Patients treated with acetaminophen should have monitoring for desired clinical effects, such as pain or fever relief. Serum concentrations are unnecessary when appropriately dosed. In overdose settings, laboratory evaluation is necessary. In acute overdoses, in which ingestion occurs over less than eight hours, a serum APAP concentration should be assessed and plotted on the Rumack-Matthew nomogram, with the time course starting at the onset of ingestion, to determine toxicity and need for treatment. Nonacute ingestions require an assessment of acetaminophen concentration and transaminases, and treatment should occur. 

Additionally, caution is necessary for patients with renal or hepatic impairment or patients with alcoholic liver disease, glucose six phosphate dehydrogenase deficiency, or severe hypovolemia. However, there is evidence that acetaminophen may be safe to use in the setting of alcoholic liver disease.[7]

Acetaminophen can cross the placental barrier, but there is no evidence of increased teratogenic effects due to the use of normal doses of acetaminophen during pregnancy.[8] Acetaminophen also is excreted into breast milk, but there have not been many observations of adverse reactions in nursing infants.

Acetaminophen is responsible for an estimated 500 deaths and 50000 emergency department visits in the United States each year.[9] It is the most common drug-related cause of acute liver failure. The mechanism of hepatic injury is due to the drug metabolism properties of acetaminophen.[10] Following therapeutic concentrations of oral acetaminophen, 60% to 90% of the drug gets metabolized in the liver to glucuronic acid- and sulfate-conjugate metabolites. A smaller fraction (approximately 5% to 15%) undergoes metabolism by the cytochrome P450 system (CYP450). Metabolism primarily via CYP2E1 results in the formation of the toxic intermediate N-acetyl-p-benzoquinone imine (NAPQI). Normally, NAPQI is neutralized by glutathione to nontoxic metabolites. However, with excessive doses of acetaminophen, the normal phase II drug metabolism pathways become depleted, and the CYP450 pathway metabolizes a higher portion of the acetaminophen taken, resulting in high levels of NAPQI formation, and the limited glutathione stores can become depleted. Without glutathione, NAPQI concentrations build-up, and NAPQI, as a reactive intermediate, can react with cellular macromolecules, proteins, lipids, and nucleic acids. This phenomenon can lead to centrilobular (Zone 3) hepatic injury and hepatocellular death. 

The only approved antidote for acetaminophen overdose and toxicity is N-acetylcysteine (NAC).[11] NAC is a precursor to glutathione synthesis and helps to restore the intracellular stores of glutathione to neutralize the NAPQI compound. N-acetyl cysteine can be administered orally or by IV. IV N-acetyl cysteine is typically preferred because vomiting is common with acetaminophen overdose. It is effective when administered within the first few hours (up to 8 to 10 hours) of a toxic ingestion of acetaminophen. N-acetyl cysteine administration has a 20-hour IV protocol or 72-hour oral protocol, and the clinician must monitor the AST/ALT during treatment.[12] One important thing to keep in mind is that most patients do not have symptoms in the first few hours of ingestion of toxic doses of acetaminophen and may only have abdominal pain and nausea as symptoms for the first 12 to 24 hours. Between 24 and 72 hours, these symptoms may dissipate, although AST/ALT concentrations may be abnormal. Patients who present more than 24 hours following ingestion of toxic doses of acetaminophen may have symptoms such as nausea, vomiting, jaundice, abdominal pain, and hypotension. These patients may require airway management, intravenous fluids, vasopressors, hemodialysis, or management of cerebral edema or other symptoms as they arise.

The most crucial aspect of acetaminophen toxicity is prevention.  Physicians, nurses, and pharmacists all share in this responsibility. Pharmacists and nurses need to emphasize the maximum dose permitted daily. Patients also need to understand how to look for acetaminophen in various medications they take and how to calculate the dose they receive when they combine products. Pharmacists need to perform medication reconciliation to look for drug interactions, as well as verify that there are not too many acetaminophen-containing drugs in the regimen. If there are concerns, the pharmacist should report them to the nurse and physician.

With the recent changes in maximum daily dosing for acetaminophen, all providers (physicians, nurses, and pharmacists) need to be aware of the new guidelines and remain current should any new guidance come out.

If toxicity occurs or there is suspicion of toxicity, management should be with an interprofessional team of physicians, nurses, and pharmacists. Specific protocols have been designed to direct an interprofessional team of healthcare providers when patients present to emergency rooms with acute acetaminophen toxicity. Emergency physicians, nurses, toxicologists, pharmacists, and psychiatrists designed one such protocol.[13] Dentists can also become involved when the overdose is secondary to dental procedures.[14] Importantly, upon discharge, patients should be provided with clear instructions on APAP medication management, as outlined above. While acetaminophen has been available for many years and is generally safe, a coordinated interprofessional team effort is necessary to avoid avoidable toxicity, by accounting for all sources of acetaminophen. [Level V]