Oxymorphone Hydrochloride

Opioid Analgesic Phenanthrene Opioid Agonist Rx Controlled Schedule II (US)

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog IV, IM, SC 0.05-0.2 mg/kg (analgesia); 0.1-0.2 mg/kg (preanesthetic) Every 2-4 hours as needed Duration: As needed for pain management
Notes: Lower doses for preanesthetic; may cause panting and bradycardia. Use with caution in patients with respiratory compromise.
Cat IV, IM, SC 0.05-0.1 mg/kg (analgesia); 0.1-0.2 mg/kg (preanesthetic) Every 2-4 hours as needed Duration: As needed for pain management
Notes: May cause excitement if used alone; often combined with acepromazine or benzodiazepines. Monitor for respiratory depression.
Horse IV, IM 0.02-0.1 mg/kg (analgesia); 0.01-0.02 mg/kg (sedation) Every 2-4 hours as needed Duration: As needed for pain management
Notes: May cause CNS stimulation and excitement; use with caution. Often combined with detomidine or xylazine for sedation.
Cattle IV, IM 0.02-0.05 mg/kg (extra-label) Every 4-6 hours as needed Duration: As needed
Notes: Limited data; use with caution. Not approved for food animals; observe withdrawal times.
Small Ruminants IV, IM 0.02-0.05 mg/kg (extra-label) Every 4-6 hours as needed Duration: As needed
Notes: Limited data; use with caution. Not approved for food animals; observe withdrawal times.
Rabbit IV, IM, SC 0.05-0.1 mg/kg (extra-label) Every 2-4 hours as needed Duration: As needed
Notes: Limited data; use with caution. May cause respiratory depression.
Bird/Poultry IM 0.1-0.5 mg/kg (extra-label) Every 2-4 hours as needed Duration: As needed
Notes: Limited data; use with caution. Not approved for food birds; observe withdrawal times.
Exotic/Other Varies Dose varies widely; consult specialist Varies Duration: As needed
Notes: Limited data; use with caution. Many exotic species are sensitive to opioids.

Clinical Indications & Species Uses

General Indications
  • Relief of moderate to severe pain
  • Preanesthetic sedation
  • Neuroleptanalgesia when combined with tranquilizers
Dog (Canine)
  • Management of moderate to severe pain
  • Preanesthetic medication
  • Sedation
  • Cough suppression (off-label)
Cat (Feline)
  • Management of moderate to severe pain
  • Preanesthetic medication
  • Sedation (may cause excitement if used alone)
Horse (Equine)
  • Management of severe pain (e.g., colic)
  • Sedation (often combined with tranquilizers)
Cattle (Bovine)
  • Analgesia (extra-label use)
Small Ruminants (Sheep / Goat)
  • Analgesia (extra-label use)
Rabbit & Small Mammals
  • Analgesia (extra-label use)
Avian & Poultry
  • Analgesia (extra-label use)
Exotic & Other Species
  • Analgesia (extra-label use)

Pharmacology & Mechanism of Action

Drug Class: Opioid Analgesic | Pharmacological Group: Phenanthrene Opioid Agonist

Mechanism of Action: Oxymorphone is a semi-synthetic opioid agonist that primarily acts on mu-opioid receptors in the central nervous system (CNS) and peripheral tissues. Activation of mu receptors leads to G-protein coupled inhibition of adenylate cyclase, decreased cAMP production, and modulation of ion channels (increased potassium efflux, decreased calcium influx), resulting in reduced neuronal excitability and inhibition of pain transmission. It also produces analgesia, sedation, and euphoria by acting on descending inhibitory pain pathways and limbic system. Additionally, it suppresses cough reflex via central action and can cause respiratory depression by reducing brainstem respiratory center sensitivity to CO2.

Pharmacodynamics: Oxymorphone produces potent analgesic effects, approximately 10 times more potent than morphine. It induces dose-dependent sedation, analgesia, and euphoria in some species, while in cats it may cause excitement. It also causes respiratory depression, bradycardia (due to central vagal stimulation), and decreased gastrointestinal motility. In dogs, it can cause panting, while in horses it may cause central nervous system stimulation and excitement. Its effects on the cardiovascular system are variable; it may cause hypotension or hypertension depending on species and dose.

⚡ Pharmacokinetics Summary

Absorption: Oxymorphone is well absorbed after parenteral administration. Oral bioavailability is low due to extensive first-pass metabolism. In dogs, subcutaneous and intramuscular absorption is rapid, with peak plasma concentrations occurring within 15-30 minutes. In horses, intravenous administration provides immediate effect.
Distribution: Oxymorphone is widely distributed throughout the body, with high affinity for tissues such as brain, lungs, liver, and kidneys. It crosses the blood-brain barrier and placenta. Volume of distribution is moderate. Protein binding is approximately 10-12% in dogs.
Metabolism: Oxymorphone undergoes extensive hepatic metabolism, primarily via glucuronidation and N-dealkylation. In dogs, the major metabolite is oxymorphone-3-glucuronide. In humans, CYP3A4 and UGT2B7 are involved, but in veterinary species, specific enzymes are less characterized. Some enterohepatic recirculation may occur.
Excretion: Metabolites are excreted primarily in urine (approximately 90%) and feces (approximately 10%). In dogs, the elimination half-life is approximately 1-2 hours after intravenous administration. In horses, the half-life is shorter, around 0.5-1 hour.
Half-Life: Dog: 1-2 hours (IV); Horse: 0.5-1 hour (IV); Cat: approximately 1-2 hours (estimated)
Bioavailability: Oral: low (<10% in dogs); IM/SC: high (nearly 100% absorbed)
Protein Binding: 10-12% in dogs

