I've been on it for 4 months, and on the full 20mg for about 1 week. It used to make me hungry, but during an anxiety led breakdown, I never ate for 4 days. This seemed to stop that certain side affect. I am not recommending that anyone else tries that though, as it's neglect.
In my experience even reducing calories doesn't work.Weight gain whilst on olanzapine is a certainty.Especially in the beginning.Olanzapine has been linked with diabetes.There's some videos about it on google video.
I gained two stone, never felt full, never stopped eating. Had to come off it in the end because the weight gain was unbearable. Within 5 days of stopping med, appetite was back to normal and, as I am waiting to start Lithium and am only taking Prozac, the difference in my appetite now compared with being on the anti pych drugs is unbelievable, but in a good way. Am due to start Lithium on Friday if my bloods are ok. Good luck, hopefully the side effect of appetite increase will wain after some time but I didnt want to wait and find out!!
Just wanted to add that I too am experiencing an appetite that cannot be satisfied whilst on Olanzapine. It's a nightmare especially when the food that I am eating are all the wrong foods!!! It wasn't nearly as bad when I was on the Quetiapine