Tylenol (Acetaminophen) Overdose Calculator — NAC Dosing

Acetaminophen Toxicity Overview

Acetaminophen (APAP, paracetamol) overdose is the most common cause of acute liver failure in the United States and the United Kingdom. At therapeutic doses, approximately 90% of acetaminophen is metabolized by glucuronidation and sulfation into non-toxic metabolites. However, about 5–10% is oxidized by the cytochrome P450 system (primarily CYP2E1) to form the highly reactive toxic metabolite NAPQI (N-acetyl-p-benzoquinone imine).

At therapeutic doses, NAPQI is quickly conjugated with glutathione and excreted as non-toxic mercapturic acid conjugates. In overdose, glucuronidation and sulfation pathways become saturated, and a larger proportion of acetaminophen is shunted through the CYP2E1 pathway. This overwhelms the glutathione supply, and unconjugated NAPQI accumulates, binding to hepatocyte proteins and causing centrilobular hepatic necrosis.

Four Stages of Acetaminophen Toxicity

Stage	Time Post-Ingestion	Clinical Features	Laboratory Findings
Stage I	0 – 24 hours	Nausea, vomiting, malaise, diaphoresis; may be asymptomatic	Normal or mildly elevated AST/ALT; elevated APAP level
Stage II	24 – 72 hours	Resolution of GI symptoms; right upper quadrant pain and tenderness; oliguria	Rising AST/ALT (may exceed 10,000 IU/L); rising bilirubin, INR, creatinine
Stage III	72 – 96 hours	Peak hepatotoxicity; return of nausea/vomiting; jaundice; confusion; possible multi-organ failure	Peak AST/ALT; severely elevated INR, bilirubin; metabolic acidosis; hypoglycemia; lactic acidosis
Stage IV	4 days – 3 weeks	Recovery phase (if survival); gradual resolution of symptoms	Normalizing liver enzymes over 1–3 weeks; complete histologic recovery expected

Rumack-Matthew Nomogram

The Rumack-Matthew nomogram is the primary tool for assessing the risk of hepatotoxicity after a single acute acetaminophen ingestion. It plots the serum acetaminophen concentration (measured 4 or more hours after ingestion) against time post-ingestion.

• Treatment line: Starts at 150 mcg/mL at 4 hours and decreases to approximately 4.7 mcg/mL at 24 hours (semi-logarithmic decline)
• Patients with levels above the treatment line should receive NAC
• The nomogram applies only to single acute ingestions with known time of ingestion
• It is NOT valid for chronic/repeated ingestions, unknown time of ingestion, or extended-release formulations

How NAC Works

N-Acetylcysteine (NAC) is the definitive antidote for acetaminophen poisoning. It works through several mechanisms:

• Glutathione precursor: NAC is a precursor to glutathione (GSH). It provides cysteine, the rate-limiting amino acid for GSH synthesis, allowing the liver to regenerate glutathione and detoxify NAPQI.
• Direct NAPQI scavenging: NAC can directly conjugate with NAPQI as a substitute for glutathione.
• Enhanced sulfation: NAC provides sulfate for the non-toxic sulfation pathway, diverting more acetaminophen away from the CYP2E1 pathway.
• Antioxidant and anti-inflammatory: NAC has additional antioxidant and anti-inflammatory properties that may protect against hepatocyte injury.
• Improved microcirculation: NAC improves hepatic blood flow and oxygen delivery, which may benefit patients even in late-stage toxicity.

NAC is most effective when administered within 8 hours of ingestion, with a success rate of nearly 100% in preventing hepatotoxicity. However, it still provides benefit even when given up to 24–36 hours post-ingestion, or later in patients with established liver failure.

IV Protocol (21-Hour)

The IV protocol is the most commonly used in hospital settings. It delivers NAC over 21 hours in three sequential infusions:

Bag	NAC Dose	Diluent	Duration	Rate Calculation
Bag 1 (Loading)	150 mg/kg	200 mL D5W	60 minutes	Total volume / 60 min
Bag 2	50 mg/kg	500 mL D5W	4 hours	Total volume / 240 min
Bag 3	100 mg/kg	1,000 mL D5W	16 hours	Total volume / 960 min

Oral Protocol (72-Hour)

The oral (Prescott) protocol is an alternative when IV administration is not available or not preferred:

Step	NAC Dose	Timing	Notes
Loading Dose	140 mg/kg	Immediately	Mixed with cola or juice to mask taste
Maintenance Doses	70 mg/kg each	Every 4 hours × 17 doses	If patient vomits within 1 hour, redose

NAC Treatment Timeline Diagram

When to Give NAC

NAC should be initiated in the following scenarios:

• Acute single ingestion: Serum APAP level at or above the treatment line on the Rumack-Matthew nomogram (level drawn 4+ hours post-ingestion)
• Unknown time of ingestion: Detectable APAP level OR elevated aminotransferases (AST/ALT > 50 IU/L)
• Repeated supratherapeutic ingestion: If APAP level is detectable or AST/ALT is elevated
• Late presentation (> 24 hours): If evidence of hepatotoxicity (elevated AST/ALT, INR, or detectable APAP)
• Empiric treatment: If there is strong clinical suspicion and a 4-hour APAP level cannot be obtained within 8 hours of ingestion

Key point: When in doubt, start NAC. It is safe and the risk of not treating a potentially toxic ingestion far outweighs the risk of adverse effects from NAC.

Worked Example

A 70 kg adult presents after acute acetaminophen ingestion. The decision is made to treat with IV NAC:

Frequently Asked Questions

What are the side effects of NAC?

The most common side effects of IV NAC are anaphylactoid reactions (not true allergy), which occur in 10–20% of patients, typically during the loading dose infusion. Symptoms include flushing, urticaria, pruritus, nausea, and occasionally bronchospasm or hypotension. These reactions are usually managed by temporarily stopping the infusion, administering antihistamines (diphenhydramine), and then restarting at a slower rate. Oral NAC commonly causes nausea and vomiting due to its unpleasant taste and smell (sulfur).

Can NAC be given after 24 hours?

Yes. While NAC is most effective within 8 hours of ingestion, it still provides significant benefit when given later, even up to 48–72 hours post-ingestion. In patients with established hepatotoxicity or liver failure, NAC improves survival and should be continued until evidence of clinical improvement.

Which protocol is preferred, IV or oral?

The IV 21-hour protocol is preferred in most emergency settings due to its shorter duration and guaranteed delivery. The oral 72-hour protocol is an alternative when IV access is not available, but has a higher rate of nausea/vomiting and requires prolonged treatment. Both protocols have similar efficacy when started within 8 hours.

What is the maximum dose for patients over 100 kg?

For patients weighing over 100 kg, some guidelines recommend capping the calculation at 100 kg for the IV protocol to reduce the risk of fluid overload and anaphylactoid reactions. However, this is controversial and institution-specific. Always follow your local toxicology service or poison center recommendations.

What lab tests should be monitored?

Recommended labs include: serum acetaminophen level (at 4+ hours post-ingestion), AST, ALT, INR/PT, total bilirubin, creatinine, BUN, electrolytes, glucose, lipase, and CBC. Serial monitoring of AST/ALT and INR every 6–12 hours is essential to track hepatotoxicity progression or resolution.