Azathioprine is an immunosuppressive medication with two primary uses: preventing rejection after a kidney transplant and treating rheumatoid arthritis. It works by dialing down the immune system’s activity, which makes it useful for a surprisingly wide range of inflammatory and autoimmune conditions beyond its official approvals. Sold under the brand name Imuran, it has been in clinical use since the 1960s.
Approved Uses
Azathioprine has two conditions it is formally approved to treat. The first is kidney transplant rejection. After receiving a donor kidney, your immune system recognizes the new organ as foreign and attacks it. Azathioprine suppresses that immune response, helping your body accept the transplant. It is typically started on the day of surgery and continued long term at a lower maintenance dose.
The second approved use is active rheumatoid arthritis, specifically for people whose joint pain and swelling haven’t responded well to other treatments. It can take up to 12 weeks before you feel the full benefit for arthritis symptoms, so it requires patience. If your symptoms haven’t improved after that initial period, your prescriber may gradually increase the dose at four-week intervals.
Common Off-Label Uses
In practice, azathioprine is prescribed for many conditions beyond transplant rejection and rheumatoid arthritis. Because it broadly suppresses immune activity, doctors use it to manage inflammatory bowel diseases like Crohn’s disease and ulcerative colitis, autoimmune hepatitis, lupus, and various skin conditions including severe eczema and pemphigus. It is also used in some neurological conditions like myasthenia gravis and neuromyelitis optica. These uses are well established in clinical practice even though they fall outside the drug’s original approvals.
How It Works in the Body
Azathioprine is what pharmacologists call a prodrug, meaning it doesn’t do much on its own. Once you take it, your body converts it into an active compound called 6-mercaptopurine (a substance originally developed in the 1950s to treat leukemia). That compound is then further broken down into molecules that mimic the building blocks of DNA.
These fake building blocks get incorporated into the DNA of rapidly dividing immune cells, particularly T-cells and B-cells, which are the main drivers of immune attacks. When these cells try to copy their DNA to multiply, the process stalls. The result is fewer immune cells available to mount an inflammatory response, whether that response is directed at a transplanted organ, your own joint tissue, or your intestinal lining.
What to Expect With Blood Monitoring
Because azathioprine suppresses the bone marrow’s ability to produce blood cells, regular blood tests are a non-negotiable part of treatment. Your medical team will check your complete blood count and liver function at frequent intervals, especially early on. The typical schedule for rheumatology patients is blood tests every two weeks until you’ve been on a stable dose for about six weeks, then monthly. Once everything looks stable, testing shifts to every three months for the duration of treatment.
The monitoring schedule can vary depending on why you’re taking the drug. Gastroenterology patients, for instance, are often tested at two weeks, then at four, eight, and twelve weeks before moving to quarterly checks. The goal is always the same: catching any drop in blood cell counts or rise in liver enzymes before it becomes a serious problem.
Genetic Testing Before Starting
Before you begin azathioprine, your doctor will likely order a genetic test for two enzymes called TPMT and NUDT15. These enzymes are responsible for breaking down the drug’s active components. About 1 in 300 people have very low activity of one of these enzymes, and for them, standard doses of azathioprine can build up to dangerously high levels, causing severe drops in white blood cells and leaving them vulnerable to infections.
The genetic test takes a simple blood draw and helps your prescriber choose the right starting dose or, in rare cases, recommend a different medication altogether. Clinical guidelines from the Clinical Pharmacogenetics Implementation Consortium specifically flag azathioprine as a drug requiring TPMT and NUDT15 genotype-guided dosing.
Common Side Effects
Nausea is the side effect people notice most often, and it sometimes improves by splitting the daily dose into two smaller doses or taking the medication with food. Other relatively common effects include vomiting, diarrhea, and general fatigue. Some people develop mouth sores or notice they pick up minor infections more easily, which reflects the drug’s intended suppression of immune activity.
The more serious risk is bone marrow suppression, where the drug reduces production of white blood cells, red blood cells, or platelets beyond the intended level. This is why the regular blood monitoring matters so much. Symptoms to watch for include unusual bruising, persistent sore throat, or fevers that won’t resolve. Long-term use also carries a small increased risk of certain cancers, particularly lymphoma and skin cancers, so consistent sun protection is important while on the drug.
A Critical Drug Interaction
One interaction stands out for its severity. Azathioprine should not be combined with gout medications that block the enzyme xanthine oxidase, most commonly allopurinol and febuxostat. The reason is straightforward: xanthine oxidase is one of the pathways your body uses to break down azathioprine’s active components. Block that pathway with a gout drug, and those active components accumulate rapidly, potentially causing life-threatening drops in blood cell counts.
If the combination truly cannot be avoided, the azathioprine dose needs to be reduced substantially and blood counts monitored very closely. But in general, this pairing is one prescribers actively work to prevent.
Use During Pregnancy and Breastfeeding
Unlike many immunosuppressive drugs, azathioprine is considered compatible with pregnancy. The European Alliance of Associations for Rheumatology (EULAR) classifies it as safe enough to continue during pregnancy when disease control requires it, based on moderate-quality evidence. It is also classified as compatible with breastfeeding. This makes azathioprine one of the few immunosuppressive options available to people who are pregnant or planning to become pregnant and need ongoing treatment for an autoimmune condition.