Fentanyl, also known as fentanil, is an opioid which is used as a pain medication and together with other medications for anesthesia.^[2] It has a rapid onset and effects generally last less than an hour or two.^[2] Fentanyl is available in a number of forms including by injection, as a skin patch, and to be absorbed through the tissues inside the mouth.^[2]

Common side effects include nausea, constipation, sleepiness, and confusion.^[2] Serious side effects may include a decreased effort to breathe (respiratory depression), serotonin syndrome, low blood pressure, or addiction.^[2] Fentanyl works in part by activating μ-opioid receptors.^[2] It is about 75 times stronger than morphine for a given amount.^[4] Some fentanyl analogues may be as much as 10,000 times stronger than morphine.