Norhydrocodone is the major metabolite of the opioid analgesic hydrocodone.[1] It is formed from hydrocodone in the liver via N-demethylation predominantly by CYP3A4. Unlike hydromorphone, a minor metabolite of hydrocodone, norhydrocodone is described as inactive.[2] However, norhydrocodone is actually an agonist of the μ-opioid receptor with similar potency to hydrocodone, but has been found to produce only minimal analgesia when administered peripherally to animals.[3] This is likely due to poor blood-brain-barrier and thus central nervous system penetration.