This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
If someone may have taken too much acetaminophen, call Poison Control at 1-800-222-1222 or 911 now. Do not wait for symptoms.
Why the level and the time both matter
Acetaminophen (paracetamol) is safe at normal doses, but in overdose part of it is turned into a toxic product, NAPQI, that the liver normally neutralizes with glutathione. When glutathione runs out, liver cells die. The antidote acetylcysteine (NAC) restores glutathione and is nearly fully protective when started within 8 hours. A single blood level is hard to read alone: the same value can be harmless 20 hours after ingestion and dangerous at 4 hours. In 1975, Barry Rumack and Henry Matthew plotted levels against time to create the nomogram still used today.
The treatment line
- At 4 hours, the US treatment line is 150 µg/mL (mg/L), equal to about 1,000 µmol/L.
- The line falls by half every 4 hours: 75 at 8 hours, 37.5 at 12 hours, about 9.4 at 20 hours and 4.7 at 24 hours.
- Formula: line = 150 × 2^(−(t − 4) ÷ 4), valid from 4 to 24 hours.
The original probable toxicity line started at 200; the 150 line adds a safety margin. The UK uses a single 100 line for all patients since 2012.
Example
A patient took about 10 g of acetaminophen at a known time. The level drawn 8 hours later is 80 µg/mL:
- Treatment line at 8 hours: 75 µg/mL
- Level: 80 ÷ 75 = 107% of the line, above it: acetylcysteine is indicated
- Dose: 10,000 mg ÷ 70 kg = 143 mg/kg, close to the 150 mg/kg threshold
When the nomogram does not apply
- Levels drawn before 4 hours: repeat at 4 hours or later.
- Unknown time of ingestion, or more than 24 hours since it.
- Repeated or staggered overdoses, such as too many doses over a day or two for pain.
- Some extended-release products and large ingestions may need a second level.
In these situations, clinicians usually treat based on the dose taken, the liver enzymes (ALT, AST) and the advice of the poison center. A detectable level with raised ALT, or any doubt, is a reason to start acetylcysteine.
Preventing accidental overdose
Many cold, flu and sleep products also contain acetaminophen. Check labels, do not exceed 4,000 mg a day for healthy adults (often 3,000 mg or less is advised), and ask a pharmacist if you drink alcohol regularly or have liver disease.