And that's also why clinical trials are staged. Some things are harmful to humans that aren't harmful to rats, mice, dogs, monkeys, or other test animals. So a few people enroll in stage 1 trials, which mostly just try to look for obvious toxicity. This will start with a few people, going to a few hundred at most. Stage 2 trials try to figure out dose response: how much of the treatment do you have to give to get a given response. This stage is usually a few thousand people. Stage 3 trials try to figure out if the treatment actually works, and if it has any noticeable safety concerns. Most of the time it doesn't work or has safety issues, but human trial data is notoriously noisy, so it takes a LOT of samples to get a recognizable signal. This stage usually has over ten thousand people. After a treatment is approved it keeps getting monitored for safety.

Something like "growing teeth" is more obvious than many outcomes. But the safety issues may not be.