let me answer it a different way. exercise doesn't affect the birth control pill.
personnally diet pills hardly ever work, what they do do is cause your heart to race and can even cause death. People have been known to have heart attacks taking stuff like that. Even at a young age. There is something that a lot of them use now since ephedra has been banned. It is something about orange, but I don't remember. It actually has then same kind of problems as ephedra.
you really can only know the answer to that question by calling pharmacy or doc, and I believe they both are going to tell you to stop taking diet pill.
Diet pills are usually a waste of money, and you can get a lot faster results by joining the gym, and actually you are pretty much paying for a gym membership when you buy those pills.