I have to sum up a thesis statement on my research topic.

So far I have thought of researching on how steroids effect a athletes on knowing how good they are in a game compared to when they are injecting themselves with steroids to boost up their stamina.

My thesis would be
When professional athletes abuse steroids they are not only causing harm to their body, but also lowering their true potential in level playing field by being immodest to themselves.

But I need something more strongerrr

For a start, restate your thesis, it's not even good English.

Also, anabolic steroids generally don't increase stamina, they are most commonly used to improve explosive strength. You see very few marathon runners using testosterone.

Just as a matter of interest, what proof do you have that steroids harm the body?