I have PCOS and was put on metformin. From what I believe, it works by regulating your blood sugar levels, as some diabetics use it too. When I first got put on it, I lost a bit of weight but then it just seemed to do nothing.
I believe that they shouldn't be taken with alcohol, which is why my consultant wanted to review things with me when I was at Uni (hehe!) but it wasn't until the last year or so i.e. finished Uni so not as much booze (!) that it's not really helped. I don't feel it's helped, so since about January I've been REALLY strict with diet and exercise and lost 2 and a half stone, but the last month or so have grinded to a halt.
Hmmm I guess I don't know why I'm telling you this, apart from to say that you're not the only one in this situation and that I know it's SO much harder to lose weight compared to 'normal' people. Also, trust me when I say your weight its not big, honestly! Nowhere near!