This information is for people who have Crohn's disease. It tells you about azathioprine and mercaptopurine, treatments used for a flare-up of Crohn's disease. It is based on the best and most up-to-date research.
Do they work?
Yes. Taking azathioprine or mercaptopurine tablets can help get rid of symptoms of Crohn's disease. However, you may need to take them for several months for them to work, and the benefits have to be weighed against some serious side effects.
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.
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?
If you're having a flare-up of Crohn's disease, taking azathioprine or mercaptopurine can help to get rid of your symptoms. 
In studies, 54 in 100 people got partly or totally better when they took azathioprine or mercaptopurine.
This compared with 33 in 100 people who improved after taking a dummy treatment (a placebo).
In studies where people were also being treated with steroids, taking azathioprine or mercaptopurine meant that people were able to take a lower dose of their steroid. 
Azathioprine and mercaptopurine worked best when people took them for 17 weeks or longer. 
How do they work?
Azathioprine and mercaptopurine affect your immune system, stopping it acting as strongly as it usually does. Drugs that do this are called immunosuppressants.
Your immune system plays an important part in causing inflammation in your body. So, treatments that stop your immune system working so hard can reduce the inflammation in your bowel and help get rid of Crohn's disease symptoms.
However, if these drugs suppress your immune system too much, it can mean your body isn't able to fight off infections very well.
Can they be harmful?
Azathioprine and mercaptopurine have side effects. In one study, 3 in 10 people got at least one side effect.  In another study, 1 in 10 people got a side effect bad enough for them to stop taking their treatment. 
Azathioprine and mercaptopurine can stop your body making as many white blood cells.  White blood cells are part of your immune system, and help to fight off bacteria and viruses. So, this side effect puts you at a higher risk of getting infections. If you take azathioprine or mercaptopurine for a long time, you'll need regular blood tests to make sure you have enough white blood cells. And if you notice an infection, make sure you see a doctor as soon as you can.