Putting on some extra weight is a small cost to pay for good health - assuming the meds were actually helping you. I put on over 20kg(44lb) after I started Lithium despite exercising and eating well. I went off the Lithium to see is I could drop the weight but my severe mixed episodes returned with a vengeance and I became acutely suicidal. This is how I was before I was on Lithium so I quickly resumed the Lithium and my symptoms stabilised. This would be a good thing to discuss with your pdoc. Maybe there are other, more weight neutral options for you.