Fentanyl
Deaths have resulted from improper medical use. Fentanyl has a relatively wide therapeutic index (270) which makes it a very safe surgical anesthetic when monitored carefully. Because it is so potent, finding the right dose to use requires great care: Fentanyl comes as a highly diluted solution. There is also a powder, but its use is impractical without advanced scientific equipment: The effective dose needed and a lethal dose of fentanyl powder placed next to each other would be difficult or impossible to differentiate with the naked eye.