I'm on loestrin fe. My dr prescribed phentermine to me for some weight loss. I am experiencing break through bleeding. Is my birth control ineffective?

Loestrin Fe works. The Loestrin Fe is still working in spite of the breakthrough bleeding as long as you are not taking the phentermine at the same time as the contraceptive. The healthiest way to lose weight is with diet modifications and exercise and not phentermine, because the weight loss on "speed" (phentermine) will return once you stop taking that pill.