Yeah, could be possible that there is a tiny mishap. Thank you for letting me know :)

Probands are the people that are volunteering to try those new drugs after they passed the animal test.
Everyone is qualified as long they are in an overall good health constitution and match specific criteria depending on the drug that they're going to take.
Often the drugs are tested only on men to avoid any damage on the female ovum to lower the risk of permanent damage. The sperm is always "freshly" produced, while the female ova develop when the person is still an embryo in the mother's womb. This is just to make sure that the offspring of the probands don't suffer any damage from the study.
And if there is the tiniest sign of a deleterious side effect, the study will be immediately stopped and the active substance gets improved or dismissed.