Fentanyl Absorption, Distribution, Metabolism, and Excretion: Narrative Review and Clinical Significance Related to Illicitly Manufactured Fentanyl

ObjectivesThis narrative review summarizes literature on pharmaceutical fentanyl's absorption, distribution, metabolism, and excretion patterns to inform research on illicitly manufactured fentanyl (IMF).ResultsFentanyl is highly lipophilic, lending itself to rapid absorption by highly perfused tissues (including the brain) before redistributing from these tissues to muscle and fat. Fentanyl is eliminated primarily by metabolism and urinary excretion of metabolites (norfentanyl and other minor metabolites). Fentanyl has a long terminal elimination, with a documented secondary peaking phenomenon that can manifest as "fentanyl rebound." Clinical implications in overdose (respiratory depression, muscle rigidity, and "wooden chest syndrome") and opioid use disorder treatment (subjective effects, withdrawal, and buprenorphine-precipitated withdrawal) are discussed. The authors highlight research gaps derived from differences in medicinal fentanyl studies and IMF use patterns, including that medicinal fentanyl studies are largely conducted with persons who were opioid-naive, anesthetized, or had severe chronic pain and that IMF use is characterized by supratherapeutic doses and frequent and sustained administration patterns, as well as adulteration with other substances and/or fentanyl analogs.ConclusionsThis review reexamines information yielded from decades of medicinal fentanyl research and applies elements of the pharmacokinetic profile to persons with IMF exposure. In persons who use drugs, peripheral accumulation of fentanyl may be leading to prolonged exposure. More focused research on the pharmacology of fentanyl in persons using IMF is warranted.