Hydromorphone Hydrochloride — Veterinary Vademecum Drug Reference

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)	q4-6h as needed	Duration: As needed for pain Notes: Lower doses for preanesthetic; monitor respiratory rate and heart rate.
Cat	IV, IM, SC	0.05-0.1 mg/kg (analgesia); 0.1 mg/kg (preanesthetic)	q4-6h as needed	Duration: As needed for pain Notes: Cats may exhibit excitement at higher doses; use lower end.
Horse	IV, IM	0.02-0.04 mg/kg (IV for colic pain)	q4-6h as needed	Duration: As needed for pain Notes: May cause excitation; use with caution and consider combination with acepromazine.
Cattle	IV, IM	0.02-0.05 mg/kg	q4-6h as needed	Duration: As needed for pain Notes: Extra-label use; observe withdrawal times.
Small Ruminants (sheep, goats)	IV, IM	0.02-0.05 mg/kg	q4-6h as needed	Duration: As needed for pain Notes: Extra-label use; monitor for respiratory depression.
Rabbit	IV, IM, SC	0.05-0.1 mg/kg	q4-6h as needed	Duration: As needed for pain Notes: Extra-label; use with caution in renal/hepatic impairment.

Clinical Indications & Species Uses

General Indications

Analgesia for moderate to severe pain
Preanesthetic medication
Sedation

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

Horse (Equine)

Management of severe pain (e.g., colic)
Sedation (in combination with tranquilizers)

Cattle (Bovine)

Management of severe pain (e.g., surgery, trauma)

Small Ruminants (Sheep / Goat)

Management of severe pain (off-label)

Rabbit & Small Mammals

Management of severe pain (off-label)

Pharmacology & Mechanism of Action

Drug Class: Opioid Agonist | Pharmacological Group: Phenanthrene Opioid

Mechanism of Action: Hydromorphone is a semi-synthetic opioid agonist that primarily acts on mu-opioid receptors in the central nervous system (CNS) and peripheral tissues. By binding to these G-protein-coupled receptors, it activates inhibitory G-proteins, leading to decreased cyclic AMP production, reduced calcium influx, and increased potassium efflux. This results in neuronal hyperpolarization and inhibition of neurotransmitter release, particularly substance P and glutamate, thereby modulating pain transmission. Additionally, it activates descending inhibitory pain pathways in the brainstem, producing analgesia. Its effects on the cough center in the medulla produce antitussive activity, and it can cause respiratory depression by reducing brainstem respiratory center sensitivity to carbon dioxide.

Pharmacodynamics: Hydromorphone produces dose-dependent analgesia, sedation, and euphoria (in some species). It also causes respiratory depression, cough suppression, and can induce nausea and vomiting via stimulation of the chemoreceptor trigger zone. In dogs, it may cause bradycardia due to increased vagal tone, while in cats it can cause mydriasis (due to CNS stimulation) and hyperthermia. It has a rapid onset of action and is more potent than morphine (approximately 5-10 times in dogs). It can cause histamine release, though less than morphine, leading to hypotension and pruritus. In horses, it can cause CNS excitation and increased locomotor activity.

⚡ Pharmacokinetics Summary

Absorption: Hydromorphone is well absorbed after subcutaneous (SC) and intramuscular (IM) administration, with peak plasma concentrations occurring within 15-30 minutes. Oral bioavailability is low (approximately 10-20% in dogs) due to extensive first-pass metabolism, making oral administration less reliable for analgesia. Rectal absorption is variable.

Distribution: It is widely distributed throughout the body, with high affinity for the brain, lungs, liver, kidneys, and spleen. It crosses the blood-brain barrier and placenta. The volume of distribution is approximately 1-2 L/kg in dogs. Protein binding is low (about 20-30%).

Metabolism: Hydromorphone undergoes extensive hepatic metabolism, primarily via glucuronidation to hydromorphone-3-glucuronide (H3G), which is inactive but may have neuroexcitatory effects at high concentrations. In dogs, it also undergoes N-demethylation to norhydromorphone. In cats, glucuronidation is deficient, leading to slower metabolism and prolonged effects.

Excretion: Metabolites and a small amount of unchanged drug are excreted primarily in the urine. In dogs, approximately 60-70% of a dose is excreted in urine within 24 hours. Biliary excretion also occurs. In cats, elimination half-life is longer due to reduced metabolism.

