Fentanyl is an opiate narcotic used for severe pain. It will stay in your system for about two to four days and will show up in a urine drug test for that length of time. It could, however, show up in a hair strand test for up to 90 days.
The injected form of fentanyl is quickly metabolized and will leave your system within about 6 hours. Other delivery systems, such as the patch, can leave residual traces of fentanyl for up to 30 days after prolonged use. It is very difficult to give a concrete answer with this drug because it comes in several different dosages and formulations and health conditions such as renal failure or liver damage can markedly change the time it takes to metabolize it.
Fifty mcg of Fentanyl can remain in your system for as long as one week. Some say four days but I will say one week to make sure you are safe.