Remember when the term "designer drug" was used in the 1980s? One of the drugs included in that group was called 3-methylfentanyl, aka, mefentanyl. Although not widely used, it killed groups of people who tried it. Fast forward 4 decades and it's now one of the 30 fentanyl analogs that are responsible for the fentanyl crisis. And it's also one of the worst. What a difference a methyl group can make.
designer drugs
In 1976, Barry Kidston, a chemistry grad student, would find out the hard way that you had better be careful with your reaction conditions when making psychoactive drugs. He got a little sloppy, and instead of making a pure derivative of Demerol, got an impurity in the batch, which gave him Parkinson's with one injection. Six years later, a group of six "frozen addicts" suffered the same fate. Crazy brain chemistry.