Fentanyl was developed as a drug that could relieve severe pain and assist doctors in performing complex surgeries. Due to its potent effects, fentanyl is often sold illegally as an ingredient in counterfeit opioid pills or mixed with heroin or cocaine. Because it is nearly impossible to gauge the strength of illegal fentanyl doses, tens of thousands of North Americans have overdosed on the drug in recent years. This book provides information about the effects of fentanyl and how it contributes to the ongoing opioid crisis. Opioids are psychoactive drugs derived from the opium poppy, such as heroin or morphine, or synthetic versions that mimic their effects, such as fentanyl or oxycodone.