My DD had taken prednisone once before for an allergic reaction. She didn't have any side effects. People take it long term for things like autoimmune disorders and I think side effects come into play more with long term use. My grandmother was on it for lupus and I know she gained a fair bit of weight an had some spaciness and mood issues, but nothing that made it a poor choice for managing the lupus.
It is a corticosteroid, not to be confused with anabolic steroids! She'll still be able to play professional baseball and ride in the Tour de France.