I am on the generic form of Imuran, it's called Azathioprine. Imuran comes in different dosages, that's the only difference. Imuran can come in 100 mg dosages whereas Azathioprine can come in 50 mg dosages.
It took about 3-4 weeks for it to really kick in, my wbc went from 40k to normal in about a month.
No side effects, it has put me into a solid remission.