I have been on azathioprine for three months now, and often feel sick.My nausea seems to mostly occur when i have the urge to go to the toilet for some reason. I was initially put on asacol which had little effect, and then with two courses of prednisolone, the steroids helped but when i came off them things went back to their normal horrid self. Im seeing my doctor soon so we will see whether i stay on azathioprine or not. as far as the long term effects are concerned most of the drugs including prednisolone have them, its a small risk compared to the discomfort you would have without. sickness it seems is common when on azathioprine