I used to take a really high dose of Lithium (1500 mg) and was size 6 on it, so it did not prevent me from losing weight. I now take much less (900 mg) and I am slowly losing weight (down to 177 lbs from 192 lbs) so again it does prevent weight loss.

But everyone is different - one can try it weighing herself every week in order to catch any gain and discontinue promptly.