Acetaminophen is the most commonly used analgesic-antipyretic medication in the United States. Acetaminophen overdose, a frequent cause of drug toxicity, has been recognized as the leading cause of fatal and non-fatal hepatic necrosis. N-Acetylcysteine is the recommended antidote for acetaminophen poisoning. Despite evidence on the efficacy of N-acetylcysteine for prevention of hepatic injury, controversy persists about the optimal duration of the therapy. Here, we describe the case of a 65-year-old male with acetaminophen overdose and opioid co-ingestion who developed a second peak in acetaminophen serum levels after completing the recommended 21-hour intravenous N-acetylcysteine protocol and when the standard criteria for monitoring drug levels was achieved. Prolongation of N-acetylcysteine infusion beyond the standard protocol, despite a significant gap in treatment, was critical for successful avoidance of hepatotoxicity. Delay in acetaminophen absorption may be associated with a second peak in serum concentration following an initial declining trend, especially in cases of concomitant ingestion of opioids. In patients with acetaminophen toxicity who co-ingest other medications that may potentially delay gastric emptying or in those with risk factors for delayed absorption of acetaminophen, we recommend close monitoring of aminotransferase enzyme levels, as well as trending acetaminophen concentrations until undetectable before discontinuing the antidote therapy.