FAST FACTS AND CONCEPTS #2: Converting to/from Transdermal Fentanyl, 2nd Edition

Introduction

Quick—what dose of the fentanyl transdermal system/patch (Duragesic â ) is equianalgesic to a 3 mg/hr morphine continuous infusion? Conversions to and from fentanyl transdermal are notoriously tricky, requiring knowledge of the published conversion data, general opioid pharmacology and a generous dose of common sense.

Step 1 - Calculate the 24 hr morphine dose: 3 mg/hr x 24 hrs = 72 mg IV morphine/24 hrs;

Step 2 - Convert the IV dose to the equianalgesic oral morphine dose using a ratio of:

1 mg IV = 3 mg oral; thus 72 mg IV = 216 mg po/24 hours;

Step 3 Convert the oral morphine dose to transdermal fentanyl. There are two methods:

A. Standard Table: look up fentanyl transdermal in the PDR and find the morphine conversion table which says that 135-224 mg of oral morphine per 24 hours = 50 mcg/hour patch. Note: this range of morphine is very broad which may result in significant under dosing.

B. Alternate Formula: In 2000, Brietbart et al published an alternative method, based on the results of a multi-center trial by Donner et al, that relied on a fixed dose conversion ratio to calculate the fentanyl transdermal dose. Brietbart recommended the ratio of:

2 mg oral morphine = 1 mcg/hour of fentanyl transdermal—rounded to the nearest patch size. In the case example above, 216 mg of oral morphine per 24 hours is approximately equianalgesic to a 100 mcg/hour fentanyl transdermal patch.

Note: using this formula, 25 mcg/hour fentanyl transdermal is roughly equivalent to 50 mg oral morphine/24 hours. This dose may be excessive when used in an opioid naïve patient and/or the elderly.

Key Considerations

1. All equianalgesic ratios/formulas are approximations; clinical judgment is needed when making dose or drug conversions.

2. The risk of sedation/respiratory depression with fentanyl transdermal is probably increased in the elderly or patients with renal impairment due to its long half-life. Thus, choose the lower end of the dosing spectrum.

3. When in doubt, go low and slow with long-acting opioids, using PRN breakthrough doses generously while finding the optimal long-acting/transdermal dosage.

Other teaching points about transdermal fentanyl:

• Start at the lowest patch dose – 12 mcg/hour – in an opioid naïve patient; there is no maximum dose.
• Therapeutic blood levels are not reached for 13-24 hours after patch application, and fentanyl will be continue to be released into the blood for at least 24 hours after patch removal.
• Opioid withdrawal symptoms can occur during dose conversions—care must be taken to avoid this by use of breakthrough opioids or other dosing systems.
• Some patients will need to have their patches changed every 48 hours.
• The recommended upward dose titration interval is no more frequently than every 72 hours.
• Place patches on non-irradiated, hairless skin.
• Direct heat applied over the patch can increase drug absorption with increased toxic effects.
• There are no clinical data that cachectic patients have reduced efficacy due to loss of subcutaneous fat.

See Fast Fact #36: Calculating opioid dose conversions.

References

1. Physicians Desk Reference, 2005
2. Donner B, et al. Direct conversion from oral morphine to transdermal fentanyl. Pain. 1996; 64:527-534.
3. Payne R, et al. QOL and cancer pain: satisfaction and side effects with transdermal fentanyl versus oral morphine. J Clinical Oncology. 1998:16:1588-1593.
4. Breitbart W. An alternative algorithm for dosing transdermal fentanyl for cancer-related pain. Oncology 2000; 14:695-702.
5. Gourlay GK. Treatment of cancer pain with transdermal fentanyl. Lancet Oncology. 2001; 2:165-172.

Publication History: 2nd Edition published July 2005. Re-copy edited and updated information about new transdermal patch dose added September 2008 by the Editor.

Fast Facts were edited by David Weissman MD, Palliative Care Center, Medical College of Wisconsin until January 2007.  For comments/questions write to the current editor, Drew Rosielle MD: drosiell@mcw.edu. The complete set of Fast Facts is available at EPERC: www.eperc.mcw.edu.

Copyright/Referencing Information: Users are free to download and distribute Fast Facts for educational purposes only. Weissman DE. Converting To/From Transdermal Fentanyl, 2 nd Ed. Fast Facts and Concept s. July 2005; 2. Available at: http://www.eperc.mcw.edu/fastFact/ff_002.htm

Disclaimer: Fast Facts and Concepts provide educational information. This information is not medical advice. Health care providers should exercise their own independent clinical judgment. Some Fast Facts cite the use of a product in a dosage, for an indication, or in a manner other than that recommended in the product labeling. Accordingly, the official prescribing information should be consulted before any such product is used.

Purpose: Teaching

Audience(s)

ACGME Competencies: Medical Knowledge

Keyword(s): Pain>opioids