"Fentanyl is a synthetic opioid analgesic acting predominately at the μ-opiate receptor. It has historically been used as a pain reliever and an anaesthetic in both human and veterinary medicine and in terms of analgesic activity it is at least 80 times more potent than morphine. Fentanyl was first synthesized by Paul Janssen in 1960 and marketed as a medicinal product for treating pain. Subsequently, many fentanyl analogues were developed including sufentanil, alfentanil, remifentanil, and carfentanil. Fentanyl was first introduced for widespread palliative use in the mid-1990s in the form of transdermal patches, and to this day, it continues to be an important and much prescribed pain management medication in many countries. Concern surrounding the fentanyls is linked to their potential for dependence and misuse, their high potency and associated risk of fatal overdose."
Jane Mounteney, Isabelle Giraudon, Gleb Denissov, and Paul Griffiths, "Fentanyls: Are we missing the signs? Highly potent and on the rise in Europe," International Journal of Drug Policy, Volume 26, Issue 7, 626 - 631. doi: 10.1016/j.drugpo.2015.04.003. Epub 2015 Apr 17.