It is well known that zyprexa causes weight gain, but if you aren't well it can be a life saver. Many who take it are also very ill and live very unhealthy lifestyles. Couple this with zyprexa and it leads to weight gain plus related medical issues.
I haven't had it myself but apparently it increases your appetite so you eat more, hence gain weight. If you keep eating no more than normal, you shouldn't gain - an interesting fact is that those suffering from anorexia nervosa don't gain weight when they take zyprexa;
Weight gain still comes down to calories consumed, and energy used up by your body - it is not that hard in concept, a bit harder in reality.