I started Imuran four weeks ago. I started on 50mg daily once my blood results came back. Your liver enzymes have to be normal before starting this medication, which is side effect doctors really worry about. I just got it increased this past week to 75mg. I was a little scared too about the side effects of this medication, which my rheumy did explain to me.
I don't think your doctor would have put you on this medication, if your immune system was not up to handling it, since it does make you prone to infections.
You will have your blood tested every 4-5 weeks to make sure everything is okay.
I know this may not ease your fears, but at least give you a chance to decide to see if will help you.