Butorphanol
**Butorphanol** is a synthetic opioid partial agonist widely used in veterinary medicine across multiple species. * **Analgesia**: Provides mild to moderate visceral analgesia. Due to its "ceiling effect," higher doses do not increase pain relief, making it unsuitable for severe pain (e.g., major orthopedic procedures). * **Antitussive**: Highly effective for suppressing chronic non-productive coughs (e.g., infectious tracheobronchitis, tracheal collapse) in dogs. * **Reversal Agent**: Uniquely useful for reversing the profound respiratory and CNS depression of pure mu-agonists (like fentanyl, methadone, or morphine) while still maintaining a degree of kappa-mediated analgesia. * **Sedation**: Frequently combined with alpha-2 agonists (like dexmedetomidine or xylazine) and benzodiazepines for synergistic sedation and chemical restraint. > **Clinical Pearl**: While butorphanol is an excellent antitussive and mild sedative in small animals, its very short duration of action (often <1 hour for analgesia in dogs) limits its utility as a primary analgesic. Conversely, it is a highly effective and commonly used visceral analgesic in horses (e.g., for colic).
Mechanism: Butorphanol exerts its effects by interacting with specific opioid receptors in the central nervous system: * **Kappa (ฮบ) and Sigma (ฯ) Agonist**: Activation of ฮบ-receptors in the limbic system and spinal cord provides visceral analgesia and sedation. * **Mu (ฮผ) Antagonist**: Competitively binds to and blocks ฮผ-receptors. This antagonism is responsible for its ability to reverse pure ฮผ-agonist drugs and is the reason for its analgesic "ceiling effect." * **Antitussive Mechanism**: Directly suppresses the **medullary cough center**, elevating the threshold to stimuli (like CO2) without depressing respiratory center sensitivity as profoundly as pure agonists. * **Cellular Pathway**: Binds to ฮบ-receptors โ inhibits adenylate cyclase โ decreases intracellular cAMP โ closes voltage-gated calcium channels and opens potassium channels โ hyperpolarization and reduced neuronal excitability.
Dosing by species
- As an analgesic ยท 0.1-0.5 mg/kg IV, IM, SQ ยท IV/IM/SC ยท PRN ยท Provides only mild to moderate analgesia; duration of sedative action 2-4 hours, but analgesic action may be 1 hour or less.
- As a postoperative CRI ยท Loading dose of 0.1-0.2 mg/kg IV, then a CRI of 0.1-0.2 mg/kg/hr ยท IV ยท CRI ยท Usually in combination with ketamine (loading dose 0.1 mg/kg IV, CRI 0.4 mg/kg/hr).
- As an epidural analgesic ยท 0.25 mg/kg diluted with preservative-free saline (0.2 mL) or local anesthetic epidurally ยท Epidural ยท Once ยท 2-4 hours ยท Onset of action less than 30 minutes.
- As reversal agent for mu-agonist opiates ยท 0.05-0.1 mg/kg IV ยท IV ยท Once ยท Does not completely reverse analgesic effects.
- In combination as an immobilizing agent (short procedures) ยท butorphanol 0.2 mg/kg; medetomidine 0.001-0.015 mg/kg; midazolam 0.05-0.2 mg/kg ยท IM ยท Once ยท For difficult cats and short procedures.
- In combination as an immobilizing agent (more sedation) ยท butorphanol 0.2 mg/kg; medetomidine 0.015-0.02 mg/kg; midazolam 0.05-0.2 mg/kg; add ketamine 1-5 mg/kg when insufficient sedation ยท IM ยท Once
- As a sedative/analgesic ยท Butorphanol alone 0.05-0.1 mg/kg IM, SC. Butorphanol/Xylazine: Butorphanol 0.2 mg/kg + Xylazine 2 mg/kg IM ยท IM/SC ยท Once
- For injectable anesthesia ยท Butorphanol 0.1 mg/kg, Ketamine 5 mg/kg, medetomidine 80 micrograms/kg. Combine in one syringe and give IM. ยท IM ยท Once ยท May need to supplement with isoflurane (0.5-1.5%) for abdominal surgery.
Routes of administration
Contraindications
- Known hypersensitivity to butorphanol
- Lower respiratory tract conditions with copious mucous production (suppressing cough prevents clearance)
- Caution in patients with head trauma, increased CSF pressure, or severe CNS dysfunction (e.g., coma)
- Caution in severe liver disease, renal insufficiency, hypothyroidism, or Addison's disease
- Caution in dogs with heartworm disease (safety not established)
Adverse effects
- Sedation and ataxia
- Anorexia or diarrhea (rare in small animals)
- Respiratory depression (mild compared to pure agonists)
- CNS excitement, head tossing, and increased ambulation (especially in horses at high doses or rapid IV administration)
- Decreased gastrointestinal motility and potential ileus (horses)
- Nystagmus, salivation, seizures, and hyperthermia (at massive overdoses in horses)
Drug interactions
- Other CNS Depressants (anesthetics, antihistamines, phenothiazines, barbiturates, tranquilizers) ยท May cause increased CNS or respiratory depression; dosage may need to be decreased.
- Erythromycin ยท Could potentially decrease the metabolism of butorphanol, prolonging its effects.
- Fentanyl (and other pure opiate agonists) ยท Butorphanol may antagonize analgesic effects, but will also reverse sedative and respiratory depressant effects.
- Pancuronium ยท May cause increased conjunctival changes when used concurrently.
- Theophylline ยท Could potentially decrease the metabolism of butorphanol.
- Anaesthetic agents ยท Reduces the doses of other drugs required for induction and maintenance of anaesthesia ยท moderate
- Full mu-opioid agonists (e.g., methadone, fentanyl) ยท Addition of butorphanol will reduce analgesia produced from the full mu agonist; combination is not recommended for analgesia ยท major
- Acepromazine ยท Synergistic sedation ยท moderate
- Alpha-2 agonists ยท Synergistic sedation and analgesia ยท moderate
- Full mu-agonist opioids (e.g., methadone, fentanyl) ยท Butorphanol's mu-antagonist properties may block or partially reverse the analgesic effects of full mu-agonists. Higher doses of full mu-agonists may be required. ยท major
Monitoring
- Analgesic and/or antitussive efficacy
- Respiratory rate and depth
- Appetite and bowel function
- CNS effects (sedation vs. excitement)
Overdose
Acute life-threatening overdoses are unlikely (LD50 in dogs is 50 mg/kg). However, because veterinary injection comes in two highly different strengths (0.5 mg/mL and 10 mg/mL), inadvertent overdoses can occur in small animals. **Clinical Signs**: CNS effects (profound sedation or excitement), cardiovascular changes, and respiratory depression. **Treatment**: * Administer **intravenous naloxone** immediately to reverse opioid effects. * Provide supportive measures: IV fluids, oxygen therapy, vasopressors, and mechanical ventilation if required. * If seizures occur and persist, use **diazepam** for control.
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.