Acetaminophen Toxicity

Acetaminophen (paracetamol) toxicity is liver injury. It comes from one large dose, or from exposure piling up above the safe daily limit. At labeled doses, acetaminophen is usually safe. So what goes wrong? The normal glucuronidation and sulfation pathways get saturated. More drug then goes down the CYP2E1 route and becomes NAPQI, a reactive metabolite. NAPQI burns through glutathione, then injures hepatocytes.

• Acute ingestion: One large dose, all at once. The 2023 US and Canada consensus calls it acute when the patient presents 4 to 24 hours after the overdose began.
• Repeated supratherapeutic ingestion: doses above the daily maximum, stacked up over days. The nomogram doesn't apply here. Lean on the level, AST, ALT and INR.
• Extended-release products: absorption can drag on, so one early level may underestimate the exposure.
• Hidden sources: combination opioid products, cold, flu and sleep remedies. The FDA puts hepatotoxicity boxed warnings on IV and prescription combination products.

Typical Presentation

• Early phase (first 24 hours): often nothing at all, or nausea, vomiting, malaise. Vitals usually look normal.
• Hepatic phase (about 24 to 72 hours): right upper quadrant pain, rising AST and ALT, jaundice, coagulopathy and, in severe cases, encephalopathy.

Red Flags

• Any intentional or unknown-time ingestion of acetaminophen, even if the patient denies it.
• Vomiting or altered mental status after a possible acetaminophen ingestion.
• Right upper quadrant pain, jaundice or bleeding in a patient with a known or suspected ingestion.

Wait at least 4 hours after the ingestion, then draw the acetaminophen level. Why the wait? Absorption isn't complete yet, so an earlier level may mislead you. Plot the result on the revised Rumack-Matthew nomogram. It's valid from 4 to 24 hours.

• Viral hepatitis or other hepatitis: many causes can push transaminases up. A detectable acetaminophen level plus a compatible history points you to acetaminophen.
• Salicylate or other co-ingestant toxicity: When the history is murky, check a salicylate level.
• Hypoglycemia or sedation from other drugs: plenty of non-acetaminophen causes can produce coma and vomiting. Review hypoglycemia and toxidromes.

1. Scene safety and history: ask what was taken, how much, and when. Bring all containers and pill bottles.
2. Airway and monitoring: support airway and breathing as needed. Monitor vital signs and mental status.
3. Vomiting: position the patient to protect the airway and suction as needed.
4. Notify early: report the time and amount to the receiving facility and the poison center. Do not delay transport for a level.
5. Transport priority: a patient with a possible intentional overdose goes to an emergency department.

Doses vary by protocol; follow local protocols and medical direction.

• IV access and labs (Paramedic/RN/CCT): establish IV access and draw blood for the level when it can be timed, along with a glucose. Send samples with the ingestion time on the requisition.
• Acetylcysteine (Paramedic/RN/CCT): IV acetylcysteine is usually stocked by hospital pharmacy, not EMS. Give it on arrival, or begin when a receiving physician or poison center directs it. See N-Acetylcysteine (NAC) for the full regimen.
• Dose (Paramedic/RN/CCT): two-bag regimen for weight 41 kg or more: 200 mg/kg IV over 4 hours, then 100 mg/kg over 16 hours. Three-bag regimen for weight 40 kg or less: 150 mg/kg over 1 hour, 50 mg/kg over 4 hours, 100 mg/kg over 16 hours. Total 300 mg/kg. The label recommends the three-bag regimen at 40 kg or less, because two-bag dosing is not established in that group. The three-bag regimen can also be used at 41 kg or more and may be preferred for a large ingestion or early signs of severe liver injury, with a higher rate of hypersensitivity reactions. The label does not establish that two-bag equals three-bag for preventing liver injury. At the 100 kg cap, the amounts are fixed: two-bag 20,000 mg then 10,000 mg; three-bag 15,000 mg, 5,000 mg, then 10,000 mg.
• Dilution and volume (Paramedic/RN/CCT): the product is hyperosmolar (2,600 mOsmol/L) and must be diluted before IV use. Label diluent volumes are weight-banded; use the label volume table for the dosing weight, and check it with a second clinician. Label diluents are 0.45% sodium chloride (preferred), or sterile water for injection or 5% dextrose in water if sodium load is a concern. The diluted solution is stable for 24 hours at room temperature. For patients under 40 kg, the label says to adjust the total volume, and it does not recommend pediatric dosing under 5 kg, which has not been studied.
• Anaphylaxis (Paramedic/RN/CCT): stop the infusion for wheezing, hypotension or angioedema, and treat per anaphylaxis protocol. Infusion reactions are common; the label states that serious reactions can be fatal.
• Monitoring (Paramedic/RN/CCT): cardiac monitor, SpO₂ and blood pressure. Recheck glucose.

