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Emergency and Acute Medicine – Acetaminophen (APAP) Poisoning
Overview and Pathophysiology
Acetaminophen is widely available as a single-agent analgesic, in combination with opioid medications, and as an ingredient in more than 200 over-the-counter cold and flu products. It is among the most common causes of both intentional and accidental poisoning and is the leading cause of acute liver failure requiring transplantation in the United States.
After ingestion, most acetaminophen is safely metabolized, but a small fraction is converted by the cytochrome P-450 system into N-acetyl-p-benzoquinoneimine (NAPQI), a highly toxic metabolite. Under normal conditions, NAPQI is neutralized by glutathione. In overdose, glutathione stores are rapidly depleted, allowing NAPQI to accumulate and cause hepatocellular injury. N-acetylcysteine (NAC) restores glutathione levels and prevents or limits liver damage. Patients with malnutrition are at higher risk because of reduced baseline glutathione reserves.
Pharmacokinetics and Toxic Thresholds
The normal acetaminophen half-life is approximately 2.5–4 hours but becomes prolonged in overdose, often exceeding 4 hours and indicating hepatic dysfunction. An acute ingestion greater than 150 mg/kg is potentially toxic. At 4 hours after ingestion, a serum acetaminophen level of 140 μg/mL or higher suggests toxicity, while therapeutic levels range from 5–20 μg/mL. The Rumack–Matthew nomogram is used to guide treatment decisions following a single, acute ingestion.
Clinical Course and Symptoms
Toxicity typically progresses through four stages.
Phase I (0.5–24 hours): Nausea, vomiting, and malaise may occur, particularly with large ingestions, though symptoms can be mild or absent early.
Phase II (24–72 hours): Gastrointestinal symptoms may improve, but liver injury evolves. Patients develop right upper quadrant pain, rising transaminases, prolonged PT/INR, elevated bilirubin, and sometimes oliguria.
Phase III (72–96 hours): This is the most critical period. Liver enzyme abnormalities peak, hepatic encephalopathy may appear, and worsening coagulopathy or renal failure signals a high likelihood of requiring liver transplantation.
Phase IV (96 hours to 10 days): Patients either recover with gradual hepatic regeneration or progress to fulminant liver failure.
Essential Evaluation
A careful history must identify all acetaminophen-containing products ingested and the timing of ingestion. A serum acetaminophen level should be obtained at 4 hours post-ingestion or immediately upon presentation if more than 4 hours have elapsed. The Rumack–Matthew nomogram applies only to single, acute ingestions and should not be used for chronic or delayed presentations. Early consultation with a poison center or toxicologist is strongly recommended.
Diagnostic Testing
Laboratory evaluation includes serum acetaminophen concentration, electrolytes, blood urea nitrogen, creatinine, glucose, liver enzymes (AST typically rises first), bilirubin, and PT/INR. Severe toxicity may result in AST/ALT levels exceeding 10,000 IU/L. A pregnancy test and toxicology screen should be obtained when appropriate.
Differential Diagnosis
Acetaminophen toxicity should always be considered as a co-ingestant in overdose. Other causes of acute liver injury include viral hepatitis, Reye syndrome, Amanita mushroom poisoning, herbal or dietary supplement toxicity, and other hepatotoxic drugs.
Initial Stabilization
Management begins with airway, breathing, and circulation assessment. Supplemental oxygen should be provided as needed. In patients with altered mental status, empiric administration of naloxone, thiamine, and dextrose (or bedside glucose testing) is appropriate.
Emergency Department Management
Supportive care includes intravenous fluids and antiemetics. A single dose of activated charcoal should be administered if the patient presents soon after ingestion.
N-acetylcysteine (NAC) is the cornerstone of therapy and is nearly 100% effective at preventing hepatotoxicity if started within 8 hours of acute overdose.
NAC Administration Options
Oral NAC has an unpleasant taste and odor and should be diluted to improve tolerability; antiemetics are often required. Vomited doses within one hour should be repeated, and persistent vomiting necessitates nasogastric or intravenous administration.
Intravenous NAC is given as a 21-hour infusion protocol. Oral NAC may be administered intravenously if the IV formulation is unavailable, with toxicology guidance.
Pregnancy Considerations
NAC is not teratogenic and crosses the placenta. Treating the mother effectively protects the fetus, particularly after 14 weeks’ gestation when fetal metabolism can produce toxic metabolites.
Disposition and Follow-Up
Admission is required for patients with hepatotoxic acetaminophen levels, abnormal liver tests, chronic ingestion with liver injury, or suicide attempts requiring psychiatric care.
Asymptomatic patients with non-toxic ingestions who do not require NAC may be discharged with counseling. Early hepatology or transplant consultation is warranted when significant liver injury is present. Patients with intentional overdose require psychiatric evaluation, while those with accidental poisoning need poison-prevention counseling.
Clinical Pearls and Pitfalls
Always consider occult acetaminophen exposure in patients presenting after opioid overdose or with unexplained liver injury. The Rumack–Matthew nomogram should not be used for chronic ingestion or late presentation. NAC should not be stopped until acetaminophen levels are undetectable and clinical and laboratory evidence of hepatotoxicity has improved or resolved.
Overview and Pathophysiology
Acetaminophen is widely available as a single-agent analgesic, in combination with opioid medications, and as an ingredient in more than 200 over-the-counter cold and flu products. It is among the most common causes of both intentional and accidental poisoning and is the leading cause of acute liver failure requiring transplantation in the United States.
