Fentanyl is a synthetic opioid drug that is similar to morphine but is 50 to 100 times more potent. It is used to treat severe pain, such as pain from cancer or surgery, and is typically administered via injection or transdermal patch. Fentanyl can also be used recreationally, and its use has been linked to a significant increase in opioid overdose deaths in recent years. Due to its high potency, fentanyl can be dangerous even in small doses, and its use should be closely monitored by a healthcare provider.