicity: Patients with chronic excessive acetaminophen ingestion may present directly with hepatotoxicity and have higher rates of renal failure 5

3. Delayed presentation: Patients may present in liver failure days after ingestion with undetectable serum acetaminophen levels 5

4. Stopping NAC too early: Complete the recommended course unless specific criteria for early discontinuation are met 1

5. Failing to recognize risk factors: Patients with risk factors require treatment at lower thresholds 2