Fentanyl is powerful synthetic opioid that can be up to 100 times more potent than morphine.

It can be prescribed in formal medical settings to relieve severe pain and treat breakthrough pain when longer-acting pain medications aren’t enough. Injections may be used during or after surgery, for example, while patches help treat persistent pain in patients that may have become tolerant to other treatments. It can also be administered through a lozenge.

But fentanyl has become the most common drug involved in overdose deaths in the United States, fueling what the US Centers for Disease Control and Prevention has identified as the “third wave” of the opioid overdose epidemic. Fentanyl is relatively cheap to make, and its high potency means that supply can stretch far, making it an attractive option in the illicit market. Illicit fentanyl is typically found in powder form or pressed into counterfeit pills that look like prescription opioids. It can also be mixed into other drugs such as heroin to raise the potency at lower cost.

Read more at CNN.com.