Fentanyl is short-acting but can be detectable longer in the body depending on use. It is typically found in blood for up to 12 hours, saliva for 1–2 days, and urine for about 1–3 days after single use. With repeated or heavy use, urine detection may extend to 4–7 days. Hair tests can detect it for up to 90 days or more. Metabolism, dose, and health affect clearance time.