espocomp:
Take your daughter to a "holistic" or "naturopathic" doctor. I started seeing one a few weeks ago, and it already has made a world of difference. A naturopathic doctor teaches us how to be healthy from the inside out and does this naturally. My holistic doctor has started me on a supplement regimen, and has given me some wonderful advice on how to eat so that I UNDERSTAND what my body is doing with all the stuff it's ingesting and what is happening to make my body put on some extra pounds. She provides "coaching" for me to help me stay on track.
Let this be your daughter's thing. Let her see a holistic doctor privately, and then let your daughter tell you all the cool things she's learning...and then don't nag her about it. Don't push her, because that just causes stress and pressure. It's hard, but something I've always heard about kids who start down this path of eating disorders (and taking diet pills at 16 when they can't stop binging on candy and junk is what I would call an eating disorder) is that the eating disorder allows them some control over their life. Controlling their weight with any means necessary because that which they "can control" in their eyes, even though we as outsiders know that they aren't controlling anything if they're managing their weight in unhealthy ways. With that said, maybe you need to look at how you interract with your daughter. Do you nag her or "remind" her that she "shouldn't" eat this or that she "should" eat that or that she "just needs" to be more active? She's at an age where she's trying to figure out who she is. As such, she's going to start exerting herself to do what "she" wants...and even though she knows that exercising is the right thing to do, she's not going to because you've told her she needs to. Does that make sense. It wasn't her decision, so she's not going to do it. She's going to not exercise because you've told her she needs to. I was the same way when I was younger. You need to find a way to get her to think SHE'S made the decision to exercise and eat better, not you...and don't make a big deal out of it when she does. Let HER make the big deal out of it, and you just need to say, "That's nice dear. I'm proud of you." and then drop it.
She wants easy, and easy is not the answer. You know that, but she doesn't. She may have to find that out the hard way, but getting her involved with a holistic doctor might do wonders, especially if you tell the doctor up front and in private what your concerns are and that you want the doctor to work with your daughter privately...that you want this to be your daughter's thing.
Here's how you can approach your daughter about going to the doctor: "I've been thinking about how you said you want to try diet pills, and I'd rather you talk to someone with better knowledge than I have. So, I've made an appointment with this lady named XXXX. She's what they call a naturopathic doctor, and she works with people to help them lose weight naturally. I'd prefer if you talked to her first about this before trying diet pills, because you know I have concerns about that. I think diet pills are a 'quick-fix' and won't correct the problem in the long run. I'd hate to see you get into an even bigger weight problem by using diet pills, so let's talk to this lady (or man) and see what she says and maybe she can find a better way to help."
Sorry so long, but this is a serious issue. Diet pills are only the beginning of a bad habit to lose weight, and they can destroy the body.