Paracetamol (Acetaminophen) Poisoning

Overview

Paracetamol is largely metabolized harmlessly by glucuronidation and sulfation at therapeutic doses; a small portion is converted by CYP2E1 to highly reactive NAPQI (N-acetyl-p-benzoquinone imine) and neutralized by glutathione. In overdose, these safe pathways are saturated, NAPQI accumulates, and glutathione stores are depleted; free NAPQI covalently binds to hepatocytes causing centrilobular (Zone 3) necrosis.

Therefore, the essence of treatment is to replace glutathione. The clinical course is described in four stages:

Stage I (0-24 hours) the patient is mostly asymptomatic or only reports nausea/vomiting and laboratory findings may be absent during this period; Stage II (24-72 hours) transaminases start to rise along with right upper quadrant pain and tenderness; Stage III (72-96 hours) is the stage where fulminant hepatic necrosis develops and AST/ALT peak values often exceed 10,000 IU/L, encephalopathy, coagulopathy, lactic acidosis, and acute kidney injury appear at this stage and most deaths occur during this period; Stage IV (4 days–2 weeks) the liver shows full histological recovery in surviving patients and does not leave chronic damage. The toxic threshold is considered in adults as a single dose of 150 mg/kg or over 7.5-10 grams (or over 12 grams in 24 hours), and in healthy children aged 1-6 years as approximately 200 mg/kg; chronic alcohol use, fasting/malnutrition, and enzyme-inducing drugs (phenytoin, carbamazepine, rifampin, chronic isoniazid) increase risk by enhancing CYP2E1 activity and reducing glutathione stores.

The consensus statement published in 2023 by the American and Canadian toxicology societies (Dart et al., JAMA Network Open 2023) suggested a more sensitive threshold by lowering the treatment line of the Rumack-Matthew nomogram from 200 mcg/mL to 150 mcg/mL at 4 hours (guiding more patients to NAC); this change remains the basis of current practice. The nomogram is VALID ONLY for a single acute ingestion when the time of ingestion is precisely known, for a single serum level taken 4-24 hours post-ingestion; it is assumed absorption is completed by 4 hours.

In cases of repeated/chronic supratherapeutic ingestion, unknown ingestion time, or extended-release (XR) form ingestion, the nomogram is unreliable; in these cases, levels, transaminases, and the clinical picture are evaluated together, and empiric NAC is initiated if necessary without waiting for levels.

Key considerations

• Viral hepatitis (primarily A, B, E) causes elevated liver enzymes and right upper quadrant pain; however, unlike acetaminophen poisoning, the history usually begins with fatigue and jaundice developing over days, and a sudden transaminase spike is not observed.
• Ischemic (shock) liver occurs after hypotension or cardiac arrest; transaminases can be extremely high, similar to acetaminophen poisoning, but lactate dehydrogenase is disproportionately elevated, and the clinical picture includes a clear history of a hypoperfusion episode.
• Autoimmune hepatitis is distinguished in female patients by a slower course of jaundice and fatigue; diagnosis is confirmed by autoantibodies and elevated immunoglobulin G.
• Amanita phalloides (mushroom) poisoning begins 6-24 hours after ingestion with severe gastroenteritis, followed by a false recovery period; unlike acetaminophen, acetaminophen level is negative and a history of mushroom ingestion should be queried.
• Other simultaneously ingested hepatotoxic substances or agents causing metabolic acidosis, such as salicylate or iron poisoning, should certainly be investigated alongside acetaminophen levels in patients with a mixed drug ingestion history.
• Toxic alcohol ingestion (methanol, ethylene glycol) causes a marked high anion gap metabolic acidosis in the early period; in paracetamol poisoning, early acidosis normally suggests massive ingestion, therefore the two should be differentiated.
• Hypoperfusion due to sepsis and septic shock can cause elevated transaminases and lactic acidosis; however, the presence of an accompanying infection focus, fever, and hemodynamic instability explains the picture independently of a history of paracetamol.
• Acute fatty liver of pregnancy and HELLP syndrome in pregnancy can mimic paracetamol poisoning in the third trimester with right upper quadrant pain, elevated transaminases, and coagulopathy; hypertension, proteinuria, and thrombocytopenia support these diagnoses.
• Drug-induced liver injury associated with other over-the-counter/prescription hepatotoxic drugs (isoniazid, valproate, some herbal supplements) is excluded by taking a drug history and by a negative paracetamol level.

