BMJ Group Medical Reference
This information is for people who have rheumatoid arthritis. It tells you about azathioprine, a treatment used for rheumatoid arthritis. It is based on the best and most up-to-date research.
Does it work?
Yes. Azathioprine may help to reduce pain and swelling in your joints. But side effects from this drug are common and can be serious. Your doctor will probably only prescribe azathioprine if you have tried other drug treatments first.
What is it?
Azathioprine belongs to a group of drugs that are used to slow down rheumatoid arthritis. These drugs are called disease-modifying anti-rheumatic drugs (DMARDs for short). They also help to reduce the pain and swelling in your joints.
Azathioprine takes a few months to work, and your doctor may prescribe it together with other DMARDs. You may need to take a combination of drugs for many months. But once your symptoms improve, you may be able to stop taking some of them. If your arthritis is not getting better or you get severe side effects from azathioprine, your doctor may advise you to stop taking it.
The brand name for azathioprine is Imuran. You have it as a tablet, which you take once a day. You will start on a low dose, probably 25 milligrams or 50 milligrams. Your doctor will then increase the dose every one or two months until your arthritis starts to improve.
How can it help?
Taking azathioprine can ease the pain and swelling in your joints. This may make it easier for you to do everyday things like going shopping, doing the housework, having sex, seeing your friends and doing other activities you enjoy.
Research shows that taking azathioprine can help in the following ways. 
It can reduce swelling in your joints: You may have fewer swollen joints when taking azathioprine. You may also find that the swelling in your joints goes down. This should help your joints move more easily. You may find it easier to pick up small things or fasten your buttons.
It can reduce pain in your joints: If your joints are less painful, you may find it easier to do everyday things like walking or working. If you have rheumatoid arthritis in your hands, you may also find that gripping things, like cups, is easier.
But studies show that azathioprine does not work as well as methotrexate.   It seems to work as well as penicillamine and ciclosporin.   
How does it work?
Azathioprine reduces the number of white blood cells that your body makes. White blood cells fight off infection. But in rheumatoid arthritis, they also make substances that attack the lining of your joints. This makes your joints swell and become painful.