Half-Life: Dogs: 1-2 hours (IV); Cats: 1-2 hours (IV); Horses: 0.5-1 hour (IV); Cattle: 0.5-1 hour (IV)

Bioavailability: Oral: 10-20% in dogs; IM/SC: nearly 100%

Protein Binding: Approximately 20-30% (species-dependent)

Available Formulations & Strengths

Injectable Solution 1 mg/mL, 2 mg/mL, 4 mg/mL, 10 mg/mL (IV, IM, SC)

Oral Tablet 2 mg, 4 mg, 8 mg (PO)

Oral Liquid 1 mg/mL (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to hydromorphone or other opioids
Severe respiratory depression
Acute asthma or chronic obstructive pulmonary disease
Head trauma or increased intracranial pressure
Concurrent use of MAO inhibitors (within 14 days)
Gastrointestinal obstruction or ileus
Biliary tract disease or pancreatitis (may cause sphincter of Oddi spasm)
Use in animals with known seizure disorders (may lower seizure threshold)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic or renal impairment
Use with caution in animals with hypothyroidism, Addison's disease, or prostatic hypertrophy
May cause respiratory depression; monitor respiratory rate and oxygen saturation
May cause bradycardia; consider anticholinergic premedication (e.g., atropine) in dogs
In cats, may cause hyperthermia; monitor body temperature
In horses, may cause CNS excitation; use with caution and consider combination with sedatives
Use with caution in debilitated or geriatric animals
May cause histamine release; administer slowly IV to minimize hypotension
Avoid extravasation at injection site
Do not use in animals with known opioid tolerance without dose adjustment
In food animals, observe withdrawal times; extra-label use requires veterinary oversight

Adverse Effects & Reactions

Common:

Sedation
Respiratory depression
Bradycardia
Vomiting
Nausea
Constipation
Urinary retention
Mydriasis (cats)
Hyperthermia (cats)
Hypothermia (dogs)
Pruritus (due to histamine release)

Serious / Severe:

Severe respiratory depression
Cardiovascular collapse
CNS excitation (especially in horses and cats)
Seizures (rare)
Ileus
Hypotension
Bradyarrhythmias

Rare:

Allergic reactions
Anaphylaxis
Paradoxical excitement
Dysphoria
Pulmonary edema (overdose)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Other CNS depressants (barbiturates, benzodiazepines, phenothiazines, anesthetics)	Additive CNS depression and respiratory depression	High
MAO inhibitors (e.g., selegiline)	Risk of serotonin syndrome or hypertensive crisis	High
Anticholinergics (e.g., atropine)	May exacerbate tachycardia or ileus	Moderate
Muscle relaxants	Enhanced neuromuscular blockade	Moderate
Beta-blockers	Increased risk of bradycardia	Moderate
Cimetidine	May increase hydromorphone plasma levels	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Severe respiratory depression
Bradycardia
Hypotension
CNS depression (coma)
Miosis (dogs) or mydriasis (cats)
Hypothermia
Muscle flaccidity
Pulmonary edema (in severe cases)

Emergency Treatment Protocol: Maintain airway and provide ventilatory support. Administer naloxone (opioid antagonist) at 0.01-0.04 mg/kg IV, IM, or SC; may repeat as needed. Monitor for renarcotization due to shorter half-life of naloxone. Provide supportive care: IV fluids, oxygen, and temperature management. In cases of severe CNS excitation, consider sedation with benzodiazepines (e.g., diazepam) if needed.

Food Animal Withdrawal Times

🥩 Meat: 7 days🥛 Milk: 3 days

Withdrawal times are not officially established for hydromorphone in food animals. The values provided are conservative estimates based on pharmacokinetic data and regulatory guidelines. Extra-label use in food animals requires veterinary oversight and extended withdrawal periods may be necessary. Consult FARAD for specific recommendations.

Storage, Handling & Regulatory Information

Storage Temperature: 15-30°C (59-86°F)

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

Handling & Special Conditions: Protect from light. Store in a tightly closed container. Do not freeze. Keep out of reach of children and animals.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Hydromorphone is not FDA-approved for veterinary use in animals; however, it is legally used in an extra-label manner under the Animal Medicinal Drug Use Clarification Act (AMDUCA) in the US. It is approved for human use.

Extra-Label (Off-Label) Use: In the US, extra-label use of hydromorphone in food animals is permitted under AMDUCA with veterinary supervision, provided there is no approved animal drug for the condition and withdrawal times are extended. In non-food animals, extra-label use is common. In the EU, similar rules apply under Regulation (EC) No 470/2009.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Hydromorphone is a potent opioid analgesic used in veterinary medicine for the management of moderate to severe pain, particularly in dogs and cats. It is often preferred over morphine due to its higher potency and lower histamine release. It is commonly used as a preanesthetic to reduce anesthetic requirements and provide analgesia. In dogs, it may cause bradycardia, which can be mitigated with anticholinergic premedication. In cats, it can cause hyperthermia and mydriasis, which may be mistaken for pain or excitement. In horses, it is used for severe colic pain but may cause excitation; combination with acepromazine is recommended. Due to its controlled substance status, strict record-keeping is required. For food animals, extra-label use is possible but requires careful consideration of withdrawal times. Always monitor respiratory rate and depth, especially in debilitated animals. Naloxone should be available for reversal if needed.

References & Literature

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