• Nomogram decision: with a known time and a level at 4 h or later, start acetylcysteine at or above the treatment line. Below the line, stop, but only when the time is known and more than 4 hours have passed. The 2023 US and Canada consensus sets the treatment line at 150 mcg/mL at 4 hours. The high-risk line sits at 300 mcg/mL at 4 hours (see Massive ingestion below).
• Known ingestion more than 8 hours ago: start the acetylcysteine loading dose now. Don't wait for the level. Draw it with the liver tests, then let those results decide whether to continue, per the Acetadote label.
• Unknown time, or level not available within 8 hours: give the full course right away, per the Acetadote label. Repeat the level. Consider stopping only after a second non-toxic level.
• Extended-release product: a 4-hour level below the line is not the final word. Take a second level at 8 to 10 hours after ingestion (Acetadote label).
• Repeated supratherapeutic ingestion: the nomogram doesn't apply, so don't plot it. Treat by level, AST, ALT and INR, with toxicology input.
• Stopping criteria: after the final bag, check the acetaminophen level, AST, ALT and INR. The 2023 consensus lists no numeric stopping criteria, so clinical markers set the duration. Common end points, to confirm with the toxicologist: acetaminophen not detectable, AST and ALT normal or falling, INR not elevated. Still detectable, or liver tests still rising? Continue, and call the poison center (1-800-222-1222, per label).
• Massive ingestion (high-risk): the 2023 consensus calls it high-risk ingestion, not massive ingestion. High-risk means at least 30 g, or a level above the high-risk line (300 mcg/mL at 4 hours). Consult a toxicologist or the poison center. The consensus says increased acetylcysteine dosing may be warranted in consultation with them. The literature still doesn't support a specific dose above 300 mg/kg. Measure PT/INR. Add laboratory testing if signs of mitochondrial dysfunction appear.
• Fomepizole: proposed as an add-on to acetylcysteine. The 2023 consensus found no data to support a standard recommendation. It sits outside standard care on this page, and it's a toxicologist decision.
• Activated charcoal: consider it early after ingestion, per poison center advice, but only if the airway is protected and the patient is alert enough to swallow safely. Don't give it with a depressed or unprotected airway, or with vomiting. Don't hold up acetylcysteine for it.
• Escalation: encephalopathy, coagulopathy or hemodynamic instability means ICU. Severe acute liver failure calls for transplant or extracorporeal assessment, per toxicology.

The level and the liver tests are the only reliable guide. The reported amount is often inaccurate, so it can't guide therapy (Acetadote label).

• Destination: take them to the nearest emergency department that can start IV acetylcysteine and monitor liver function. Critical-care transport fits patients with encephalopathy, shock, or a dose that needs a toxicologist waiting on arrival.
• In transit: monitor continuously. If the acetylcysteine infusion is already running, keep it going and watch for hypersensitivity reactions. Log which bag is running, when it started and how much volume is left. Don't interrupt between bags. The label schedule is 1, 4 and 16 hours for the three-bag regimen, and 4 and 16 hours for the two-bag regimen. Have the next bag ready before you leave, and tell the receiving unit the total time on acetylcysteine.
• Handoff: time of ingestion, amount, product names, time of any acetaminophen level, doses given, acetylcysteine start time and volume remaining, and the poison center contact.
• Equipment: infusion pump with weight-based calculation checked by a second clinician, and a dilution record.

• Pediatrics: the IV acetylcysteine regimen is weight-based, and the label tables start at 5 kg. Weights under 5 kg haven't been studied. Use weight-based dosing and the poison center. Children follow the same nomogram and treatment rules. Small children carry the diluent risk. Work out the weight-banded diluent volume from the label table, and have a second clinician check it. The label says to reduce total volume for patients under 40 kg or with fluid restriction. It also says to keep diluted osmolarity at generally not less than 150 mOsmol/L in pediatric patients, which limits fluid and sodium load.
• Pregnancy: the 2023 consensus keeps evaluation and management the same in pregnancy, standard nomogram and acetylcysteine dosing included. The label (section 8.1) warns that delaying treatment may increase maternal and fetal morbidity and mortality. So don't delay treatment. Document a pregnancy test for females of childbearing age, and bring in a toxicologist and obstetric care.
• Older Adults: no specific adjustment has been established. Keep an eye on reduced body mass and renal function.
• Renal Impairment: the label gives no specific acetylcysteine dose adjustment. Hemodialysis can strip out some acetylcysteine, so get the poison center to set dosing during dialysis. Monitor renal function and electrolytes through the whole course.
• Hepatic Impairment: in hepatic impairment, the label reports the acetylcysteine half-life rose about 80% and exposure about 1.6-fold. The label doesn't consider those changes clinically meaningful, so no dose adjustment is given. The label advises continuing dosing beyond 300 mg/kg after a prolonged or massive ingestion, or with preexisting liver disease, if acetaminophen, ALT, AST or INR stay abnormal.
• Obesity: the label tables cap weight-based amounts at 100 kg. Data above 100 kg are limited, the label notes.
• Chronic Alcohol Use and Malnutrition: the nomogram may underestimate risk here. Consider treatment, and call the poison center even when the level is below the line.
• Patients on Isoniazid or Other CYP2E1 Inducers: the nomogram may underestimate risk here too. Consider treatment, and call the poison center even when the level is below the line.

• Error: a level drawn before 4 hours can look negative, and the negative gets trusted. Do instead: repeat the level at 4 hours or later.
• Error: treating off the reported amount. Do instead: treat off the level, the time and the liver tests.
• Error: stopping acetylcysteine because one level is nontoxic, in an unknown-time or early-draw case. Do instead: finish the course or repeat the level, per the label.
• Error: weight-based math going wrong. Do instead: work from the label tables, have a second clinician check the calculation, and write down the dosing weight.
• Error: combination products or repeat doses slipping past you. Do instead: ask about cold, flu and opioid products, and add up the daily intake.
• Warning: acetylcysteine causes infusion reactions, and it can cause anaphylaxis. For serious reactions (wheezing, hypotension, angioedema), stop the infusion and treat. Mild reactions, like flushing or rash, need a toxicologist's decision on whether to pause, treat and restart. Don't interrupt therapy for a mild reaction without that advice.
• Error: discharging after an early presentation that looks normal. Do instead: finish the assessment and time the level before disposition.