Only your doctor can tell you why he prescribed the Zyprexa, but here is a link to some information about it. Reading this may help you understand how it works. We aren't medical professionals here, so can't give you a definitive answer, but your doctor must have felt, from your symptoms and what you've told him, that this medication would help you. You mention that you have bad mood swings, so he might be thinking that you could possibly be bipolar.
Not all medications cause you to gain weight and even the ones that can may not bother you as an individual. Everyone is different. If you have concerns about that, please talk to your doctor.
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