|A||Partial opioid agonist|
|B||Pure opioid agonist|
|C||Pure opioid antagonist|
|D||Opioid with strong analgesic property|
(Ref. KDT Pharmacology 6th eD. pg. 465; Harrison's Principles of Internal Medicine 16th Edition pg. 2567)
a. The purely synthetic opioids and their cousins include meperidine, propoxyphene, diphenoxylate, fentanyl, buprenorphine, tramadol, methadone, and pentazocine.
b. Endogenous opioid peptides (i.e., enkephalins, endorphins, dynorphins, and others) have distinct distributions in the central nervous system (CNS) and appear to be natural ligands for opioid receptors.
c. The receptors with which opioid peptides interact differentially produce analgesia, respiratory depression, constipation, euphoria, and other actions.
d. Substances capable of antagonizing one or more of these actions include nalorphine, levallorphan, cyclazocine, butorphanol, buprenorphine, and pentazocine, each of which has mixed agonist and antagonist properties, as well as naloxone, nalmefene, and naltrexone, which are pure opiate antagonists.
Buprenorphine is the longest acting opioid and binds tightly to the mu receptors; the binding is so tight that even naloxone can not reverse its actions.