Acetaminophen-d5 is deuterated-labeled Acetaminophen (HY-66005). Acetaminophen (Paracetamol) is a selective cyclooxygenase-2 (COX-2) inhibitor with an IC50 of 25.8 µM, is a widely used antipyretic and analgesic agent[1][2][3]. Acetaminophen is a potent hepatic N-acetyltransferase 2 (NAT2) inhibitor[4].