Then you'll be fine. I put on half a stone at max and Ive been on them for 2 years now. I actually think that weight gain was because I was in the last months of my degree and I was eating rubbish as I was chained to my pc at every waking moment!
I dont think the meds make you gain weight, I think they just make you want to eat more. So as long as you can keep that in check, you'll be fine