Intravenous fentanyl exhibits an elimination half-life of approximately 2 to 4 hours in adults, meaning it takes approximately 11 to 22 hours to eliminate from the plasma.
On the other hand, transdermal fentanyl (patches), as well as transmucosal fentanyl (lozenges), exhibit half-lives of approximately 7 hours. This suggests that both should be cleared from plasma circulation in around 1.6 days after discontinuation.
Some sources suggest that there is a greater discrepancy between transdermal and transmucosal fentanyl in regards to elimination half-life. One source estimates that transdermal fentanyl has a longer elimination half-life at around 17 hours (within the range of 13 to 22 hours) and transmucosal fentanyl has a shorter one at around 6.6 hours (within the range of 5 to 15 hours). If this is accurate, it would take around 3.89 days to eliminate the transdermally administered fentanyl and approximately 1.51 days to eliminate the transmucosal fentanyl.