What happens when fentanyl 50mg kicks in?

Pain relief: Fentanyl works on pain receptors. Its primary use is to alleviate pain. How fast it works depends on how it is given (IV, IM, transdermal). I'll assume you are referring to IV. Works in 1-5 mins, lasts ~ 45-90 mins. Some very serious, but expected side effects include slowed or even stopped breathing, low blood pressure, sedation. Expect excellent pain relief with Fentanyl when used properly