Paracetamol Overdose Calculator – Acetaminophen Toxicity Assessment
Assess paracetamol overdose risk and determine treatment needs
How to Use
- Enter the total amount of paracetamol ingested in milligrams
- Enter the time elapsed since ingestion in hours
- Enter the patient's weight in kilograms
- Click calculate to assess overdose risk and treatment needs
What is Paracetamol Overdose?
Paracetamol (acetaminophen) overdose is one of the most common causes of poisoning worldwide. While safe at therapeutic doses, excessive amounts can cause severe liver damage and potentially fatal hepatotoxicity.
Toxicity typically occurs with single ingestions exceeding 150 mg/kg in adults, though chronic excessive use at lower doses can also cause harm. The toxic metabolite NAPQI overwhelms the liver's detoxification capacity, leading to hepatocellular injury.
Understanding Toxic Doses
| Dose (mg/kg) | Risk Level | Clinical Significance |
|---|---|---|
| < 75 | Minimal | Unlikely to cause toxicity |
| 75-150 | Possible | May cause toxicity; monitoring recommended |
| 150-250 | Probable | Likely toxic; treatment usually indicated |
| > 250 | High | Severe toxicity risk; emergency treatment essential |
Maximum safe therapeutic dose is 4000 mg per day (1000 mg per dose, up to 4 times daily) for adults. A standard paracetamol tablet contains 500 mg.
Symptoms and Timeline
Paracetamol overdose progresses through distinct stages:
- Stage 1 (0-24 hours): Nausea, vomiting, pallor, malaise. Symptoms may be minimal or absent.
- Stage 2 (24-72 hours): Right upper quadrant pain, liver enzyme elevation, reduced symptoms (deceptive improvement)
- Stage 3 (72-96 hours): Peak liver damage, jaundice, coagulopathy, encephalopathy, possible kidney failure
- Stage 4 (4 days-2 weeks): Recovery or progression to liver failure requiring transplantation
Early presentation is crucial as antidote effectiveness decreases significantly after 8-10 hours post-ingestion.
Treatment: N-Acetylcysteine (NAC)
N-acetylcysteine (NAC) is the specific antidote for paracetamol poisoning. It works by:
- Replenishing glutathione stores in the liver
- Providing alternative pathway for toxic metabolite detoxification
- Reducing oxidative stress and hepatocellular damage
- Most effective when started within 8 hours of ingestion
- Can still provide benefit if started later, especially within 24 hours
NAC can be administered orally or intravenously. IV administration is preferred in cases of severe toxicity, vomiting, or when presentation is delayed.
Rumack-Matthew Nomogram
The Rumack-Matthew nomogram is the standard tool used in emergency departments to guide treatment decisions. It plots serum paracetamol concentration against time since ingestion.
Treatment lines on the nomogram:
- Treatment line starts at 150 mg/L at 4 hours post-ingestion
- Patients above this line require NAC treatment
- Some guidelines use a lower threshold (100 mg/L) for high-risk patients
- Nomogram is only valid for single acute ingestions
- Cannot be used if time of ingestion is unknown or for staggered overdoses
Special Considerations
Certain factors increase susceptibility to paracetamol toxicity:
- Chronic alcohol use (induces CYP2E1 enzyme)
- Malnutrition or eating disorders (depleted glutathione)
- Concurrent medications (carbamazepine, phenytoin, rifampicin)
- Pre-existing liver disease
- Fasting state
- Genetic variations in drug metabolism
For these high-risk patients, treatment may be initiated at lower thresholds.
When to Seek Emergency Help
Seek immediate medical attention if:
- Any intentional overdose or suicidal ingestion
- Ingestion exceeds 75 mg/kg or 7.5 grams in total
- Any symptoms develop after paracetamol ingestion
- Uncertain about amount or time of ingestion
- Child under 6 years ingests adult paracetamol products
- Staggered overdose over multiple hours/days
- Co-ingestion with other substances
Contact poison control (US: 1-800-222-1222, UK: 111) or go directly to emergency department. Do not wait for symptoms to develop.
Frequently Asked Questions
- How accurate is this calculator for determining treatment needs?
- This calculator provides a preliminary assessment only. Actual treatment decisions must be based on serum paracetamol levels measured at the appropriate time (4+ hours post-ingestion), plotted on the Rumack-Matthew nomogram, and interpreted by healthcare professionals. This tool cannot replace medical evaluation.
- When should NAC treatment be started?
- NAC is most effective when started within 8 hours of ingestion. However, if there's significant concern for toxicity (dose >150 mg/kg), treatment may be started empirically while awaiting serum levels. NAC can still provide benefit even if started after 24 hours in cases of established hepatotoxicity.
- What if I don't know the exact time or amount ingested?
- If the time of ingestion is unknown or if there was a staggered overdose (multiple ingestions over >8 hours), the nomogram cannot be used reliably. In these cases, medical evaluation is essential, and treatment decisions may be based on clinical presentation, liver function tests, and INR (clotting) results.
- Can children tolerate higher doses than adults?
- No, this is a myth. The toxic threshold (mg/kg) is similar for children and adults, though children may be at slightly lower risk due to different metabolism. However, any significant paracetamol ingestion in a child requires immediate medical evaluation. Never assume a child can tolerate an overdose.
- Is activated charcoal helpful for paracetamol overdose?
- Activated charcoal can be beneficial if given within 1-2 hours of ingestion and the patient can safely take it orally. However, it should not delay transfer to medical care or administration of NAC. Charcoal is contraindicated if the patient has altered mental status or is vomiting.
- What are the long-term effects of paracetamol overdose?
- If treated promptly with NAC, most patients recover fully without long-term liver damage. However, untreated or delayed treatment can lead to acute liver failure, which may require liver transplantation or can be fatal. Survivors of severe hepatotoxicity usually recover complete liver function if they survive the acute phase.