Difference among a pure agonist partial agonist and an agonist

What is the difference among a pure agonist partial agonist and an agonist?

Pure agonists are opioid drugs that bind to mu-opioid receptors in the body. That binding then produces naturally occurring endorphins, analgesia, euphoria, and other well-known opioid properties. Examples of full agonists include morphine, oxycodone, hydrocodone, fentanyl, methadone, and several others. Partial agonists are opioids that bind to mu-opioid receptors; however, they produce endorphins to a much lesser extent than previously discussed full agonists. When the dosage of a partial agonist is increased, the production of endorphins is not proportionately increased. There is only a small increase, and with receptor saturation these medications begin to take on properties of antagonists. The term “partial agonist” should not be interpreted to be equivalent to partial analgesic effect—the term refers to receptor pharmacology.

An example of a partial agonist is buprenorphine. An antagonist is a medication that binds to the opioid receptor but does not stimulate endorphin production at all. Examples of opioid antagonists are naltrexone and naloxone. An antagonist/antagonist is a product that can stimulate the opioid receptors in both ways. Buprenorphine can act as both for reasons previously discussed; it is a partial agonist at mu receptors and antagonist at kappa receptors. Nalbuphine is an opioid agonist at kappa receptors and an antagonist or partial agonist at mu receptors. There are also co-formulated products such as buprenorphine/naloxone (Suboxone) that contain both a partial agonist and an antagonist. However, at most doses the buprenorphine has a higher mu-receptor affinity compared with naloxone, therefore it will not be displaced by naloxone.


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