A thiopurine immunosuppressant
Azathioprine
A long-term medicine that calms an overactive or transplant-rejecting immune system — used in inflammatory bowel disease, rheumatoid arthritis and after transplants, with regular blood monitoring.
What is Azathioprine?
Azathioprine dampens the immune system. It is used to control conditions where the immune system is overactive — such as inflammatory bowel disease (Crohn’s and ulcerative colitis), rheumatoid arthritis and other autoimmune diseases — and to prevent rejection after an organ transplant. It works slowly over weeks to months, needs regular blood tests to watch the blood count and liver, and must never be combined with allopurinol without major dose changes.
Education and reference only. This is a plain-language guide to Azathioprine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Azathioprine is a long-term "immunosuppressant" and disease-modifying medicine. By calming an overactive immune system, it controls a range of autoimmune and inflammatory conditions — inflammatory bowel disease, rheumatoid arthritis, some skin, liver and neurological conditions — and it is widely used after organ transplantation to stop the body rejecting the new organ. It is a steroid-sparing treatment, meaning it can allow steroid doses to be reduced. It is taken as a daily tablet, started and supervised by a specialist, and works gradually rather than quickly.
How it works
Azathioprine is converted in the body into active substances (thiopurine nucleotides) that interfere with the production of DNA in rapidly dividing cells — particularly the white blood cells that drive immune and inflammatory responses. Damping these cells reduces the immune attack on the body’s own tissues or on a transplanted organ. Because it works on cell turnover, its full effect builds up over weeks to months, and the same action explains its main risk: lowering the blood count.
What it treats
Conditions Azathioprine is used for
Practical use
How to take Azathioprine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it once or twice daily as prescribed, with or after food to reduce nausea; it works slowly, so keep taking it even before you feel the benefit.
- Attend all monitoring blood tests — these are essential and non-negotiable for safe use.
- Never take allopurinol (or febuxostat) alongside it without your specialist adjusting the dose — this combination can be dangerous.
- Report fever, sore throat, mouth ulcers, or unusual bruising or bleeding promptly — these can signal a low blood count.
- Protect your skin from the sun, keep vaccinations up to date, and avoid live vaccines while taking it.
Weighing it up
Advantages & disadvantages of Azathioprine
Advantages
- Effective long-term control of many autoimmune and inflammatory conditions, and prevention of transplant rejection.
- Allows steroid doses to be reduced ("steroid-sparing").
- Well established, once- or twice-daily, and low cost.
Disadvantages
- Lowers the blood count and can affect the liver — needs regular blood monitoring.
- Slow to work (weeks to months), and raises infection and some cancer risks with long use.
- A dangerous interaction with allopurinol, and nausea can limit tolerance.
Practical use
Good to know
Two things dominate safe use of azathioprine. First, it lowers the blood count, so regular blood tests are essential — frequently at first, then at intervals — to catch any drop in white cells, red cells or platelets early; report any fever, sore throat, unusual bruising or bleeding promptly. Before starting, many people have a blood test (TPMT, and sometimes NUDT15) that shows how they break the drug down, which guides the starting amount. Second, azathioprine must not be taken with allopurinol (a gout medicine) at normal doses — the combination can cause dangerous, sometimes fatal, bone-marrow suppression. It also slightly raises the long-term risk of certain infections and skin cancers, so sun protection and up-to-date vaccinations (avoiding live vaccines) matter.
Who should not take it / use with caution
- People taking allopurinol or febuxostat, unless the dose is specifically and drastically adjusted under specialist care.
- People with very low blood counts, active serious infection, or (rarely) an inherited very low TPMT enzyme level.
- Pregnancy and breastfeeding are considered individually by a specialist — it is sometimes continued when the benefit is clear, but always under guidance.
Monitoring
- Full blood count (often weekly at first, then regularly)
- Liver function tests
- TPMT (and sometimes NUDT15) enzyme level before starting; skin checks with long use
Side effects
- Common early on: nausea, being sick and feeling generally off — often eased by taking it with food or splitting the dose.
- Important: a fall in the blood count (picked up on monitoring) and, sometimes, effects on the liver or, rarely, the pancreas — report severe stomach pain.
- Increased risk of infections, and a small increased long-term risk of certain skin cancers and lymphoma — hence sun protection and skin awareness.
Key interactions
- Allopurinol and febuxostat (gout medicines) dangerously increase azathioprine levels — the combination is avoided or the dose is cut to about a quarter with close monitoring.
- Other medicines that suppress the bone marrow, and some that affect the immune system, add to the risks.
- Live vaccines are avoided; the response to some other vaccines may be reduced.
Available as: Tablets; a liquid and an injection are available in some settings.
Answers
Azathioprine: frequently asked questions
Why do I need so many blood tests on azathioprine?
Azathioprine can lower your blood count and affect your liver, usually without symptoms at first, so regular blood tests catch any problem early while it is easily managed. Testing is frequent when you start or change the dose, then settles to longer intervals once you are stable.
Why can’t I take allopurinol with azathioprine?
Allopurinol blocks an enzyme that breaks azathioprine down, so azathioprine levels can rise dramatically and cause dangerous, sometimes fatal, suppression of the bone marrow. If you genuinely need both, a specialist must reduce the azathioprine dose substantially and monitor you closely — never combine them at normal doses.
How long until azathioprine works?
It works slowly, often taking several weeks to a few months to reach its full effect, which is why it is sometimes started alongside a faster-acting treatment such as a steroid. Keep taking it consistently even before you notice a benefit.
What should I report straight away?
Contact your team promptly if you develop a fever, sore throat, mouth ulcers, or unusual bruising or bleeding — these can signal a low blood count — or severe stomach pain, which can rarely signal the pancreas. Also mention any new or changing skin lesions.
The wider class
About Thiopurines (immunosuppressants)
Azathioprine belongs to the thiopurines (immunosuppressants) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
Browse by body system
Authoritative sources
- BNF: Azathioprine.
- electronic Medicines Compendium (SmPC): Azathioprine (Imuran).
- NICE: Crohn’s disease; Ulcerative colitis; Rheumatoid arthritis.
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