They say being ON bc makes you gain weight. I just got mirana a few months ago, and Im still losing weight. BC has estrogen, while the mirana just gives off a low dose of progestren (sp?). From what I've heard it is the high level of estrogen that makes you gain weight, so I would assume switching to mirana would actually be a good thing. EVERYone's body is different, and I am no doctor, just someone who switched to mirana :}
p.s. Getting the mirana in hurts..bad.

but you'll live.