Fentanyl is a lipophilic Opioid that is hundred times as strong as morphine.
In the IV form Fentanyl is often used in ICUs for pain control due to its rapid onset of action and due to the minimal histamine release associated with fentanyl in comparison to morphine.
Fentanyl is also available as a sustained release transdermal patch which when applied to a fleshy (non-bony) region of the body functions as a basal pain medication for 72 hours. The fentanyl in the patch diffuses through the skin and depots in the adipose tissue and provides sustained pain relief. Very hairy skin may need to be shaved locally before applying the patch.
The efficacy of absorption of the fentanyl is dependant on the skin temperature and the availability of adequate adipose tissue. Therefore some patients who are cachetic or hypothermic may not get adequate pain relief. In contrast patients who are febrile may absorb more of the medication thus necessitating either a dose decrease (if they are experiencing undue side effects) or a need for the patch to be changed as often as every 48 hours.
Therapeutic blood levels are not reached for 13-24 hours after patch application. So when converting from an oral Opioid, remember to have enough prn opioids available for breakthrough analgesia during the window between stopping the oral medication and the interval of time for the fentanyl to reach therapeutic blood levels
The recommended upward dose titration interval is every 72 hours. Doses should be increased or decreased by 25-50% of the current 24-hour dose.
The fentanyl will continue to be effective for 13-24 hours after the patch is removed (as a part of the drug is already depot in the adipose tissue).