At first glance, this looks like a circular definition.
Is there some way to definitively determine if a given molecule is or is not an opioid? (Medically or scientifically, not legally).
I'm hoping for something more scientific than "if it's on this list, it is; and if it's not, it isn't."
Wikipedia's article Opioid receptor:
Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. The opioid receptors are ~40% identical to somatostatin receptors (SSTRs). Opioid receptors are distributed widely in the brain, and are also found in the spinal cord and digestive tract. (emphasis added)
Wikipedia's article on Opioid:
Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, treating addiction, reversing opioid overdose, suppressing cough, and suppressing opioid induced constipation. Extremely strong opioids are approved only for veterinary use such as immobilizing large mammals. Opioids are also frequently used non-medically for their euphoric effects or to prevent withdrawal. (emphasis added)