"A recent enforcement operation by the Drug Enforcement Administration's (DEA) Indianapolis District Office (IDO) resulted in the seizure of 970 purported prescription pills that were various shades of blue and appeared to be oxycodone. Laboratory analysis subsequently revealed that the pills were counterfeit; the dark blue pills contained carfentanil and the light blue pills contained fentanyl. Fentanyl is a Schedule II opioid analgesic approximately 50 to 100 times more potent than morphine. Carfentanil is a Schedule II substance, similar in structure to fentanyl, with an analgesic potency 10,000 times that of morphine and is used in veterinary practice to immobilize certain large animals, including elephants. Both can be fatal in very small amounts."
DEA-CHI-BUL-167-17; Drug Enforcement Administration Chicago Division Bulletin 167-17
Drug Enforcement Administration: https://www.dea.gov/