Tylenol (Acetaminophen) Overdose Risk and NAC Dosing Calculator

What counts as a Tylenol overdose?

An acute acetaminophen overdose is generally defined as a single ingestion that delivers 150 mg/kg or more of body weight, or roughly 7.5-10 g in an average-sized adult, within an 8-hour window. Below 75 mg/kg, risk is low in otherwise healthy individuals. The gray zone between 75 and 150 mg/kg requires a serum acetaminophen level drawn at exactly 4 hours post-ingestion, then compared to the Rumack-Matthew nomogram to decide whether N-acetylcysteine is needed. Children aged 1-6 are at significant hepatotoxicity risk at 150-200 mg/kg; infants and groups with reduced glutathione reserves (malnourished, alcohol-dependent, or fasting patients) may be harmed at lower doses.

The Rumack-Matthew nomogram

The Rumack-Matthew nomogram, published in 1975, is the standard clinical tool for deciding whether N-acetylcysteine is needed after an acute ingestion. A blood sample is drawn at exactly 4 hours post-ingestion (or later if the time is unknown) and the serum acetaminophen concentration is plotted against time on a semi-logarithmic graph. The "treatment line" starts at 150 mcg/mL at 4 hours and declines to 4.69 mcg/mL at 24 hours. Patients whose levels plot above this line are treated with NAC; those below it are monitored. The nomogram is valid only for acute single ingestions measured between 4 and 24 hours - it cannot be applied to chronic ingestion, unknown timing, or levels drawn outside that window.

N-acetylcysteine (NAC): how it works and when to give it

Acetaminophen is metabolized in the liver mainly to non-toxic sulfate and glucuronide conjugates. A small fraction is converted by cytochrome P450 enzymes (primarily CYP2E1) to a highly reactive metabolite called NAPQI. Glutathione normally neutralizes NAPQI, but in overdose the glutathione supply is overwhelmed and NAPQI binds directly to liver proteins, causing cell death. N-acetylcysteine works by replenishing glutathione and directly scavenging NAPQI. It is most effective when given within 8 hours of ingestion, but trials and clinical experience show meaningful benefit up to 24 hours and beyond, especially in severe cases. The IV 3-bag protocol delivers 150 mg/kg loading, then 50 mg/kg over 4 hours, then 100 mg/kg over 16 hours. The oral 72-hour protocol uses a 140 mg/kg loading dose followed by 70 mg/kg every 4 hours for 17 additional doses. Dosing weight is capped at 100 kg: the acetylcysteine injection label lists fixed doses of 15,000 mg, 5,000 mg, and 10,000 mg for patients weighing 100 kg or more, and notes that dose adjustment above 100 kg has not been studied.

The four stages of acetaminophen toxicity

Stage I (0-24 hours): The patient may seem well or have mild nausea, vomiting, and malaise. Liver enzymes are typically normal. Stage II (24-72 hours): Liver injury begins to manifest as rising transaminases (AST, ALT) and right upper quadrant pain. Patients often appear to improve clinically, which can be misleading. Stage III (72-96 hours): Peak hepatotoxicity, with transaminases often exceeding 10,000 IU/L, jaundice, encephalopathy, coagulopathy, and acute renal failure. This is the window of highest mortality. Stage IV (Recovery, day 4 onward): In survivors, liver function normalizes. Complete histologic recovery is expected by 3 months with no lasting cirrhosis or fibrosis, provided fulminant failure was avoided.