Azathioprine weakens your body's immune system, to help keep it from "rejecting" a transplanted organ such as a kidney. Organ rejection happens when the immune system treats the new organ as an invader and attacks it.

Azathioprine is used to prevent your body from rejecting a transplanted kidney. Azathioprine is also used to treat symptoms of rheumatoid arthritis.

Azathioprine may also be used for purposes not listed in this medication guide.

Azathioprine may cause a rare type of lymphoma (cancer) of the liver, spleen, and bone marrow that can be fatal. This has occurred mainly in teenagers and young men with Crohn's disease or ulcerative colitis.

Azathioprine comes as a tablet to take by mouth. It is usually taken once or twice a day after meals. Take azathioprine at around the same time(s) every day. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take azathioprine exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.

If you are taking azathioprine to treat rheumatoid arthritis, your doctor may start you on a low dose and gradually increase your dose after 6-8 weeks and then not more than once every 4 weeks. Your doctor may gradually decrease your dose when your condition is controlled. If you are taking azathioprine to prevent kidney transplant rejection, your doctor may start you on a high dose and decrease your dose gradually as your body adjusts to the transplant.

Azathioprine controls rheumatoid arthritis but does not cure it. It may take up to 12 weeks before you feel the full benefit of azathioprine. Azathioprine prevents transplant rejection only as long as you are taking the medication. Continue to take azathioprine even if you feel well. Do not stop taking azathioprine without talking to your doctor.

Your doctor will perform blood tests to make sure you do not have conditions that would prevent you from safely using azathioprine.

Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose. Use the medicine exactly as directed.

When given for kidney transplant, azathioprine is usually given right before or on the day of transplant. For rheumatoid arthritis, azathioprine is taken on a daily basis.

It may take up to 8 weeks before your symptoms improve. Keep using azathioprine as directed and tell your doctor if your symptoms have not improved after 12 weeks of use.

Take with food if azathioprine upsets your stomach.

You may not be able to continue taking other arthritis medications together with azathioprine. Do not change your dose or dosing schedule without your doctor's advice.

Azathioprine affects your immune system. You may get infections more easily, even serious or fatal infections. Your doctor will need to examine you on a regular basis.

Store at room temperature away from moisture, heat, and light. Keep the bottle tightly closed when not in use.

You should not take azathioprine if you are allergic to azathioprine.

You should not use azathioprine to treat rheumatoid arthritis if you are pregnant. This medicine can harm an unborn baby. Use effective birth control to prevent pregnancy while using this medicine.

Azathioprine may cause a rare type of lymphoma (cancer) of the liver, spleen, and bone marrow that can be fatal. This has occurred mainly in teenagers and young men with Crohn's disease or ulcerative colitis. However, anyone with an inflammatory autoimmune disorder may have a higher risk of lymphoma. Talk with your doctor about your own risk.

While taking azathioprine, you may have a higher risk of developing skin cancer. Ask your doctor about skin symptoms to watch for.

Tell your doctor if you have ever had:

• kidney disease, or a kidney transplant (if you are using azathioprine for rheumatoid arthritis);

• any type of viral, bacterial, or fungal infection;

• liver disease; or

• chemotherapy with medications like cyclophosphamide, chlorambucil, melphalan, busulfan, and others.

You should not breast-feed while you are using azathioprine.

Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

Keep this medication in the container it came in, tightly closed, and out of reach of children. Store it at room temperature and away from excess heat and moisture (not in the bathroom).

Unneeded medications should be disposed of in special ways to ensure that pets, children, and other people cannot consume them. However, you should not flush this medication down the toilet. Instead, the best way to dispose of your medication is through a medicine take-back program. Talk to your pharmacist or contact your local garbage/recycling department to learn about take-back programs in your community. See the FDA's Safe Disposal of Medicines website (http://goo.gl/c4Rm4p) for more information if you do not have access to a take-back program.

It is important to keep all medication out of sight and reach of children as many containers (such as weekly pill minders and those for eye drops, creams, patches, and inhalers) are not child-resistant and young children can open them easily. To protect young children from poisoning, always lock safety caps and immediately place the medication in a safe location – one that is up and away and out of their sight and reach. http://www.upandaway.org

Unless your doctor tells you otherwise, continue your normal diet.

Avoid sunlight or tanning beds. Azathioprine can increase your risk of developing skin cancer. Wear protective clothing and use sunscreen (SPF 30 or higher) when you are outdoors.

Avoid being near people who are sick or have infections. Tell your doctor at once if you develop signs of infection.

Do not receive a "live" vaccine while using azathioprine. The vaccine may not work as well and may not fully protect you from disease. Live vaccines include measles, mumps, rubella (MMR), polio, rotavirus, typhoid, yellow fever, varicella (chickenpox), and zoster (shingles).

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Azathioprine may cause a serious brain infection that can lead to disability or death. Call your doctor right away if you have problems with speech, thought, vision, or muscle movement. These symptoms may start gradually and get worse quickly.

Stop using azathioprine and call your doctor right away if you have any of these symptoms of lymphoma:

• fever, swollen glands, body aches, night sweats, not feeling well;

• pale skin, rash, easy bruising or bleeding;

• cold hands and feet, feeling light-headed or short of breath;

• pain in your upper stomach that may spread to your shoulder; or

• feeling full after eating only a small amount, weight loss.

Also call your doctor at once if you have:

• signs of infection (fever, chills, weakness, flu symptoms, sore throat, cough, pain or burning when you urinate);

• severe nausea, vomiting, or diarrhea;

• easy bruising, unusual bleeding;

• rapid heartbeats, shortness of breath;

• pale skin, cold hands and feet; or

• dark urine, jaundice (yellowing of the skin or eyes).

Common side effects of azathioprine may include:

• nausea, diarrhea, stomach pain;

• hair loss; or

• skin rash.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Tell your doctor about all your current medicines. Many drugs can affect azathioprine, especially:

• allopurinol;

• febuxostat; or

• ribavirin.

This list is not complete and many other drugs may affect azathioprine. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.

Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.

It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.