Why it matters

N-acetylcysteine replenishes glutathione synthesis by providing cysteine, and also supports harmless metabolism by directly binding NAPQI with its sulfhydryl group; when given early (especially before 8 hours), it prevents hepatotoxicity almost entirely, and has been shown to reduce mortality even in established liver injury, therefore it should be started even in late-presenting patients. The 2023 consensus statement prepared by the American and Canadian toxicology societies (Dart et al., JAMA Network Open 2023;6(8):e2327739) provides an algorithm for treatment decisions based on the timing of ingestion and the reliability of the history, and updates the treatment line of the modified Rumack-Matthew nomogram to 150 mcg/mL at the 4 hour; with this change, starting NAC at a lower level is recommended to increase sensitivity.

The most critical variable when deciding is the time of ingestion: in a single acute ingestion, levels between 4-24 hours are interpreted using the nomogram, but if the time is unknown, the ingestion is delayed, or repeated, the nomogram is unreliable and NAC is started empirically. The structure of the IV NAC regimen in three phases (150+50+100 mg/kg) and its total duration of 21 hours is based on the label of Acetadote approved in the United States; in the United Kingdom, however, the SNAP study by Bateman et al. (Lancet 2014) showed that a shortened 12-hour regimen with two bags and a slower loading rate reduced the frequency of anaphylactoid reactions compared to the classic three-bag regimen; therefore, there is a significant divergence in the number and rate of NAC regimens between U.S. and U.K. guidelines, and the clinician should know which regimen they are using.

Anaphylactoid reactions are reported in the literature over a very wide range (varying between approximately 4% and 50% across series), and this large difference is thought to be largely due to differences in definitions; it has been reported that this reaction is more common in patients with low paracetamol levels and less common in those with high levels, and most reactions occur within the first 15 minutes of the infusion; therefore, slowing the infusion rate and giving an antihistamine is sufficient in most cases, and it is rarely necessary to stop NAC completely. In cases of massive ingestion (cumulative over 30-50 grams or a 4. hour level above 250-500 mcg/mL), the standard NAC may be insufficient, so the maintenance dose is increased, the CYP2E1 inhibitor fomepizole is considered as an adjuvant, and hemodialysis is added according to the 2014 recommendations of the EXTRIP working group: if the APAP level is above 1000 mg/L and NAC has not been given; if there are signs of mitochondrial dysfunction (altered consciousness and severe metabolic acidosis preceding hepatic failure) along with APAP over 700 mg/L and NAC has not been given; or if the APAP level is over 900 mg/L with the same mitochondrial dysfunction signs and this condition persists despite NAC administration, hemodialysis is indicated.

In those developing fulminant failure, the King's College criteria (arterial pH <7.3 alone; or INR >6.5 with creatinine >300 µmol/L and the presence of Stage III-IV encephalopathy) indicate a low chance of spontaneous recovery and require urgent liver transplant evaluation. The previously used approach in some centers of 'if a single level is normal, NAC can be stopped' has been abandoned; the current recommendation is to indefinitely extend the maintenance infusion if ongoing hepatotoxicity signs (rising ALT, abnormal INR, detectable level) are present, because there is no guarantee that the damage has completely stopped at the end of the fixed 21-hour duration.

