Fentanyl can be found in your body for different periods of time, depending on what is being tested: Urine: Fentanyl stays in the urine for up to 3 days after its last use. Blood and plasma: The drug can be found in the blood for up to 12 hours after its last use. Further, its major breakdown product norfentanyl can be found in the blood for up to 10 hours after the last use. Saliva: It is not clear how long fentanyl is detectable in saliva. Hair: Fentanyl is present in hair. In general, 1.5 inches of hair growth shows the past 90 days of drug use.