Analgesia

Methadone

Full µ-opioid agonist with additional NMDA-receptor antagonism, useful for moderate-to-severe acute pain and for premedication where opioid analgesia is desired without the histamine release of morphine. Weight-based bolus, premedication, and CRI dosing for dogs and cats (extra-label). Stock 10 mg/mL (no US veterinary product; human-labeled vials).

Stock: 10 mg/mL · Schedule II

No US veterinary product. Use human methadone HCl injection 10 mg/mL (Dolophine® or generic). Verify label, concentration errors are high-stakes. DEA Schedule II, record all use.
Bradycardia risk. Methadone has greater cardiodepressant effect than morphine. Monitor HR and blood pressure. Use caution with pre-existing bradyarrhythmias or when combined with alpha-2 agonists.
Incompatible with meloxicam at Y-sites. Do not run in the same line as NSAIDs. Compatible in syringe with acepromazine.

Awaiting input

Enter a patient weight to see the result.

Formula

All three use cases follow the same relationship: dose × weight ÷ stock = volume.

Bolus / premedication

$$\text{volume}_{\text{mL}} = \frac{\text{dose}_{\text{mg/kg}} \times \text{weight}_{\text{kg}}}{\text{stock}_{\text{mg/mL}}}$$

Enter a patient weight to see the worked example.

CRI maintenance (undiluted from vial)

$$\text{mL/hr} = \frac{\text{weight}_{\text{kg}} \times \text{rate}_{\text{mg/kg/hr}}}{\text{stock}_{\text{mg/mL}}}$$

Enter a patient weight to see the worked example.

CRI bag prep

Enter a patient weight to see the worked example.

Sources

Plumb DC. Plumb's Veterinary Drugs. Methadone monograph (current edition). All dosages extra-label in dogs and cats. Bolus: 0.1–0.5 mg/kg IV/IM/SC q4–8h (dogs), 0.1–0.6 mg/kg q4–6h (cats). CRI: 0.1–0.2 mg/kg IV loading, then 0.12 mg/kg/hr. Bag prep: 60 mg / 500 mL IV fluid at 1 mL/kg/hr. plumbs.com
No US veterinary-labeled product. Use human methadone HCl injection 10 mg/mL. DEA Schedule II controlled substance, follow applicable federal, state, and local controlled substance regulations. NMDA antagonist activity (both isomers) distinguishes methadone from other mu-agonists.