There was a recent post about using bacteria to kill cancer. They selected 50 strains of bacteria. With in vitro test they reduced thenumber to 9. Withmice test they reduced the number to 1. I think they are testing only that bacteria in other animals, because it may still be a bad idea (or hopefuly a brilant one).
This is _sometimes_ true, but we will also miss opportunities by not trusting a substance that would be fine for humans but not for mice.
> miss opportunities
It's totally possible, but if it already killed 50/50 rats, 15/15 dogs and 5/5 monkeys, and they offer me to be the fist human subject, I'd run away faster than Forest Gump.
I imagine there are exceptions. For example, we understand very well why ivermectine kills a lot of invertebrates but not (most) vertebrates and is a very good horse and human dewormer. There may be something that kills everything except humans, and we may understand why theoretically, but I'd not like to be the first to try it.
its not perfect, but the alternatives is forcing it on humans. are you going to voluenteer? you'll be the only one in that queue.