A patient on long-term oral morphine 60 mg/day (equianalgesic dose 60 mg/day) needs to be converted to transdermal fentanyl due to swallowing difficulties. What is the correct fentanyl patch dose?
- A 12 mcg/hour patch
- B 50 mcg/hour patch
- C 25 mcg/hour patch (approximately equivalent to oral morphine 60 mg/day) ✓
- D 100 mcg/hour patch
Explanation
The standard equianalgesic conversion is: oral morphine 60 mg/day ≈ transdermal fentanyl 25 mcg/hour (using the simplified rule: divide oral morphine 24-hour dose in mg by 2 to 3 to get fentanyl mcg/hour). Alternatively, the widely used ratio is oral morphine 90 mg/day ≈ 25 mcg/hour fentanyl patch in some references; oral morphine 60 mg = ~25 mcg/hour is the standard British National Formulary/palliative care guideline. A 10–25% dose reduction is applied when switching opioids for incomplete cross-tolerance. The patch takes 12–17 hours to reach therapeutic serum levels, so oral morphine should be continued during this transition period.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.