If they did, there's always going to be a debate over whether something was caused by the trial. You get a rash and have to go see a dermatologist, was it a result of the trial or unrelated? Or more seriously, you develop cancer. Was it from the trial, or from an unrelated issue? Other people might show symptoms, but it would still take time you don't have for them to sort out whether other participants have cancer, and whether that's a result of the trial or if they just had a higher than average number of people with risk factors.