Methadone
Methadone is a synthetic, highly efficacious **mu-opioid receptor agonist** used primarily as a preanesthetic and analgesic in dogs and cats. Compared to morphine, it causes significantly less histamine release when administered intravenously, and generally results in less vomiting and sedation. **Clinical Pearls & Pharmacological Advantages:** * **Triple Mechanism:** Unlike most traditional opioids, methadone is not only a mu-agonist but also a **non-competitive NMDA receptor antagonist** and a **monoamine (serotonin and norepinephrine) reuptake inhibitor**. * **Neuropathic Pain:** Because of its NMDA antagonism, methadone is particularly valuable for treating refractory pain, neuropathic pain, and preventing central sensitization ("wind-up" pain). * **Bioavailability:** It has very poor oral bioavailability in dogs, making parenteral administration (IV, IM, SC) the standard route in veterinary medicine. * **Regulatory:** It is a Schedule II (C-II) controlled substance due to its high potential for human abuse.
Mechanism: Methadone provides analgesia through a unique, multi-modal mechanism of action: * **Mu-Opioid Receptor (MOR) Agonism:** Binds to MORs in the central nervous system โ inhibits adenylyl cyclase โ decreases cAMP โ promotes opening of inward-rectifying potassium channels and closing of voltage-gated calcium channels โ hyperpolarization of neurons and decreased release of nociceptive neurotransmitters (e.g., Substance P, glutamate). * **NMDA Receptor Antagonism:** Acts as a non-competitive antagonist at the **N-methyl-D-aspartate (NMDA)** receptor โ blocks glutamate binding โ reduces central sensitization and hyperalgesia. * **Monoamine Reuptake Inhibition:** Inhibits the presynaptic reuptake of **serotonin (5-HT)** and **norepinephrine (NE)** โ increases monoamine concentrations in the synaptic cleft โ enhances descending inhibitory pain pathways in the spinal cord.
Dosing by species
- For perioperative pain control ยท 0.05-0.5 mg/kg IV , IM or SC q4-6h ยท IV, IM, SC ยท q4-6h
- As a pre-anesthetic ยท 0.1-0.2 mg/kg SC, IM; or a combination of methadone 0.1-0.3 mg/kg with acepromazine 0.02-0.05 mg/kg SC, IM ยท SC, IM
- For moderate to severe pain ยท 0.1-0.2+ mg/kg IM or SQ ยท IM, SQ, IV ยท Duration of effect is 2-6 hours (IM/SQ) or 1-4 hours (IV) ยท For IV dosing use ยฝ the low end dose, titrate over 35 minutes
- For pain ยท 0.1-0.2 mg/kg SC, IV ยท SC, IV ยท Duration of effect 2-3 hours
- Analgesia ยท 0.1-0.5 mg/kg ยท IM ยท prn ยท Duration of action is typically 3-4 hours.
- Analgesia ยท 0.1-0.3 mg/kg ยท IV ยท prn ยท Monitor respiratory function if anaesthetized.
- Analgesia ยท 0.6 mg/kg ยท OTM ยท prn ยท Authorized preparation contains preservative and causes salivation when given by the OTM route.
- As an analgesic ยท 0.1-0.2 mg/kg PO ยท PO ยท Anecdotal; author has not prescribed this drug
- As a pre-anesthetic ยท 0.2-0.5 mg/kg SC, IM; or a combination of methadone 0.1-0.3 mg/kg with acepromazine 0.02-0.05 mg/kg SC, IM ยท SC, IM
Routes of administration
Contraindications
- Late-term pregnancy (due to risks of respiratory depression and stillbirths)
- Known hypersensitivity to methadone
- No specific contraindications available in the monograph, but use with caution in patients with severe respiratory compromise or hepatic impairment.
Adverse effects
- Panting
- Whining or vocalization
- Sedation
- Defecation
- Constipation
- Bradycardia
- Respiratory depression
- Pronounced CNS excitement (in horses at IV doses โฅ 0.1 mg/kg)
- Respiratory depression (primarily under general anaesthesia)
- Transient excitation (when given IV)
- Salivation (when given via oral transmucosal route in cats due to preservatives)
- Constriction of gastrointestinal sphincters (e.g., pyloric sphincter)
- Reduction in gastrointestinal motility (with prolonged use)
- Neonatal sedation (crosses the placenta)
Drug interactions
- Class I & III Antiarrhythmics (e.g., lidocaine, procainamide, quinidine, amiodarone) ยท May increase risks for arrhythmias
- Alpha2-Agonists (e.g., medetomidine, xylazine) ยท Potentiates sedative and analgesic effects; combination with medetomidine may cause severe hypoxemia in dogs breathing room air
- Azole Antifungals (fluconazole, itraconazole, ketoconazole) ยท May increase methadone levels in humans (does not appear to be true for dogs)
- Calcium Channel Blockers ยท May increase risks for arrhythmias
- CNS Depressants (anesthetics, antihistamines, phenothiazines, barbiturates, tranquilizers) ยท May cause increased CNS or respiratory depression
- Corticosteroids (Mineralocorticoids) ยท May increase potential for electrolyte abnormalities
- Diuretics ยท Opiates may decrease diuretic efficacy in CHF patients
- Macrolide Antibiotics (erythromycin, clarithromycin) ยท May inhibit metabolism of methadone and increase levels in humans (does not appear to be true for dogs)
- Monoamine Oxidase Inhibitors (MAOIs) (e.g., amitraz, selegiline) ยท Potential for severe CNS/behavioral reactions; concomitant use should be avoided
- Skeletal Muscle Relaxants ยท Methadone may enhance neuromuscular blockade
- Phenobarbital, Phenytoin ยท May decrease methadone levels in humans (probably does not affect dogs)
Monitoring
- Analgesic or preanesthetic efficacy
- Respiratory rate and depth (monitor for respiratory depression, especially at higher doses)
- Heart rate (bradycardia)
- CNS depression or excitation
- Respiratory rate and depth (especially under general anaesthesia)
- Pain scores (due to individual variation in response)
- Level of sedation
- Gastrointestinal motility (with prolonged use)
Overdose
Overdosage may produce profound **respiratory and/or CNS depression** in most species. Newborns are particularly susceptible. Other toxic effects include cardiovascular collapse, hypothermia, and skeletal muscle hypotonia. **Treatment:** * **Naloxone** is the reversal agent of choice for treating respiratory depression. In massive overdoses, naloxone doses may need to be repeated because naloxone's effects might diminish before sub-toxic levels of methadone are attained. * Mechanical respiratory support should be considered in severe cases. * Dialysis, charcoal hemoperfusion, or forced diuresis do not appear to be beneficial.
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.