I've been on weight loss medicatons several times in my life (I was always at least 75 lbs overweight when I was prescribed them), and the side effects can be pretty gruesome, but the biggest problem with them is that they eventually (and rather quickly) stop working.
When the pills stop working, or when you stop taking them, the hunger is back worse than before (that's been my experience, anyway, and that of a lot of people I've talked to in weight loss support groups and online forums).
Some doctors won't prescribe them to anyone. Other doctors have specific guidelines (not always just how much you have to lose, but also whether you have specific health issues).
Personally, I'd advise against them in most cases (I wish I hadn't taken them).