Critical complications

• Fulminant hepatic failure develops if treatment is delayed or in cases of massive ingestion; it is recognized by encephalopathy, coagulopathy, and hypoglycemia, and is managed in intensive care with glucose support, prognostic evaluation by INR monitoring instead of fresh frozen plasma, and early communication with a transplant center.
• Acute kidney injury can develop independently of liver damage due to the formation of NAPQI-like metabolites by paracetamol in renal tubular cells; it is managed with creatinine monitoring and renal replacement therapy if necessary.
• Anaphylactoid reactions to NAC infusion (itching, urticaria, bronchospasm, rarely hypotension) are managed by slowing the infusion rate and giving antihistamines/bronchodilators if necessary; NAC does not need to be completely stopped unless the reaction is severe.
• Hyponatremia and fluid overload due to rapid NAC loading infusion occur, especially in small children, when the dilution volume is kept the same as the adult protocol; this is prevented by reducing pediatric diluent volumes according to weight.
• Hypoglycemia develops in fulminant hepatic failure due to impaired gluconeogenesis and is identified early with frequent blood glucose monitoring and corrected with dextrose infusion.
• Aspiration of activated charcoal can occur if charcoal is given to a patient with impaired consciousness without airway protection; therefore, assessment of consciousness and airway is mandatory before charcoal administration.
• Cerebral edema can develop in the advanced stages of fulminant hepatic failure together with encephalopathy and may require intracranial pressure monitoring/management; these patients should be referred to a transplant center early.
• In the long term, the risk of immunosuppression and graft rejection after liver transplantation is a complication seen in patients who meet King's criteria and undergo transplantation, and requires follow-up at the transplant center.

Special considerations

• Paracetamol poisoning during pregnancy should be treated according to the standard approach without delay; NAC is considered safe in pregnancy, passes through the placenta, and also protects the fetus. If treatment is delayed, the risk of fetal loss significantly increases, so pregnancy is not a reason to DELAY starting NAC.
• In pediatric patients, accidental single-dose ingestions usually involve low doses and prognosis is better compared to adults; however, if the dilution volume in the IV NAC regimen is not reduced according to weight, there is a risk of hyponatremia and seizures, therefore the pediatric diluent chart must be used.
• In patients with chronic alcohol use, NAPQI production is increased due to CYP2E1 induction while glutathione stores are also reduced due to malnutrition and fasting; in these patients, toxicity can develop at lower doses and even with repeated supratherapeutic ingestions, so empirical NAC should be considered if ALT is elevated even when the level appears normal.
• In patients with pre-existing liver disease (cirrhosis, chronic viral hepatitis), baseline transaminase elevation may obscure the picture, and since liver reserve is lower, the same level of toxic NAPQI accumulation can lead to more severe clinical damage; in these patients, the NAC threshold should be lower and hepatology should be involved early.
• In patients with renal failure, no change in NAC dose is required, but paracetamol's own nephrotoxicity may progress faster in patients with impaired baseline kidney function, and the decision for hemodialysis may arise earlier based on EXTRIP criteria.
• Patients using enzyme-inducing drugs (phenytoin, carbamazepine, rifampicin, chronic isoniazid) produce more NAPQI with the same dose of paracetamol and may develop serious hepatotoxicity even below standard toxic thresholds; in these patients, NAC should be started with a low threshold of suspicion.
• In older adults, due to polypharmacy and the frequency of chronic illness, paracetamol is more often ingested in a pattern of repeated supratherapeutic intake (accidentally with multiple combination drugs); in these patients, since the timing and dose history is unreliable, empirical NAC should be preferred over nomogram.

Follow-up plan

Serial ALT/AST, INR, creatinine, glucose, blood gas, and lactate should be monitored; the paracetamol level should be reassessed at the end of the regimen. NAC should not be automatically stopped because 21 hours have passed; if there are ongoing signs of damage (ALT rising, INR abnormal, detectable level, encephalopathy), the maintenance infusion should continue until the level becomes undetectable, ALT returns to normal or is clearly improving, and INR normalizes (since the exact termination thresholds may vary by institution, they should be confirmed with the local protocol).

Hypoglycemia should be closely monitored and corrected immediately if detected. King's College criteria should be checked serially; if met, hepatology/transplant center and nephrology should be contacted for hemodialysis indication.

Discharge criteria are a single acute ingestion with a precisely known ingestion time, level below the nomogram line, normal ALT, and patient asymptomatic; if these criteria are not met, the patient should remain under observation. A discharged patient should be told to seek care immediately if nausea, vomiting, or right upper quadrant pain develops within a few days.

In cases of intentional overdose, a psychiatric evaluation must be completed before discharge. Throughout the process, contact should be maintained with the Turkish National Poison Consultation Center (UZEM) at 114.