1) Synthetic opioid 
Mirfentanil is a fentanyl derivative with strong selectivity for the μ opioid receptor. At lower doses, it antagonizes the analgesic effects of alfentanil and substitutes for naloxone in morphine-treated monkeys; however, it also reverses naloxone-precipitated withdrawal in pigeons trained to discriminate morphine from naloxone. At high doses, it...