Fast Five Quiz: Acetaminophen

Mary L. Windle, PharmD


July 22, 2021

Most patients who have taken an overdose of acetaminophen will initially be asymptomatic because clinical evidence of end-organ toxicity often does not manifest until 24-48 hours after an acute ingestion. However, because antidotal therapy is most effective when initiated within 8 hours after an ingestion, it is important to obtain an accurate history of the time(s) of ingestion, the quantity, and the formulation of acetaminophen ingested. Knowledge of the maximum recommended and minimum toxic dosages, as well as underlying conditions that increase susceptibility, can help the clinician determine the risk for hepatotoxicity. In addition, the history should include any co-ingestants, such as salicylates or medications that may delay gastric emptying and acetaminophen absorption (eg, anticholinergic drugs or opioids).

The clinical course of acetaminophen toxicity generally is divided into four phases. Physical findings vary depending on the degree of hepatotoxicity. In phase 1 (0.5-24 hours after ingestion), patients may be asymptomatic or report anorexia, nausea or vomiting, and malaise. Physical examination may reveal pallor, diaphoresis, malaise, and fatigue. In phase 2 (18-72 h after ingestion), patients generally develop right upper quadrant abdominal pain, anorexia, nausea, and vomiting. Right upper quadrant tenderness may be present. Tachycardia and hypotension may indicate extracellular fluid volume. Some patients may report decreased urine output (oliguria).

Phase 3 (72-96 h after ingestion) is also called the hepatic phase. Patients may have continued nausea and vomiting, abdominal pain, and a tender hepatic edge. Hepatic necrosis and synthetic dysfunction manifest as jaundice, coagulopathy, hypoglycemia, and hepatic encephalopathy. Acute renal failure develops in some critically ill patients. Death from multiorgan failure may occur. Phase 4 is the recovery phase (4 d to 3 wk after ingestion). Patients who survive critical illness in phase 3 have complete resolution of symptoms and resolution of organ failure. Clinical recovery may take up to 21 days; however, complete hepatic histologic recovery requires several months.

Read more about the presentation of acetaminophen toxicity.


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