Converting Transdermal Fentanyl to Oral Morphine
Use a conversion ratio of 2:1 (transdermal fentanyl mcg/hour to oral morphine mg/day), meaning each 1 mcg/hour of transdermal fentanyl equals approximately 2 mg/day of oral morphine, and then reduce the calculated dose by 25-50% to account for incomplete cross-tolerance. 1, 2
Standard Conversion Table
The following conversions are based on established guideline recommendations 1, 2:
- 25 mcg/hour fentanyl patch = 60 mg/day oral morphine
- 50 mcg/hour fentanyl patch = 120 mg/day oral morphine
- 75 mcg/hour fentanyl patch = 180 mg/day oral morphine
- 100 mcg/hour fentanyl patch = 240 mg/day oral morphine
Step-by-Step Conversion Algorithm
Step 1: Calculate the Baseline Oral Morphine Equivalent
Identify the current transdermal fentanyl dose and use the 2:1 ratio to determine the oral morphine equivalent 1, 2. For example, a patient on a 50 mcg/hour patch would calculate to 120 mg/day oral morphine.
Step 2: Apply the Safety Reduction
Reduce the calculated oral morphine dose by 25-50% to account for incomplete cross-tolerance 3, 4. This critical step prevents overdosing when switching between opioids. For the example above, 120 mg/day reduced by 25-50% yields 60-90 mg/day oral morphine as the starting dose.
Step 3: Divide into Scheduled Doses
Divide the total daily oral morphine dose into 4-6 doses throughout the day 4. Using the example of 60 mg/day: this equals 15 mg every 6 hours or 10 mg every 4 hours.
Step 4: Prescribe Breakthrough Medication
Provide immediate-release morphine at 10-20% of the 24-hour dose for breakthrough pain 3, 4. For a patient on 60 mg/day, prescribe 6-12 mg of immediate-release morphine as needed.
Critical Timing Considerations
Remove the fentanyl patch and initiate oral morphine 12-18 hours later 1. Fentanyl has a prolonged half-life after patch removal, with serum concentrations declining gradually over 17+ hours 2. Starting oral morphine too early risks opioid overdose, while starting too late may cause withdrawal symptoms.
Important Clinical Caveats
Avoid Reverse Conversion
Never use these conversion ratios in reverse to convert from oral morphine to transdermal fentanyl 2. The FDA explicitly warns that these tables are conservative for converting TO fentanyl and will overestimate doses when converting FROM fentanyl, potentially causing fatal overdose 2.
Alternative Conversion Ratios in Literature
While guidelines recommend the 2:1 ratio (100:1 for total daily dose), some research suggests a 70:1 ratio may be more accurate based on actual clinical titration 5. However, the FDA-approved and guideline-recommended 100:1 ratio (2:1 hourly) remains the safest starting point 1, 2.
Monitor for Withdrawal
Approximately 3-10% of patients may experience opioid withdrawal symptoms within the first 24 hours of conversion 5. Watch for agitation, diaphoresis, tachycardia, and increased pain. If withdrawal occurs, provide supplemental immediate-release morphine and reassess the conversion calculation.
Titration Requirements
Reassess pain control and adverse effects within 24-48 hours of conversion 4. Patients with uncontrolled pain may require upward titration of 25-50% of the baseline dose 4. More frequent assessment is needed for actively dying patients 3, 4.
Special Populations
The conversion ratio may vary based on prior opioid exposure and individual patient factors 6, 7. Patients on very high fentanyl doses (>300 mcg/hour) may require more conservative reductions and closer monitoring 8.