How do you convert from intravenous (IV) fentanyl to oral morphine?

Converting IV Fentanyl Infusion to Oral Morphine

Yes, you can convert from IV fentanyl infusion to oral morphine using a two-step calculation: first convert IV fentanyl to IV morphine using a 100:1 ratio, then convert IV morphine to oral morphine using a 1:3 ratio, with a 25-50% dose reduction to account for incomplete cross-tolerance. 1, 2

Step-by-Step Conversion Algorithm

Step 1: Calculate Total Daily IV Fentanyl Dose

• Multiply the hourly IV fentanyl infusion rate (in mcg/hour) by 24 to get the total daily dose in mcg 3
• Example: 100 mcg/hour × 24 = 2400 mcg/day IV fentanyl

Step 2: Convert IV Fentanyl to IV Morphine

• Use the equianalgesic ratio of 100:1 (100 mcg IV fentanyl = 10 mg IV morphine) 2
• Divide the total daily IV fentanyl dose (in mcg) by 100 to get the equivalent IV morphine dose in mg 2
• Example: 2400 mcg IV fentanyl ÷ 100 = 24 mg IV morphine per day

Step 3: Convert IV Morphine to Oral Morphine

• Use the conversion ratio of 1:3 (1 mg IV morphine = 3 mg oral morphine) 1
• Multiply the IV morphine dose by 3 to get the oral morphine equivalent 1
• Example: 24 mg IV morphine × 3 = 72 mg oral morphine per day

Step 4: Apply Dose Reduction for Incomplete Cross-Tolerance

• Reduce the calculated oral morphine dose by 25-50% if pain was well-controlled on the original opioid 1, 2
• This accounts for incomplete cross-tolerance between different opioids and patient variability 1
• Example: 72 mg oral morphine × 0.5 to 0.75 = 36-54 mg oral morphine per day
• If pain was poorly controlled, you may use 100% of the calculated dose or even increase by 25% 2

Step 5: Divide Into Scheduled Doses

• Divide the total daily oral morphine dose into regular intervals (typically every 4 hours for immediate-release or every 12 hours for sustained-release formulations) 4
• Example: 36-54 mg/day ÷ 6 doses = 6-9 mg oral morphine every 4 hours

Critical Clinical Considerations

Breakthrough Pain Management

• Always prescribe breakthrough medication during the conversion period, typically 10-20% of the total 24-hour dose as short-acting opioid 4, 2
• Breakthrough doses should be available every 1-2 hours as needed 4
• Review total daily consumption (scheduled plus rescue doses) every 24 hours and adjust accordingly 4

Monitoring and Titration

• Clinical judgment must be used as these conversion ratios are approximate due to patient variability 1
• Monitor closely for signs of inadequate pain control or opioid toxicity during the first 24-48 hours 3
• Adjust doses based on breakthrough medication requirements after 24 hours 4

Important Caveats and Pitfalls

Wide Variation in Practice

• A 2022 international survey of palliative care professionals found significant variation in conversion ratios used worldwide, highlighting the lack of standardization 5
• Some clinicians use more conservative ratios (up to 150-200 mcg fentanyl for 10 mg morphine) based on clinical experience 6

Special Populations

• Elderly patients and those with renal insufficiency may require further dose reductions 4
• Patients on high-dose opioids (>30 mg/day IV equivalents) may require lower conversion ratios 7

Safety First Approach

• When in doubt, start with a lower dose (use the 50% reduction) and titrate upward based on response 1
• It is safer to underestimate and supplement with breakthrough medication than to overdose 1, 2