It doesn't weaken them. Do you know what steroids are? They are suppose to give you muscles, and make you have a lot of energy. People who take them have side effects: they become irritable and emotional, and if they are a boy... A certain body part that every boy wants long becomes really short, if you know what I mean. It's really bad to take, but it doesn't weaken you.
I'm not sure if it would weaken them over time, but even if it does, what matters to the athlete is to be strong for whatever sport or pass-time they have. After they are done, it wouldn't really matter if they're weak or not, because they would be done.