Opioid and opioid-like receptors respond to a variety of endogenous peptides including [Met]enkephalin, [Leu]enkephalin, β-endorphin, α-neodynorphin, dynorphin A, dynorphin B, big dynorphin, nociceptin/orphanin FQ; endomorphin-1 and endomorphin-2 are also potential endogenous peptides. The human N/OFQ receptor, NOP, is considered ‘opioid-related’ rather than opioid because, while it exhibits a high degree of structural homology with the conventional opioid receptors, it displays a distinct pharmacology. Currently there are numerous clinically used drugs, such as morphine and many other opioid analgesics, as well as antagonists such as naloxone, however only for the μ receptor.