Morphine
Morphine sulfate
Also known as: Astramorph PF · Avinza · DepoDur · Infumorph · Kadian · MSIR · MS Contin · Oramorph SR · RMS · Roxanol · Morphini sulfas · Morphine sulfate · Morphine hydrochloride
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceAnalgesia (extra-label) — CRI dosing
Verified clinician required
NA
Governing clinical context (5)
- 1. Preservative-free morphine formulations should be used for epidural administration.
- 2. Do not confuse CRI dosages listed as mg/kg/hour with those listed as µg/kg/minute.
- 3. For additional dosages/protocols for using morphine in combination with other drugs (eg, ketamine, lidocaine, dexmedetomidine) for pain/sedation in dogs or cats, see the dosages and their accompanying references in the Dexmedetomidine, Ketamine, and Lidocaine monographs.
- Analgesia (extra-label):
- Morphine injection should be stored at room temperature (20°C-25°F [68°F-77°F]), protected from light; do not freeze. 19 Morphine gradually darkens in color when exposed to light. Morphine does not appear to adsorb to plastic or polyvinyl chloride (PVC) syringes, tubing, or bags.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Morphine is the prototypical opiate analgesic used in veterinary medicine for the treatment of moderate to severe acute pain. It is a naturally occurring pure mu-opioid receptor agonist derived from opium.
Clinical Pearls & Species Differences:
- Dogs & Primates: Morphine typically causes predictable CNS depression, sedation, and analgesia. It is a potent emetic in dogs due to direct stimulation of the chemoreceptor trigger zone (CTZ). Dogs often defecate immediately following an initial dose, followed by decreased GI motility.
- Cats, Horses, & Ruminants: These species may exhibit paradoxical CNS excitation (dysphoria, mania, pacing) rather than depression, especially if administered without a concurrent sedative or tranquilizer (like acepromazine or an alpha-2 agonist). Cats require significantly higher doses than dogs to induce vomiting.
- Histamine Release: Morphine causes non-immunologic histamine release from mast cells. Intravenous administration must be performed slowly to avoid severe hypotension, vasodilation, and bronchoconstriction.
- Thermoregulation: Morphine can cause hypothermia in dogs and rabbits, but hyperthermia in cats, horses, cattle, and goats.
Mechanism of action
Morphine acts primarily as a full agonist at the mu (μ) opioid receptors in the central nervous system, with some secondary activity at delta receptors.
- Mechanism: Binding to the G-protein coupled mu-receptor → inhibits adenylate cyclase → decreases intracellular cAMP.
- Presynaptic effect: Closes voltage-gated calcium channels → decreases the release of excitatory nociceptive neurotransmitters (e.g., Substance P, glutamate).
- Postsynaptic effect: Opens inward-rectifying potassium channels → hyperpolarizes the neuron → inhibits ascending pain transmission pathways.
- Secondary effects: Direct stimulation of the chemoreceptor trigger zone (CTZ) causes emesis. Central vagal stimulation leads to bradycardia. Increased anti-diuretic hormone (ADH) release can reduce urine production.
Safety & warnings
Contraindications
- Hypersensitivity to narcotic analgesics
- Patients receiving monoamine oxidase inhibitors (MAOIs)
- Diarrhea caused by a toxic ingestion (until toxin is eliminated)
- Scorpion envenomation (Centruroides spp. - potentiates venom)
- Conditions where vomiting is contraindicated (e.g., raised intraocular pressure)
Adverse effects
- Histamine release (hypotension, vasodilation)
- Bronchoconstriction
- CNS depression (dogs, primates) or excitation (cats, horses, ruminants)
- Physical dependence (with chronic use)
- Hyperthermia (cattle, goats, horses, cats)
- Hypothermia (dogs, rabbits)
- Nausea and vomiting
- Decreased intestinal peristalsis and constipation
- Initial defecation (dogs)
- Panting (dogs)
- Bradycardia or tachycardia
- Histamine release (if given rapidly IV)
- Vomiting (common in non-painful pre-operative patients)
- Transient excitation (IV administration)
- Respiratory depression (especially under general anaesthesia)
- Constriction of gastrointestinal sphincters (e.g., pyloric sphincter)
- Reduction in gastrointestinal motility (with prolonged use)
- Neonatal sedation (crosses the placenta)
Precautions
Extreme Caution: Patients with head injuries, increased intracranial pressure, acute abdominal conditions (e.g., colic - may mask clinical signs), and respiratory disease or acute respiratory dysfunction.
Caution: Hypothyroidism, severe renal insufficiency (acute uremia), adrenocortical insufficiency (Addison's), and geriatric or severely debilitated patients.
Important Notes:
- IV Administration: Must be given slowly to avoid significant hypotension and histamine release.
- Envenomation: Avoid in envenomation situations, as clinical signs associated with histamine-release can be confused with anaphylaxis.
- Uremia: Because of its effects on vasopressin (ADH), urine flow can decrease by up to 90% in dogs given large doses.
Drug interactions
May cause increased CNS or respiratory depression when used with morphine.
Opiates may decrease diuretic efficacy in congestive heart failure patients.
