Conversion calculations between opioids - morphine

The reference opioid

Morphine is the reference opioid against which all other opioid conversion calculations are anchored - "oral morphine equivalent (OME)" is the standard unit used to compare doses across different opioids and routes
The most widely used strong opioid for cancer and palliative pain worldwide, and the historical reference point for the WHO analgesic ladder

Metabolised hepatically to morphine-6-glucuronide (M6G, active, analgesic) and morphine-3-glucuronide (M3G, inactive/neuroexcitatory) - both renally excreted, so accumulate in renal impairment, a key reason morphine is used cautiously (or avoided in favour of fentanyl/buprenorphine) in significant CKD

Available in immediate-release (for breakthrough dosing and initial titration) and modified-release (for background/regular dosing) oral formulations, plus parenteral (SC/IV) forms

Oral to subcutaneous/IV conversion: approximately 2-3:1 (oral morphine dose divided by ~2-3 gives the parenteral equivalent dose) - reflecting reduced oral bioavailability (~30-40%) compared with the parenteral route
Breakthrough dose is typically 1/6 to 1/10 of the total 24-hour background dose, given as needed alongside regular background dosing
Dose titration should be individualised and reviewed regularly, particularly as renal function changes or disease progresses

Renal impairment (metabolite accumulation risk), opioid-induced constipation (near-universal, requires proactive laxative co-prescription), opioid toxicity (see opioid toxicity)

Morphine remains the benchmark opioid for cost, availability, and familiarity, but is not always the best individual choice - renal impairment, intolerance, or specific symptom patterns may favour an alternative (fentanyl, oxycodone, buprenorphine, hydromorphone) despite morphine's reference status

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