Hello there,
Azathioprine was good for me at the start but unfortunately I became unwell with side effects after a few months, I then tried Mercaptopurine, which really helped my symptoms but unfortunately made my white cell count far too low so I had to stop taking it because of infection risks. They think I was just unlucky and am in the 10% of people that are just over sensitive to these drugs.
Despite the issues that I've had with them I would still recommend trying them, they can really make a positive differences and although they do come with some risks, these risks are out weighed by the benefits. Leaving Crohns untreated can be really dangerous so its best to get things under control as soon as possible. They will keep a close eye on you while you are on these medications and should give you regular blood tests to monitor the way your body is responding to the medication. Remember that the list of side effects is just there as a maybe, its mostly there to protect the drug company if anything does happen, none of them are a guarantee and most are totally reversable when the medication is stopped.