I'm on 150mg Azathioprine a day now, gradually increasing it as they were afraid that my liver wouldn't hack it.
As far as side effects after reading the info sheet the only ones other than tiredness that I suffer is restless leg syndrome, but as far as the fatigue goes, I've just kinda gotten used to it, I remember it was alot worse earlier on in the treatment, don't know if I've gotten used to it or it gets better
Sorry I don't have any better news
EDIT
(But wait, I do!)
I forgot to mention that once I got put up to 150mg the majority of my symptoms cleared up, I didn't notice the Pentasa/Mesazaline's effects, it was only until I got put up to 100 that I started to feel a bit better, then 150 I am a hundred times healthier than before.