N-Acetylcysteine Dose Calculator
N-Acetylcysteine Dose Calculator
Work out the total intravenous acetylcysteine dose for the standard three-bag 21-hour regimen, with each bag's dose, diluent volume and infusion time, and the reasons an infusion reaction is not a reason to stop.
Acetylcysteine 21-hour total
Weight → 21-hour totalA 70 kg adult on the standard three-bag intravenous regimen
Formula
Bag 1: 150 mg/kg over 60 minutes · Bag 2: 50 mg/kg over 4 hours · Bag 3: 100 mg/kg over 16 hours
- 150 mg/kg
- the loading dose, in 200 mL of 5% dextrose over 60 minutes — the original regimen used 15 minutes, and the slower loading infusion reduces anaphylactoid reactions
- 50 mg/kg
- the second bag, in 500 mL of 5% dextrose over 4 hours
- 100 mg/kg
- the third bag, in 1000 mL of 5% dextrose over 16 hours
- diluent volume
- reduced in proportion to weight below 40 kg and in fluid restriction; the mg/kg dose itself does not change
Worked example
A 70 kg adult on the standard three-bag intravenous regimen
Bag 1: 150 × 70 = 10 500 mg in 200 mL of 5% dextrose over 60 minutes
Bag 2: 50 × 70 = 3500 mg in 500 mL over 4 hours
Bag 3: 100 × 70 = 7000 mg in 1000 mL over 16 hours
Total = 300 × 70 = 21 000 mg over 21 hours in 1700 mL of diluent
The three-bag regimen worked through for a 70 kg adult
| Bag | Dose | For 70 kg | Diluent (5% dextrose) | Infusion time |
|---|---|---|---|---|
| 1 — loading | 150 mg/kg | 10 500 mg | 200 mL | 60 minutes |
| 2 | 50 mg/kg | 3500 mg | 500 mL | 4 hours |
| 3 | 100 mg/kg | 7000 mg | 1000 mL | 16 hours |
| Total | 300 mg/kg | 21 000 mg | 1700 mL | 21 hours |
Reactions during the loading infusion
| Feature | What it is | Response |
|---|---|---|
| Flushing, urticaria, itch | Direct histamine release, rate-related | Antihistamine; slow or briefly pause the infusion, then continue |
| Nausea and vomiting | Common, often at the start of the load | Antiemetic; the infusion usually continues |
| Bronchospasm | Anaphylactoid, not IgE-mediated | Stop, treat with a bronchodilator, restart at a slower rate once settled |
| Hypotension | Uncommon but the most serious form | Stop, treat, seek toxicology advice before restarting |
Getting the three bags, the volumes and the timing right
Intravenous acetylcysteine is given as three consecutive infusions over 21 hours. The loading dose is 150 mg/kg in 200 mL of 5% dextrose over 60 minutes, followed by 50 mg/kg in 500 mL over 4 hours, then 100 mg/kg in 1000 mL over 16 hours, giving 300 mg/kg in total. The three bags differ in concentration as well as in dose, so the commonest preventable error is a rate or a volume carried across from the wrong bag. Write the regimen out in full for each patient rather than reconstructing it from memory at the bedside.
The volumes are not fixed. In patients under 40 kg the diluent volumes are reduced in proportion to weight, because the adult volumes represent a large free-water load for a child and have caused hyponatraemia and seizures; the same reduction applies to anyone who is fluid-restricted. At the other end, many services cap the weight used for the calculation at 110 kg, since the regimen was never validated at extremes of weight and the total otherwise rises without evidence of added benefit.
Reactions during the loading infusion are common. Flushing, itch and urticaria are typical, with bronchospasm and hypotension less often. These are anaphylactoid rather than allergic: they arise from direct histamine release, are related to infusion rate and concentration rather than prior sensitisation, and are more frequent when the paracetamol concentration is low. The response is to slow or pause the infusion and treat symptomatically, then restart more slowly — not to abandon the antidote. A previous reaction is not a contraindication.
Timing dominates outcome. Acetylcysteine started within 8 hours of ingestion is close to fully protective against hepatotoxicity and its benefit declines steadily after that, so treatment should begin on the dose history rather than wait on a confirmatory concentration that will not return inside that window. The 21-hour course is a minimum rather than a fixed endpoint: it is extended if transaminases are rising or paracetamol remains detectable at the end of the third bag. Poisoning management is time-critical and should be directed by a poisons centre or clinical toxicology service, and this calculator supports that advice rather than replacing it.
Frequently asked questions
What is the intravenous acetylcysteine dose?
The standard three-bag regimen gives 150 mg/kg in 200 mL of 5% dextrose over 60 minutes, then 50 mg/kg in 500 mL over 4 hours, then 100 mg/kg in 1000 mL over 16 hours. That is 300 mg/kg in total over 21 hours — 21 000 mg for a 70 kg adult.
Should the diluent volume change for a child or a fluid-restricted patient?
Yes. Below 40 kg, and in anyone who is fluid-restricted, the diluent volumes are reduced in proportion to body weight. The mg/kg dose itself is unchanged. Excessive free water in small children has caused hyponatraemia and seizures.
What do I do if the patient reacts during the loading infusion?
Slow or pause the infusion and treat the symptoms. These reactions are anaphylactoid, caused by direct histamine release and related to infusion rate rather than true allergy, so treatment is restarted at a slower rate rather than abandoned. A previous reaction is not a contraindication.
Should acetylcysteine wait for the paracetamol concentration?
Not when the result will not be available within 8 hours of ingestion. Treatment started inside that window is close to fully protective and its benefit falls away steadily afterwards, so a plausible significant ingestion is treated on the dose history and stopped later if the concentration turns out to be low.
Does the course always stop at 21 hours, and who should I be talking to?
No. The course is extended if transaminases are rising or paracetamol is still detectable at the end of the third bag. Poisoning management is time-critical and directed by a poisons centre or clinical toxicology service; this calculator supports that conversation and never replaces it.
Related calculators
References
- Acetadote (acetylcysteine) injection, for intravenous use — prescribing information. Cumberland Pharmaceuticals Inc.
- Prescott LF, Illingworth RN, Critchley JA, Stewart MJ, Adam RD, Proudfoot AT. Intravenous N-acetylcysteine: the treatment of choice for paracetamol poisoning. Br Med J. 1979;2(6198):1097–1100.
- Chiew AL, Reith D, Pomerleau A, et al. Updated guidelines for the management of paracetamol poisoning in Australia and New Zealand. Med J Aust. 2020;212(4):175–183.