There was a famous case in the UK a few years ago with a double blind trial that had an extremely serious reaction by the volunteers. Participants and staff went through the panic of not knowing who was going to react next, looking at each other, waiting for the next reaction.

Eight men took part, the drug had looked positive in animal testing but reacted very badly in humans. Here’s one article, you can use it to find more, possibly the documentary is online.

And there is an ethical dilemma, giving potentially healthy people something that might harm them. Risk versus reward, the greater good.