I knew a former member that was bipolar - II, and had also been on Zyprexa, for awhile, I can't remember how long he said, but did gain a lot of weight adding to his depression.
I do not remember if he was prescribied an alternative, but remember his pdoc took him off the Zyprexa.
Next visit you have, talk to your pdoc about your concern, it's good to let them know how you feel with and about your meds. he or she may be able to switch you to an alternative.
Take care,
DE