The opioid fentanyl is not only used in medical practice as a painkiller, but is also a popular recreational drug. In the United States in particular, fentanyl plays an important role in the opioid crisis that claims many victims every day. UvA chemists have now shown, in close cooperation with the Netherlands Organisation for Applied Scientific Research (TNO), that it should be possible to determine the origin of the drug by analyzing blood samples of users.