Dr. Michael Lowenstein of the Waismann Method has a piece published on The Hill on fentanyl. Here is an excerpt:
Often times, the illicit drug heroin is thought of as one of the more powerful opioids on the planet. It might be surprising to hear that a relatively common prescription medication, fentanyl, is even more powerful. Furthermore, when the two are combined, it can become a fatal combination.
Heroin has been around since the late 1800s. It is a powerful semi-synthetic opiate derived from the opium poppy and is used illicitly as a recreational drug. Heroin delivers an intense rush or euphoria and is more powerful than most opioid analgesics. Although historically heroin was marketed as a cough suppressant, it has since become a Schedule I (illegal) substance in the United States.
On the other hand, fentanyl is a synthetic opioid compound created in the early 1960s. It is one of the few opioids that is even more powerful than heroin, yet available with prescription.