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The Abilify is likely the major source of the problems, although Zoloft may contribute. The are no natural remedies that have been shown to help. I would talk to MD about quitting Abilify first. If ineffective, I would consider changing Zoloft to something with less side effects, such as Prozac. It is effective with OCD but can potentially increase anxiety for first couple weeks.
Immodium for diarrhea is fine.
Are you saying that the Abilify is the source of the GI problems and the weight gain? If so, would stopping the Abilify eventually get her back to her normal weight? I have done some research and found many people saying they gained weight while on Zoloft.