If you are taking zyprexa, what mental condition are you using it for? Weed can cause depression as chainer3k said, but it can get you stuck in a negative thought loop which can help the depression along. If you have a disorder that is exacerbated by psychedelics, I would also be careful with cannabis.
If you are curious, you can try smoking a very tiny amount and see how you feel afterwards. Very small though, just a couple of hits...if you overdo it and get too high, you might feel uncomfortable.