In 2006, the U.S. Food and Drug Administration (FDA) began investigating several respiratory deaths, but doctors in the United Kingdom were not warned of the risks with fentanyl until September 2008.
Pada 2006, U.S. Food and Drug Administration (FDA) mulai menginvestigasi banyak kematian respirasi tetapi dokter di Inggris tidak diperingatkan risiko fentanil hingga September 2008.