Divide by two from oral to subcutaneous morphine, and remember the breakthrough dose
Expansion
Common conversions, all to oral morphine equivalent
| From | To oral morphine |
|---|---|
| Oral codeine or tramadol | Divide by 10 |
| Oral morphine to subcutaneous morphine | Divide by 2 |
| Oral morphine to subcutaneous diamorphine | Divide by 3 |
| Oral morphine to oral oxycodone | Divide by 1.5 to 2 |
Breakthrough dose = one sixth of the total daily dose, given as required, usually up to hourly.
Worked example: a patient on 60 mg of modified release morphine twice daily takes 120 mg in 24 hours. The breakthrough dose is 20 mg. Converting to a subcutaneous syringe driver gives 60 mg of morphine or 40 mg of diamorphine over 24 hours.
“Halve it for the needle, and a sixth for breakthrough.”
Always co-prescribe
- A laxative, since tolerance never develops to constipation
- An antiemetic for the first few days
- The breakthrough dose
Reduce doses in renal impairment: morphine metabolites accumulate and cause sedation and myoclonus. Oxycodone, alfentanil and buprenorphine are preferred, and alfentanil is the usual choice in severe renal failure.
Check conversions against a reference every time; errors here are a recognised cause of serious harm.