A 24-year-old woman presents 30 hours after taking repeated doses of paracetamol for a dental infection. She estimates taking 10–12 g daily for four days; her plasma paracetamol concentration is 65 mg/L, alanine aminotransferase is 420 U/L (less than 40 U/L) and INR is 1.7 (less than 1.3). She is alert and has no abdominal tenderness. Which one of the following is the most appropriate next step?