Opioids: The prototypical painkillers
If the trenches of everyday veterinary medicine have you feeling blasé about the power and purpose of opioids, perhaps you just need a reminder of what they are to remember why you love them.
Opioids have a wide range of analgesic action—from ultra-short (like remifentanil) to very long (like hydromorphone). Their general reversibility makes them especially attractive in higher-risk patients, and they are often relatively inexpensive. Opioids are also extremely versatile in that they can be administered via oral tablets, intermittent injections, constant-rate infusions, epidurals and transdermal patches.
The effects of opioid analgesics depend on the receptors at which they’re acting. There are three major classes of opioid receptors currently recognized within the central nervous system—mu, delta and kappa—and all three produce some level of analgesia. Drugs acting on opioid receptors have their own classifications: agonists, partial agonists, mixed agonists/antagonists and antagonists.
Agonists (like morphine, hydromorphone, oxymorphone, fentanyl and methadone) have a high affinity for the mu opioid receptors responsible for analgesia and sedation. If a patient is considered to be painful, agonist opioids should be utilized.
Partial agonists (like buprenorphine) are, by definition, only partially as effective as the agonists above. This is because their binding with mu opioid receptors produces a less pronounced effect than that of opioid agonists like fentanyl.
Mixed agonist/antagonists (like butorphanol and nalbuphine) work by exerting an agonist effect at kappa receptor sites responsible for sedation and some analgesic properties while acting as antagonists at mu receptor sites. These drugs can also be used to reverse some of the unwanted side effects of full agonist opioids, such as excessive sedation.1
Antagonists (like naloxone and naltrexone) work to fully antagonize and reverse the effects of opioids at the mu and kappa receptors. These drugs will cause increased alertness and will also reverse the analgesic effects of opioid agonists, so use with caution in the painful patient.
1. Wagner AE. Opioids. In: Gaynor J, Muir W, eds. Handbook of veterinary pain management. 2nd ed. St. Louis: Elsevier, 2009.