Question 1

Why is there no fixed conversion ratio for methadone?

Accepted Answer

Methadone's half-life (~24–36 hours) is much longer than its analgesic duration (6–8 hours), and its NMDA-receptor antagonism creates non-linear cross-tolerance. As the oral morphine equivalent dose increases, the methadone conversion ratio decreases (e.g., 3:1 at low doses, 20:1 at very high doses). Methadone conversions should never be done via simple equianalgesic calculation alone.

Question 2

Do I need to reduce the dose when converting to transdermal fentanyl?

Accepted Answer

Yes — and the reduction is already built into the manufacturer-recommended conversion table. The additional 50–75% safety reduction on top of that table is recommended by multiple safety organisations. Also note that fever, heating pads, and saunas increase fentanyl absorption by up to one-third.

Question 3

Should the same conversion ratios be used for opioid-naive patients?

Accepted Answer

No. Equianalgesic tables are designed for patients on chronic opioids requiring conversion. For opioid-naive patients requiring a strong opioid, use standard starting doses (e.g., morphine 5–10 mg oral every 4 hours) rather than converting from another analgesic.