Partial Opioid Agonists



Partial opioid agonists are a class of drugs that include buphrenophine, nalbuphine, pentazocine, and butorphanol. They all work by binding to mu-opioid receptors with relatively low activity. Partial opioid agonists are essentially weak opioid agents. 

By activating opioid receptors, partial opioid agonists can be used clinically for analgesia, or pain relief. Since they activate opioid receptors to a lesser extent than full agonists, these drugs are often used to help wean patients off of opioids. 

However, partial opioid receptors also have side effects. Firstly, like all opioids agonists, partial opioid agonists can cause CNS and respiratory depression. Secondly, because partial opioid agonists do not fully activate opioid receptors, they can lead to acute withdrawal.

Key Points

  • Partial Opioid Agonists
    • Drug Names
      • Buprenorphine
        • Partial mu opioid receptor agonist
        • Kappa and delta-opioid receptor antagonist
      • Nalbuphine
        • Partial mu opioid receptor agonist
        • Kappa receptor partial agonist
      • Pentazocine
        • Partial mu opioid receptor agonist
        • Kappa receptor agonist
      • Butorphanol
        • Partial mu opioid receptor agonist
        • Kappa receptor agonist
    • Mechanism
      • Partial Mu-opioid receptor agonist
        • Buprenorphine and Nalbuphine have mixed agonist/antagonist activity
        • Buprenorphine binds with high affinity but low activity at the receptor
        • In general, mu vs. kappa vs. delta distinctions are not important clinically; takeaway is that these are weak opioid agents
    • Clinical Use
      • Analgesia
        • Usually combined with naloxone to prevent abuse
        • May be used to wean patients off full opioid agonists, while avoiding withdrawal from sudden termination of opioids
    • Adverse Effects
      • Opioid withdrawal
        • Seen in patients with tolerance to opioids, since it may displace full opioid agonists
      • Causes adverse effects typically seen with all opioids (e.g. respiratory and CNS depression)