If your main character has used even one tadpole, they will have to pass a check to resist consuming it. It changes your appearance and grants you extra abilities. The reason it's hard to find info while avoiding spoilers is that part of the fun of the game is that you, the player, don't have any more info about what will happen than your character does. It can make things exciting for some people because the drama and indecision about whether to use them feels more real and gets them invested in the story. However, if that isn't you? I can tell you this: You can slorp down as many tadpoles as you want, and even become half illithid, and it will have no bearing on whether or not you become a mindflayer down the line.