This information is for people who have Crohn's disease. It tells you about azathioprine and mercaptopurine, treatments used to prevent symptoms of Crohn's disease. It is based on the best and most up-to-date research.
Do they work?
Yes. Azathioprine and mercaptopurine help to prevent symptoms of Crohn's disease. If you take them regularly for several months or even a year, it's less likely that you'll have a flare-up.
Unfortunately, azathioprine and mercaptopurine can cause side effects. The side effects are not all that common, but if you do get them, they could be serious. So, you and your doctor will have to weigh the benefits of treatment against the risks.
What are they?
Azathioprine and mercaptopurine are drugs that suppress your immune system. Azathioprine is often used for people who've had an organ transplant. It stops their immune system rejecting the new organ. Mercaptopurine is used as a treatment for some kinds of cancer. Both of these drugs are also used to treat Crohn's disease.
The brand name for azathioprine is Imuran. The brand name for mercaptopurine is Puri-Nethol.
Doctors sometimes recommend these drugs as an alternative to steroids. So, you might take these drugs if you'd rather avoid steroids, perhaps because you've been getting side effects. Or you might be offered them if steroids haven't been working for you.
Your doctor will probably recommend that you take these medicines as tablets. You can also get azathioprine as an injection, but it can cause severe irritation at the spot where the needle goes in. 
How can they help?
Both azathioprine and mercaptopurine can help prevent symptoms of Crohn's disease. Most of the research has looked at azathioprine.
In several studies, taking azathioprine regularly for a long time (say, between six months and two years) helped stop people getting flare-ups of Crohn's disease.  At the start of the studies, no one was getting symptoms of Crohn's disease. As time went on:
A few people in the studies had also been taking steroid treatment. Taking azathioprine helped people to either stop needing steroids or switch to a lower dose.
We only found one study looking at mercaptopurine. It didn't seem to work as well as azathioprine, but that might be because people were taking a low dose. 
Researchers have also looked at people with Crohn's who take these drugs after an operation to remove an inflamed section of their small bowel. One review of studies found that: 
There hasn't been much long-term research on azathioprine or mercaptopurine.  So, it's difficult to say how long you should take these drugs for, or whether the side effects could outweigh the benefits if you take them for a long time. And we don't know whether your symptoms are likely to come back when you stop treatment.