What is the treatment protocol for acetaminophen toxicity in cats including N-acetylcysteine dosing?

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N-acetylcysteine (NAC) is the primary antidote for acetaminophen (APAP) toxicosis in cats, but standard human-derived dosing regimens are insufficient due to faster drug clearance in cats and must be adjusted accordingly.1 Cats are uniquely susceptible to APAP toxicosis. The predominant clinical syndrome in cats is hematotoxicity — methemoglobinemia, Heinz body formation, and hemolytic anemia — rather than the hepatic necrosis more commonly seen in dogs.69 Methemoglobinemia exceeding 20% is associated with hemoglobinuria.6 Hepatotoxicity occurs via glutathione depletion and accumulation of the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI); NAC acts as a glutathione precursor to restore this depleted pool.68 The critical pharmacokinetic problem with standard human NAC protocols in cats is faster clearance. Cats have a shorter elimination half-life than humans, meaning the standard human maintenance interval of every 6 hours produces subtherapeutic plasma concentrations in cats. To achieve the same area under the curve (AUC) as the human intravenous protocol, a constant rate infusion of 40.5 mg/kg/h would be required in cats, compared to 23 mg/kg/h in humans.1 For practical dosing, the maintenance interval should be every 3 hours in cats rather than every 6 hours to account for this faster clearance.1 For chronic or long-term oral NAC administration in cats, a dose of 100 mg/kg PO is appropriate when target therapeutic plasma concentrations below 5 μg/mL are acceptable.1 This recommendation assumes linear pharmacokinetics and minimal drug accumulation with repeated dosing.1 NAC adverse effects are generally mild and include nausea, vomiting, pruritus, and tachycardia.2 Acetaminophen is absolutely contraindicated in cats — it is explicitly excluded from feline perioperative analgesic protocols at all stages.7 | Parameter | Human Protocol | Cat-Adjusted Protocol | |---|---|---| | IV constant rate infusion | 23 mg/kg/h | 40.5 mg/kg/h 1 | | Maintenance dosing