I've been on Lithium before and I didn't gain weight on it personally, I was thirsty all the time more than hungry. But I know meds can lead to weight gain and troubles. My pdoc had told me to keep a lot of healthy snacks, like raw veggies, around the house. And to try to avoid having less healthy options around so you aren't as tempted. Going on an eating binge of carrots is better than a bag of cheetos.

She also said that often when meds change your mood, it can also change your activity level, which effects your weight. For example, if you were primarily depressed you may start moving around more than before. But if you were manic you might start sitting down more. It is cliche but she said to start exercising. (Of course only if you're healthy for exercising.) And she said a balanced diet and exercise will also help improve mood.

That's all I know on the subject. I know weight gain is frustrating and doesn't help anyone feel better!