Opioids and Analgesics Overview

Opioids and Analgesics

• Opioids affect MU and Kappa receptors throughout the body, impacting pain pathways. MU receptors have a more profound effect on pain management.
• Common opioid disadvantages include hypersalivation, nausea, vomiting, respiratory and cardiovascular depression, decreased GI motility.
• Opioid potency is measured relative to morphine.
• Morphine is a MU agonist with low Kappa agonist activity. It can be administered intravenously (IV), subcutaneously (SQ), intramuscularly (IM), or epidurally. IV administration results in less emesis (vomiting) as it reaches the emetic center faster. It's classified as a class 1 narcotic and is effective for visceral pain.
• Hydromorphone is a pure MU agonist, stronger than morphine (5x potency), and can lessen vomiting.

Specific Opioids

• Hydromorphone: A strong analgesic with some sedation, effective at reducing the need for anesthetic sparing. It has minimal cardiovascular effects and reduced histamine release. However, it can cause hyperthermia.
• Fentanyl: A potent MU agonist (100x morphine potency) with profound analgesia. It’s available as an IV, continuous-release intravenous (CRI), and transdermal patch. Quick onset (30 minutes to 2 hours) and reduced vomiting. Patch lasts 12-24 hours, up to 72 hours. Client education is critical regarding proper patch placement and handling to prevent accidental consumption. Potential for dysphoria.
• Meperidine: A weak MU agonist with a moderate analgesic effect but high cardiovascular depressant effects. Shorter acting than other opioids, not the ideal opioid option. Low potency (0.1x morphine).
• Butorphanol: A partial MU agonist and Kappa agonist. Transported by G-glycoprotein in dog brains. Partially reversible activity via naloxone and is a mild analgesic. Helps with GI issues (less GI motility issues and ileus) and can function as a cough suppressant, but cannot be used with other opioids. Short duration. Duration is 60 minutes in dogs and up to 90 minutes in cats.
• Buprenorphine: A partial MU agonist and Kappa antagonist. Moderate analgesia with a slower onset. More gentle on the cardiovascular system; useful for mild to moderate pain. Less effective with severe pain and has a ceiling effect; cannot be combined with pure MU drugs. Can be administered intravenously (IV), subcutaneously (SQ), intramuscularly (IM), or transmucosally (cats).
• Methadone: A MU agonist that also binds NMDA receptors. Provides profound analgesia and is available via IV or IM injection. It has no histamine release, but may cause bradycardia, respiratory depression, ileus, and nausea.
• Tramadol: Requires metabolization to be effective. An M1 metabolite functions as an MU agonist. Prevents serotonin and acetylcholine reuptake. An analgesic, but not indicated for cats due to poor metabolism. Cannot be given with serotonin reuptake inhibitors.
• Apomorphine: A dopamine agonist. Used to induce emesis (vomiting) via subcutaneous (SQ), intravenous (IV), or transmucosal (cats) administration.