I do not recommend diet pills. They don't even burn fat. They contain extremely high doses of caffeine, which flush water right through your system. You will be dehydrated, jittery, weak and you won't lose any actual fat, despite what manufacturers want you to believe.
Try adjusting your diet. If you are already exercising enough, that's the only other thing you can do.
If you have tried really hard to lose weight with no success, go see your doctor about getting your thyroid and blood glucose levels tested. You may have some type of disorder.
I have no idea about the athsma, but if you have any type of health issue it's even more of a reason to stay away from diet pills.