Methadone
Full mu-opioid agonist with additional NMDA-receptor antagonism. Useful for neuropathic or wind-up pain where pure mu-agonists are insufficient. Available as bolus, premedication, or CRI. DEA C-II.
Clinical background
Methadone is a synthetic full mu-opioid agonist with an additional mechanism that distinguishes it from most veterinary opioids: it is also an NMDA-receptor antagonist. This dual action makes it particularly useful for patients with neuropathic or chronic pain, where central sensitization (wind-up) plays a role, and for multimodal perioperative analgesia. It has been used in veterinary medicine for decades and is available in human-labeled formulations.
Pharmacology
Mu-opioid agonism accounts for the primary analgesic effect, as well as the typical opioid adverse effects: dose-dependent respiratory depression, bradycardia, decreased GI motility, and sedation. In dogs at analgesic doses, sedation is moderate and generally useful clinically.
NMDA-receptor antagonism blocks the glutamate receptor responsible for central sensitization and wind-up. This mechanism is shared with ketamine and is thought to contribute to methadone’s efficacy in pain states where pure mu-agonists are insufficient. Clinically, this translates to better performance in post-amputation pain, chronic osteoarthritis flares, and cancer pain than morphine or hydromorphone alone.
Pharmacokinetics are complex. Methadone has a long and variable half-life (4–8+ hours in dogs; variable in cats) but a clinical duration of action that is shorter, typically 4–6 hours in dogs. The discrepancy means accumulation can occur with frequent redosing. Bioavailability after IM/SC injection is high (>80%). Extensive hepatic metabolism; renal excretion is a minor route, so dose adjustment for renal disease is not required.
DEA Schedule II (C-II) controlled substance. Documentation and disposal requirements apply.
Indications
- Perioperative analgesia: premedication, intraoperative, and early postoperative
- Acute moderate-to-severe pain: trauma, postoperative, pancreatitis
- Neuropathic or wind-up pain: where the NMDA-antagonist component provides added benefit over other opioids
- Chronic cancer pain: often used in palliative protocols when oral opioids are insufficient
- CRI: appropriate for patients requiring sustained analgesia in the ICU
Dosing
- Dogs: 0.1–0.5 mg/kg IV/IM/SC q4–8h for bolus; 0.2–0.3 mg/kg IM for premedication
- Cats: 0.1–0.6 mg/kg IV/IM/SC q4–6h for bolus; 0.1–0.6 mg/kg IM for premedication
- CRI (both species): 0.1–0.2 mg/kg loading dose, then 0.12 mg/kg/hr
For CRI delivery, a common bag preparation is 60 mg in 500 mL (0.12 mg/mL), run at 1 mL/kg/hr.
Stock concentration is typically 10 mg/mL (human-labeled).
Adverse effects
- Respiratory depression: dose-related; monitor respiratory rate and effort
- Bradycardia: vagally mediated; treat with anticholinergics if hemodynamically significant
- Dysphoria: particularly in cats; may manifest as restlessness or vocalization. More common at higher doses.
- GI effects: decreased motility, constipation with prolonged use; nausea and vomiting less common than with morphine (no histamine release)
- Accumulation: with repeated dosing due to long half-life; titrate conservatively in geriatric patients
Drug interactions
- CNS depressants: additive sedation and respiratory depression with benzodiazepines, α₂ agonists, acepromazine, and inhalant anesthetics
- Incompatibilities (Y-site): do not co-administer with meloxicam, thiopental, or pentobarbital
- MAO inhibitors: serotonin syndrome risk; avoid
Sources
- Plumb’s Veterinary Drugs, methadone monograph.
- KuKanich B, Papich MG: Pharmacokinetics of methadone and a metabolite in healthy Greyhound dogs. J Vet Pharmacol Ther 2004.
- Ingvast-Larsson C et al: Clinical pharmacology of methadone in dogs. Vet Anaesth Analg 2010.