After ingestion, most acetaminophen is safely metabolized, but a small fraction is converted by the cytochrome P-450 system into N-acetyl-p-benzoquinoneimine (NAPQI), a highly toxic metabolite. Under normal conditions, NAPQI is neutralized by glutathione. In overdose, glutathione stores are rapidly depleted, allowing NAPQI to accumulate and cause hepatocellular injury. N-acetylcysteine (NAC) restores glutathione levels and prevents or limits liver damage. Patients with malnutrition are at higher risk because of reduced baseline glutathione reserves.
Pharmacokinetics and Toxic Thresholds
The normal acetaminophen half-life is approximately 2.5–4 hours but becomes prolonged in overdose, often exceeding 4 hours and indicating hepatic dysfunction. An acute ingestion greater than 150 mg/kg is potentially toxic. At 4 hours after ingestion, a serum acetaminophen level of 140 μg/mL or higher suggests toxicity, while therapeutic levels range from 5–20 μg/mL. The Rumack–Matthew nomogram is used to guide treatment decisions following a single, acute ingestion.
Clinical Course and Symptoms
Toxicity typically progresses through four stages.
Phase I (0.5–24 hours): Nausea, vomiting, and malaise may occur, particularly with large ingestions, though symptoms can be mild or absent early.
Phase II (24–72 hours): Gastrointestinal symptoms may improve, but liver injury evolves. Patients develop right upper quadrant pain, rising transaminases, prolonged PT/INR, elevated bilirubin, and sometimes oliguria.
Phase III (72–96 hours): This is the most critical period. Liver enzyme abnormalities peak, hepatic encephalopathy may appear, and worsening coagulopathy or renal failure signals a high likelihood of requiring liver transplantation.
Phase IV (96 hours to 10 days): Patients either recover with gradual hepatic regeneration or progress to fulminant liver failure.
Essential Evaluation
A careful history must identify all acetaminophen-containing products ingested and the timing of ingestion. A serum acetaminophen level should be obtained at 4 hours post-ingestion or immediately upon presentation if more than 4 hours have elapsed. The Rumack–Matthew nomogram applies only to single, acute ingestions and should not be used for chronic or delayed presentations. Early consultation with a poison center or toxicologist is strongly recommended.
Diagnostic Testing
Laboratory evaluation includes serum acetaminophen concentration, electrolytes, blood urea nitrogen, creatinine, glucose, liver enzymes (AST typically rises first), bilirubin, and PT/INR. Severe toxicity may result in AST/ALT levels exceeding 10,000 IU/L. A pregnancy test and toxicology screen should be obtained when appropriate.
Differential Diagnosis
Acetaminophen toxicity should always be considered as a co-ingestant in overdose. Other causes of acute liver injury include viral hepatitis, Reye syndrome, Amanita mushroom poisoning, herbal or dietary supplement toxicity, and other hepatotoxic drugs.
Initial Stabilization
Management begins with airway, breathing, and circulation assessment. Supplemental oxygen should be provided as needed. In patients with altered mental status, empiric administration of naloxone, thiamine, and dextrose (or bedside glucose testing) is appropriate.
Emergency Department Management
Supportive care includes intravenous fluids and antiemetics. A single dose of activated charcoal should be administered if the patient presents soon after ingestion.
N-acetylcysteine (NAC) is the cornerstone of therapy and is nearly 100% effective at preventing hepatotoxicity if started within 8 hours of acute overdose.
- If the acetaminophen level cannot be obtained within 8 hours and toxicity is suspected, NAC should be started empirically and stopped if levels prove non-toxic.
- For presentations ≥8 hours after ingestion, NAC should be started immediately while awaiting laboratory confirmation.
- For late (>24 hours) or chronic ingestions, NAC is indicated if ingestion exceeds 150 mg/kg, symptoms are present, or liver tests are abnormal. Therapy may be discontinued once acetaminophen is undetectable and liver enzymes remain normal.
NAC Administration Options
Oral NAC has an unpleasant taste and odor and should be diluted to improve tolerability; antiemetics are often required. Vomited doses within one hour should be repeated, and persistent vomiting necessitates nasogastric or intravenous administration.
Intravenous NAC is given as a 21-hour infusion protocol. Oral NAC may be administered intravenously if the IV formulation is unavailable, with toxicology guidance.
Pregnancy Considerations
NAC is not teratogenic and crosses the placenta. Treating the mother effectively protects the fetus, particularly after 14 weeks’ gestation when fetal metabolism can produce toxic metabolites.
Disposition and Follow-Up
Admission is required for patients with hepatotoxic acetaminophen levels, abnormal liver tests, chronic ingestion with liver injury, or suicide attempts requiring psychiatric care.
Asymptomatic patients with non-toxic ingestions who do not require NAC may be discharged with counseling. Early hepatology or transplant consultation is warranted when significant liver injury is present. Patients with intentional overdose require psychiatric evaluation, while those with accidental poisoning need poison-prevention counseling.
Clinical Pearls and Pitfalls
Always consider occult acetaminophen exposure in patients presenting after opioid overdose or with unexplained liver injury. The Rumack–Matthew nomogram should not be used for chronic ingestion or late presentation. NAC should not be stopped until acetaminophen levels are undetectable and clinical and laboratory evidence of hepatotoxicity has improved or resolved.
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