Use with extreme caution; contraindicated in humans due to risk of severe opiate overdose signs.
Morphine may enhance neuromuscular blockade.
Morphine may exacerbate the effects of tricyclic antidepressants.
Opiates may potentiate anticoagulant activity.
Increased CNS or respiratory depression
Monitoring
- CNS level of depression or excitation
- Blood pressure (especially with IV use)
- Analgesic activity
- Respiratory rate and depth (especially under general anaesthesia)
- Pain scores (to assess individual efficacy)
- Signs of histamine release (hypotension, tachycardia) during IV administration
- Gastrointestinal motility
Pharmacokinetics
Half-life
Absorption
Absorbed when given by IV, IM, SC, and rectal routes. Very low oral bioavailability (<20%) in dogs due to a high first-pass effect, limiting the clinical usefulness of oral morphine in canines.
Distribution
Concentrates in the kidney, liver, and lungs; lower levels are found in the CNS. The majority of free morphine is found in skeletal muscle. Crosses the placenta and distributes into maternal milk. Volume of distribution in dogs is ~7.5 L/kg.
Metabolism
Major route of elimination is by metabolism in the liver, primarily by glucuronidation. Cats are deficient in this metabolic pathway, leading to prolonged half-lives.
Elimination
The active glucuronidated metabolite (M6G) is excreted by the kidney. Clearance in dogs is approximately 83 mL/min/kg.
Overdose
Signs of Toxicity: Overdosage may produce profound respiratory and/or CNS depression in most species. Newborns are more susceptible. Parenteral doses >100 mg/kg are thought to be fatal in dogs. Other toxic effects include cardiovascular collapse, hypothermia, and skeletal muscle hypotonia. Horses, cats, swine, and cattle may demonstrate CNS excitability (hyperreflexia, tremors) and seizures at high doses or if given rapidly intravenously.
Treatment:
- Naloxone is the agent of choice for treating respiratory depression. Doses may need to be repeated as naloxone's effects might diminish before sub-toxic levels of morphine are attained.
- Mechanical respiratory support should be considered in severe cases.
- Pentobarbital has been suggested for CNS excitement and seizures in cats, but extreme caution is required as barbiturates and narcotics have additive respiratory depressant effects.
Available products
Formulations
- Morphine Sulfate for Injection (1-50 mg/mL)
- Liposomal Extended-release Injection (10 mg/mL)
- Preservative-free Injection (0.5-25 mg/mL)
- Soluble Tablets for Injection
- Oral Tablets (15 mg, 30 mg)
- Extended/Controlled Release Tablets (15-200 mg)
- Extended/Sustained Release Capsules (20-200 mg)
- Oral Solution (2-20 mg/mL)
- Rectal Suppositories (5-30 mg)
- Injectable solution: 10 mg/ml, 15 mg/ml, 20 mg/ml, 30 mg/ml
- Oral tablets: 10 mg, 30 mg, 60 mg, 100 mg
- Oral suspensions
- Slow-release capsules
- Granules
Human-labeled
- Morphine Sulfate for Injection (1 mg/mL to 50 mg/mL)
- Morphine Sulfate Liposomal Extended-release Injection (DepoDur, 10 mg/mL)
- Morphine Sulfate for Injection, preservative-free (Infumorph, Astramorph PF, 0.5 mg/mL to 25 mg/mL)
- Morphine Sulfate Soluble Tablets for Injection (10 mg, 15 mg, 30 mg)
- Morphine Sulfate Oral Tablets (15 mg, 30 mg)
- Morphine Sulfate Extended/Controlled Release Tablets (MS Contin, Oramorph SR, 15 mg to 200 mg)
- Morphine Sulfate Extended/Sustained Release Capsules (Avinza, Kadian, 20 mg to 200 mg)
- Morphine Sulfate Oral Solution (MSIR, Roxanol, 2 mg/mL to 20 mg/mL)
- Morphine Sulfate Rectal Suppositories (RMS, 5 mg to 30 mg)
- Morphine sulfate injectable solutions
- Morphine sulfate tablets and slow-release capsules
- Morphine suppositories
Regulatory status
Methadone is the licensed alternative preferred for bolus administration.
Controlled Drug Schedule 2. Methadone is the licensed alternative.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Oral morphine products should be stored in tight, light-resistant containers at room temperature unless otherwise labeled. Morphine injection should be stored at room temperature, protected from light; do not freeze. Morphine gradually darkens in color when exposed to light; protect from prolonged exposure to bright light.
Client information
Morphine is a potent, highly regulated pain-relieving medication typically administered in a hospital setting under direct veterinary supervision.
- What to Expect: Your pet may appear sedated, sleepy, or unusually quiet. Dogs often pant heavily and may vomit or defecate shortly after receiving the drug.
- Cats and Horses: May sometimes appear restless, pace, or have wide-open eyes instead of appearing sleepy. This is a known species difference.
- At-Home Care: If your pet is sent home with oral morphine, keep it strictly out of reach of children and other animals. It is a highly controlled substance (Schedule II). Do not adjust the dose without consulting your veterinarian.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.