Morphine to Methadone: how to convert safely
No single conversion factor exists
Methadone's potency is dose-dependent and non-linear (≈4:1 vs morphine at low doses, up to 12:1–20:1 at high doses), and its half-life outlasts its analgesia by days. Fixed-factor conversion is a recognized cause of fatal overdose — this site's calculator intentionally refuses to produce a single number for methadone.
Why calculators disagree about methadone
Published methods (Ripamonti, Ayonrinde, Fudin's multiplying factors, and institutional tables) use different dose-tiered ratios and none is validated across populations. Two calculators can give answers that differ several-fold at higher morphine doses — which is the strongest argument for treating any output as a hypothesis, not an order.
- Baseline ECG: methadone prolongs QTc; recheck after dose changes.
- Interaction screen: CYP3A4/2B6 inhibitors and other QT-prolonging drugs.
- Titrate no faster than every 5–7 days — steady state takes days, and early "failure" often becomes late toxicity.
- Involve palliative care, pain, or addiction medicine unless methadone rotations are part of your routine practice.
What this site's tools will and won't do
The opioid regimen builder totals MME for mixed regimens and flags methadone lines as "specialist conversion required" rather than guessing. Use it for the rest of the regimen; use a human for this switch.
Frequently asked questions
- Why is there no single morphine-to-methadone conversion factor?
- Methadone's potency relative to morphine is dose-dependent and non-linear: roughly 4:1 at low morphine doses but as steep as 12:1–20:1 at high doses. Its half-life (8–59 hours) far outlasts its analgesic effect, so doses stack for days before full effect is seen. Applying a fixed factor from a standard equianalgesic table is a recognized cause of fatal overdose.
- Why do methadone calculators disagree with each other?
- Different references use different dose-tiered ratios (Ripamonti, Ayonrinde, Fudin's, and others), and none is validated across populations. Any tool that outputs a single methadone number without a tiered method, QT assessment, and titration plan is oversimplifying — which is why this calculator declines to convert methadone automatically.
- What should be checked before starting methadone?
- A baseline ECG for QTc (methadone prolongs QT), a review for interacting drugs (CYP3A4/2B6 inhibitors, other QT-prolonging agents), and a titration plan that increases doses no faster than every 5–7 days. Involve palliative care, pain medicine, or addiction medicine unless you rotate patients to methadone regularly.
Créé et maintenu par Yasmine Abbey, MD, MSc · Dernière revue:
Aide éducative à la décision destinée aux cliniciens. Ne constitue pas un avis médical — vérifiez les résultats au regard de votre jugement clinique, des protocoles de votre établissement et des recommandations en vigueur.