What oral morphine tablet dose is equivalent to 5 µg intravenous fentanyl?

Converting 5 mcg IV Fentanyl to Oral Morphine

5 mcg IV fentanyl converts to approximately 0.15–0.23 mg oral morphine (150–230 mcg), which is clinically negligible and not practical for tablet administration.

Conversion Calculation

The National Comprehensive Cancer Network (NCCN) provides a two-step conversion algorithm for IV fentanyl to oral morphine 1:

Step 1: Convert IV fentanyl to IV morphine

• Use the 100:1 potency ratio (100 mcg IV fentanyl = 10 mg IV morphine) 1
• 5 mcg IV fentanyl = 0.5 mg IV morphine

Step 2: Convert IV morphine to oral morphine

• Use the 1:3 ratio (1 mg IV morphine = 3 mg oral morphine) 2, 1
• 0.5 mg IV morphine = 1.5 mg oral morphine

Step 3: Apply incomplete cross-tolerance reduction

• NCCN recommends reducing the calculated dose by 25–50% when rotating opioids if pain was well-controlled 1, 3
• 1.5 mg × 0.5 (50% reduction) = 0.75 mg oral morphine
• 1.5 mg × 0.75 (25% reduction) = 1.125 mg oral morphine

Clinical Reality and Practical Considerations

This dose is far below the minimum recommended starting dose for oral morphine tablets:

• NCCN guidelines recommend an initial oral dose of 5–15 mg of short-acting morphine sulfate for opioid-naïve patients 2
• The calculated conversion (0.75–1.5 mg oral morphine) represents only 5–10% of the minimum recommended starting dose 2
• Standard morphine immediate-release tablets are available in 15 mg, 30 mg, and higher strengths, making accurate dosing of sub-milligram amounts impossible 2

Critical Context

5 mcg IV fentanyl represents an extremely small opioid dose:

• This is equivalent to approximately 1/400th of a standard 25 mcg/hour transdermal fentanyl patch 4
• A single 5 mcg IV bolus would provide analgesia for only 30–60 minutes due to fentanyl's redistribution-limited duration of action 5
• The conversion assumes a single bolus dose, not a continuous infusion 1

Recommendation for Clinical Practice

If you need to convert ongoing IV fentanyl analgesia to oral morphine, you must calculate the total 24-hour fentanyl dose, not a single 5 mcg bolus:

• Calculate the total daily IV fentanyl dose (hourly rate × 24 hours) 2, 1
• Multiply by 100 to get IV morphine equivalents 1
• Multiply by 3 to get oral morphine equivalents 1
• Reduce by 25–50% for incomplete cross-tolerance 1, 3
• Divide by 6 for every-4-hour immediate-release dosing, or by 2 for every-12-hour extended-release dosing 1

For a single 5 mcg IV fentanyl dose, oral morphine conversion is not clinically appropriate—consider repeating the IV dose or using a different short-acting analgesic strategy instead.