Fentanyl Overdose Signs Fentanyl is a highly dangerous and, at times, deadly substance. Learn about the signs of a fentanyl overdose in this article. Article Contents What is Fentanyl? According to the National Institute on Drug Abuse, overdose deaths involving opioids more than doubled from 2010 to 2017. The deaths remain at an astronomical level of almost 50,000 deaths each year. Many of these deaths include the opioid drug, fentanyl. In California alone, fentanyl accounted for almost a third of all overdose deaths.1 The