Norfentanyl NHS Ester is an excellent amino-reactive building block for developing antibodies, assays and tests for detecting norfentanyl substances. Norfentanyl is a monocarboxylic acid amide resulting from the formal condensation of the aryl amino group of 4-(N'-phenyl)piperidin-4-amine with propanoic acid. It is a major metabolite of fentanyl. It has a role as an opioid analgesic and a drug metabolite. It is an anilide, a member of piperidines and a monocarboxylic acid amide.