What Is The Half Life Of Fentanyl – When referring to an opioid’s elimination half-life, this means how long it takes for 50% of the original opioid dose to be metabolized and processed out of the body. Knowing the half-life, or relative half-life of one opioid compared to another can assist in understanding how long it will take for the substance to fully leave one’s body. When it comes to opioid addiction treatment, knowing with the type of opioid(s) that were consumed will help give the caregivers an idea of when withdrawal symptoms should set in and what the withdrawal timeline may look like.