Fentanyl is "a synthetic opioid that is 80-100 times stronger than morphine. Pharmaceutical fentanyl was developed for pain management treatment of cancer patients, applied in a patch on the skin. Because of its powerful opioid properties, Fentanyl is also diverted for abuse. Fentanyl is added to heroin to increase its potency, or be disguised as highly potent heroin. Many users believe that they are purchasing heroin and actually don’t know that they are purchasing fentanyl – which often results in overdose deaths. Clandestinely-produced fentanyl is primarily manufactured in Mexico." It is currently in Schedule II of the Controlled Substances Act, meaning that while highly addictive, dangerous and restricted, it is recognized as medically useful for the treatment of pain in extreme cases.
Fentanyl is usually an opiate of last resort, used when a patient is highly tolerant to most other opiates including morphine. When used in accordance with a doctor's orders and properly prescribed, it can be safe for patients recovering from extreme pain, or in cases of terminal cancer.
In addition to being commonly added to heroin, fentanyl can be purchased directly from China through the Dark Web. 2 milligrams of pure fentanyl is fatal.