Acetaminophen Overdose With NAC Dosing Calculator

Dosages N-acetylcysteine in case of ingestion of high paracetamol dose in order to avoid hepatic risk.

There is in depth information below the calculator on how to perform the dosage and about the original study the method is based on.

The acetaminophen overdose with NAC dosing calculator is used to determine the N-acetylcysteine dosage (for IV or PO administration) needed to ensure there are no hepatic risks (hepatotoxicity) after ingestion of a high dosage of acetaminophen (APAP) containing drugs (Paracetamol, Tylenol etc).

Between 1976 and 1981, 662 patients with acetaminophen overdose have been treated with N-acetylcysteine, with the following incidence of hepatotoxicity in high risk overdoses:

■ 7% with treatment at 10 hours of ingestion;

■ 29% with treatment at 10 to 16 hours of ingestion;

■ 62% with treatment 16 to 24 hours of ingestion.

Oral administration of N-acetylcysteine starts with a loading dose calculated at 140mg/kg. Every 4 hours, 70mg/kg will be administered, 17 consecutive times.

Intravenous dosing starts with a loading dose and is followed by two more doses, all of which are calculated based on patient weight in kg. the recommendation table can be found in the text below the calculator.

Route of NAC (N-Acetylcysteine)*
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1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.

Variables considered

The acetaminophen overdose with NAC dosing calculator consists of two variables, the method of administration and the patient weight. The first allows you to choose from IV an PO while the second allows weight input in either kg or lbs.

The algorithm used by the calculator follows the recommended management procedure of acetaminophen overdose, meaning the administration of acetadote or NAC (N-acetylcysteine or N-acetyl-L-cysteine), within 8 hours post ingestion (the sooner, the better), to avoid hepatic risks. Hepatotoxicity is defined in relation with levels of AST or ALT higher than 1,000 IU/L.


PO or IV NAC dosing

Oral administration of N-acetylcysteine commences with a loading dose calculated at 140 mg/kg. Every 4 hours, 70 mg/kg will be administered, 17 consecutive times. For this type of administration, in case vomiting occurs within one hour of dose administration, the dose will be repeated.

Intravenous administration is exemplified in the below table, (where diluents of choice are either 5% dextrose D5W or ½ normal saline):

Weight/Dose Loading dose Dose II Dose III
≤ 20 150 mg/kg in 3 mL/kg diluent (60 min) 50 mg/kg in 7 mL/kg diluent (4 hr) 100 mg/kg in 14 mL/kg diluent (16 hr)
20 – 40 kg 150 mg/kg in 100 mL diluent (60 min) 50 mg/kg in 250 mL diluent (4 hr) 100 mg/kg in 500 mL diluent (16 hr)
≥ 40 kg 150 mg/kg in 200 mL diluent (60 min) 50 mg/kg in 500 mL diluent (4 hr) 100 mg/kg in 1000 mL diluent (16 hr)

Dosage guidelines

When the exact time of ingestion is known and intervention is at around 4 hours after ingestion, the acetaminophen toxicity can be plotted on the Rumack-Matthew nomogram. This is a toxicology tool that facilitates the outcome prediction and the risk of hepatotoxicity after high intake.

This type of overdose is one of the most common types of poisoning, especially because of the general conception that this pain medication is a safe one. Due to this, people are most likely to superimpose or take higher doses without respecting the time in between doses (the recommended half time of the medication).

Acetylcysteine is a generic medication, present in the World Health Organization's List of Essential Medicines. It is also used in the treatment of chronic obstructive pulmonary disease.

There are certain contraindications for its usage, such as hypersensitive patients, patients with previous anaphylactoid reaction or history of bronchospasm.


About the original study

Rumack conducted a study between 1976 and 1981 that assessed the effectiveness of NAC dosing in acetaminophen overdose cases, thus establishing the first guidelines or intervention. 662 consecutive patients with acetaminophen overdose, within the previous 24 hours, have been included.

The results have shown that the incidence of hepatotoxicity in high risk patients (toxic range acetaminophen concentration), treated with NAC, was:

■ 7% when treated within 10 hours of ingestion.

■ 29% when treated within 10 to 16 hours of ingestion.

■ 62% when treated within 16 to 24 hours of ingestion.

This has established the highest possible efficiency of the treatment within 8 hours of ingestion.

Barry H. Rumack, MD, is director emeritus of the Rocky Mountain Poison and Drug Center and director, Office of Professionalism at UC Denver.


Original reference

Rumack BH, Peterson RC, Koch GG, Amara IA. Acetaminophen overdose. 662 cases with evaluation of oral acetylcysteine treatment. Arch Intern Med. 1981; 141(3 Spec No):380-5.

Other references

1. Prescott LF, Illingworth RN, Critchley JA, Stewart MJ, Adam RD, Proudfoot AT. Intravenous N-acetylcystine: the treatment of choice for paracetamol poisoning. Br Med J. 1979; 2(6198):1097-100.

2. Smilkstein MJ, Knapp GL, Kulig KW, Rumack BH. Efficacy of oral N-acetylcysteine in the treatment of acetaminophen overdose. Analysis of the national multicenter study (1976 to 1985) N Engl J Med. 1988; 319(24):1557-62.

Specialty: Toxicology

System: Dosage

Objective: Dosage

Type: Calculator

No. Of Variables: 2

Year Of Study: 1981

Article By: Denise Nedea

Published On: March 16, 2017

Last Checked: March 16, 2017

Next Review: March 10, 2023