I gained 100 pounds on antipsychotic medications. I was never overweight before I started taking them. I'm happy to say that now I've lost 80 pounds of that! It has taken me two years but I'm maintaining it. I think that being off the antipsychotics has been a big contributor to my being able to do this. I also had to start a good exercise program, and change my diet quite a bit. I have low thyroid too, so it hasn't been fast. I'm doing the Weight Watchers program to help monitor my food intake. I lost about ten pounds before going off the meds, and then discontinued the meds. I lost 50 more pounds that year, and 20 last year. I'd still like to lose about ten more.
Just wanted to encourage you all, that if you've gained weight from the meds, you can lose it again.