A 24-year-old man with a history of depression is brought to the emergency room because of a drug overdose. He is experiencing some nausea and vomiting, but no other symptoms. Physical examination and vital signs are normal. Six hours prior to presentation, he intentionally took 40 tablets of acetaminophen (500 mg/tablet). Baseline acetaminophen level, liver enzymes and function are drawn, and he is admitted to the hospital. Which of the following is the most appropriate next step in management?