Oral to Intravenous Morphine Conversion
The standard conversion ratio from oral to intravenous morphine is 3:1, meaning you divide the oral dose by 3 to obtain the equivalent IV dose. 1
Conversion Ratio and Clinical Application
The average relative potency ratio of oral to parenteral (subcutaneous or intravenous) morphine ranges between 1:2 and 1:3, with 1:3 being the most commonly recommended conversion factor. 1, 2
When converting from oral to IV morphine, divide the total daily oral morphine dose by 3 to calculate the roughly equianalgesic IV dose, though upward or downward adjustment may be required based on individual patient response. 1
This 3:1 ratio accounts for the first-pass metabolism that oral morphine undergoes, which significantly reduces its bioavailability (20-30%) compared to IV administration. 3
Practical Conversion Examples
Using the conversion tables from cancer pain guidelines 1:
- 60 mg oral morphine daily = 20 mg IV/subcutaneous morphine daily
- 120 mg oral morphine daily = 40 mg IV/subcutaneous morphine daily
- 180 mg oral morphine daily = 60 mg IV/subcutaneous morphine daily
These examples consistently demonstrate the 3:1 conversion ratio across different dose ranges. 1
Important Clinical Considerations
When Converting from Oral to IV:
If pain was well-controlled on oral morphine, consider reducing the calculated IV dose by 25-50% to account for incomplete cross-tolerance and individual variability. 1
If pain was poorly controlled, you may use 100% of the calculated equianalgesic IV dose or even increase it by 25%. 1
Dosing Intervals:
IV morphine has a duration of action of approximately 4 hours, similar to immediate-release oral morphine, so dosing intervals remain consistent when converting between routes. 3, 2
Peak effects occur within 15-30 minutes for IV administration compared to 1 hour for oral immediate-release formulations. 3
Common Pitfalls and Caveats
Large interindividual variation exists in morphine pharmacokinetics, so these conversion ratios represent averages rather than guarantees for every patient. 3
The conversion ratio may vary depending on the total daily dose and clinical context—some sources suggest ratios ranging from 1:2 to 1:3, with higher oral doses potentially requiring less aggressive conversion. 1, 4
Always monitor patients closely after conversion for both adequate pain control and adverse effects, particularly respiratory depression with IV administration. 1, 5
In patients with renal impairment, avoid morphine entirely or use extreme caution due to accumulation of renally cleared metabolites. 1, 2
Route Selection Guidance
Reserve IV/subcutaneous routes for patients unable to take oral medication or requiring rapid titration for severe, uncontrolled pain. 3
For patients with severe pain requiring urgent relief, IV morphine is superior to oral administration, providing peak effects within 15-30 minutes versus 1 hour for oral routes. 3, 6
Once pain is stabilized on IV morphine, convert back to oral administration using the 3:1 ratio (multiply IV dose by 3 to get oral dose) for long-term management. 1, 7