Available Formulations & Strengths

Injectable Solution 1 mg/mL, 2 mg/mL, 5 mg/mL (IV, IM, SC)
Suppository (human formulation, not commonly used in vet med) 5 mg (Rectal)

Contraindications & Clinical Warnings

Contraindications:
  • Known hypersensitivity to oxymorphone or other opioids
  • Severe respiratory depression or compromised respiratory function
  • Head trauma or increased intracranial pressure
  • Concurrent use of MAO inhibitors (within 14 days)
  • Severe hepatic or renal insufficiency (use with caution)
  • Paralytic ileus or gastrointestinal obstruction
  • Use in animals with biliary tract disease (may cause spasm of sphincter of Oddi)
Warnings & Clinical Precautions:
  • Use with caution in animals with hypothyroidism, adrenocortical insufficiency, or shock
  • May cause bradycardia; consider anticholinergic premedication (e.g., atropine) if needed
  • Monitor respiratory rate and depth, especially in cats and brachycephalic breeds
  • May cause excitement in cats and horses; use with tranquilizers if needed
  • Use with caution in debilitated or geriatric animals; reduce dose
  • May cause constipation; use with stool softeners if prolonged therapy
  • In food animals, extra-label use is prohibited in the US; consider alternatives
  • Avoid rapid IV administration to reduce risk of histamine release and hypotension
  • Use with caution in animals with seizure disorders; may lower seizure threshold
  • Tolerance and dependence may develop with prolonged use

Adverse Effects & Reactions

Common:

  • Sedation
  • Respiratory depression
  • Bradycardia
  • Panting (dogs)
  • Mydriasis (cats)
  • Gastrointestinal stasis/constipation
  • Vomiting (especially at higher doses)
  • Urinary retention

Serious / Severe:

  • Severe respiratory depression
  • Hypotension or hypertension
  • CNS excitement (cats, horses)
  • Cardiac arrhythmias
  • Seizures (rare)
  • Anaphylactoid reactions (rare)

Rare:

  • Allergic reactions
  • Hepatotoxicity (with prolonged use)
  • Paralytic ileus
  • Addisonian crisis in stressed animals

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Other CNS depressants (barbiturates, benzodiazepines, phenothiazines, anesthetics) Additive CNS and respiratory depression; reduce doses of both agents High
MAO inhibitors (e.g., selegiline) Risk of serotonin syndrome or hypertensive crisis; avoid concurrent use High
Anticholinergics (e.g., atropine) May exacerbate tachycardia or ileus; use with caution Moderate
Muscle relaxants Enhanced muscle relaxation and respiratory depression Moderate
Beta-blockers May increase risk of bradycardia and hypotension Moderate
Cimetidine May increase oxymorphone plasma levels due to reduced hepatic metabolism Mild
Tricyclic antidepressants May potentiate opioid effects and cause sedation Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe respiratory depression (slow, shallow breathing)
  • Bradycardia
  • Hypotension
  • CNS depression progressing to coma
  • Miosis (dogs) or mydriasis (cats)
  • Hypothermia
  • Muscle rigidity
  • Pulmonary edema (rare)

Emergency Treatment Protocol: Maintain airway and provide assisted ventilation if needed. Administer naloxone (opioid antagonist) at 0.01-0.04 mg/kg IV, IM, or SC; repeat as needed. Monitor for withdrawal signs (excitement, tachycardia) after naloxone administration. Supportive care: IV fluids, thermoregulation, and monitoring of vital signs. In severe cases, consider continuous rate infusion of naloxone.

Food Animal Withdrawal Times

Oxymorphone is not approved for use in food animals in the US. Extra-label use is prohibited in food-producing animals. If used in an emergency, consult FARAD (Food Animal Residue Avoidance Databank) for withdrawal recommendations. No established withdrawal times.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F), excursions permitted between 15-30°C (59-86°F)

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light. Store in a secure, locked cabinet as a controlled substance. Keep out of reach of children and animals.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs and cats in the US (e.g., Numorphan). Not approved for food animals. Availability may vary by country.

Extra-Label (Off-Label) Use: In the US, oxymorphone is a Schedule II controlled substance. Extra-label use in animals is permitted under AMDUCA for non-food animals, but prohibited in food animals. In the EU, similar restrictions apply. Prescriptions must comply with controlled substance regulations.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Oxymorphone is a potent opioid analgesic used primarily for managing moderate to severe pain in small animals, especially dogs and cats. It is also used as a preanesthetic and for sedation in horses. Due to its high potency, accurate dosing is critical. It should be used with caution in patients with respiratory compromise, head trauma, or hepatic/renal disease. In cats, it may cause excitement; combining with a tranquilizer like acepromazine is recommended. In horses, it can cause CNS stimulation; use with alpha-2 agonists (e.g., xylazine) is common. As a controlled substance, strict record-keeping is required. Naloxone should be available for emergency reversal. For prolonged pain management, consider multimodal analgesia (e.g., NSAIDs, local anesthetics) to reduce opioid requirements. In food animals, oxymorphone is not permitted; use approved alternatives. Always monitor vital signs and adjust doses based on individual response.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)