I also am on 150mg and have no problems. I did start on 50mg and my Doctor gradually increased it by 50 mg each time so that may have helped. It is possible you could not tolerate it. I hope you get a solution like the others suggested 6-mp or maybe you can try 50mg of Imuran at first. I gradually got used to mine I was nauseous at the beginning.
Diagnosed w/ Crohn’s Disease March 2007 On 150mg Azathioprine (generic Imuran), Pentasa, & Entocort (take zofran for nausea now)
Diagnosed w/ Fibromyalgia May 2007 also on Soma
Also have Arthritis, and feel like I am falling apart sometimes...