Some meds are weight neutral - my doc said that Lamictal is weight neutral and I found that to be true.
However, I gained 15 lbs in 6 weeks on Zyprexa and Depakote and from what I've read, I'm lucky it's not more. I find that I am always hungry. I have also read that these meds actually change your metabolism to cause weight gain but I would say my gain is a result of being hungry every waking minute.
It's particularly discouraging because early this year I worked very hard and lost 20 lbs over 8 weeks then I regained 15 of those pounds.
Good luck to you.