Oxymorphone
Oxymorphone is a potent, semi-synthetic **opiate agonist** used primarily as a sedative/restraining agent, analgesic, and preanesthetic in veterinary medicine. * **Potency:** It is approximately 10 times more potent than morphine on a per-weight basis. * **Cardiovascular Effects:** Its effects on the cardiovascular system are usually not clinically significant, making it a relatively safe choice for patients with cardiovascular disease. * **Histamine Release:** Unlike morphine or meperidine, oxymorphone does not appear to cause significant histamine release when administered intravenously. * **Clinical Pearl:** It is a Schedule II (C-II) controlled substance due to its high potential for abuse. While highly effective for moderate to severe pain, availability and expense can sometimes be limiting factors in veterinary practice.
Mechanism: Oxymorphone acts primarily as a full agonist at **mu (ฮผ) opioid receptors**, with some possible activity at **delta (ฮด) receptors**. * **Pathway:** Binds to G-protein coupled mu-opioid receptors in the CNS (limbic system, spinal cord, thalamus, midbrain) and peripheral tissues (GI tract, urinary tract) โ inhibits adenylate cyclase โ decreases intracellular cAMP โ promotes opening of potassium channels and inhibits voltage-gated calcium channels โ hyperpolarizes nociceptive neurons and inhibits the release of pain neurotransmitters (e.g., Substance P). * **Effects:** This results in profound analgesia, sedation, respiratory depression, and altered GI motility.
Dosing by species
- As a preanesthetic/analgesic ยท 0.05-0.1 mg/kg IM ยท IM ยท Once ยท may cause dysphoria or excitement, add sedative.
- As an analgesic (acute pain) ยท 0.05-0.1 mg/kg IM, SC or IV ยท IM/SC/IV ยท q1-3h ยท concomitant tranquilization recommended
- As an analgesic (acute pain) for animals with cardiovascular disease ยท 0.05-0.1 mg/kg IV, IM or SC ยท IV/IM/SC ยท q2-4h
- As an analgesic (acute pain) ยท 0.025-0.1 mg/kg IV (IM or SC) ยท IV/IM/SC ยท q2-6h
- As an analgesic (acute pain) ยท 0.02-0.1 mg/kg IV, IM, or SC ยท IV/IM/SC ยท q3-4h
- Analgesia ยท 0.05-0.2 mg/kg IV or IM ยท IV/IM ยท 2-4 times daily
- Analgesia ยท 0.05-0.2 mg/kg SQ or IM ยท SQ/IM ยท q8-12 hrs
- As an analgesic ยท 0.01-0.02 mg/kg IV ยท IV ยท Once
- As an analgesic ยท 0.01-0.022 mg/kg IV ; up to 15 mg total ยท IV ยท Once ยท divide dose into 3-4 increments and give several minutes apart
- As an analgesic ยท 0.02-0.03 mg/kg IM ยท IM ยท Once
Routes of administration
Contraindications
- Hypersensitivity to narcotic analgesics
- Patients receiving monoamine oxidase inhibitors (MAOIs)
- Diarrhea caused by a toxic ingestion (until toxin is eliminated)
- Scorpion stings (Centruroides sculpturatus and C. gertschi)
Adverse effects
- Respiratory depression
- Bradycardia
- Decreased GI motility (constipation)
- Panting (commonly seen in dogs)
- Ataxia, hyperesthesia, and behavioral changes (cats at high dosages)
- CNS excitement (horses)
Drug interactions
- Butorphanol, Buprenorphine, Nalbuphine ยท Potentially could antagonize opiate effects
- CNS Depressants ยท Additive CNS effects possible
- Diuretics ยท Opiates may decrease efficacy in CHF patients
- Monoamine Oxidase Inhibitors (MAOIs) ยท Extreme caution; may cause signs of opiate overdose
- Muscle Relaxants, Skeletal ยท Oxymorphone may enhance effects
- Phenothiazines ยท May antagonize analgesic effects and increase risk for hypotension
- Tricyclic Antidepressants ยท Oxymorphone may exacerbate the effects of tricyclic antidepressants
- Warfarin ยท Opiates may potentiate anticoagulant activity
Monitoring
- Respiratory rate/depth
- CNS level of depression/excitation
- Blood pressure (especially with IV use)
- Analgesic activity
- Cardiac rate
Overdose
Massive overdoses may produce profound respiratory and/or CNS depression in most species. Other effects may include cardiovascular collapse, hypothermia, and skeletal muscle hypotonia. * **Antidote:** **Naloxone** is the agent of choice in treating respiratory depression. * **Management:** In massive overdoses, naloxone doses may need to be repeated. Animals should be closely observed as naloxone's effects sometimes diminish before sub-toxic levels of oxymorphone are attained. Mechanical respiratory support should be considered in cases of severe respiratory depression.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.