Rheumatology
Azathioprine
A thiopurine immunosuppressant — A long-term immune-suppressing drug for autoimmune disease and transplantation.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Azathioprine dampens an overactive immune system. It is used as a "steroid-sparing" agent in autoimmune and inflammatory conditions (such as inflammatory bowel disease, autoimmune hepatitis and some rheumatological diseases) and to prevent rejection after organ transplantation.
How it works
It is converted in the body into active products that interfere with the DNA building blocks dividing immune cells need, reducing the proliferation of the lymphocytes that drive autoimmune attack and transplant rejection. The enzyme TPMT is central to how each person handles the drug, which is why it is measured first.
In practice
In practice the key safety steps with azathioprine are done before and throughout treatment. Before starting, the enzyme TPMT is checked: people with low or absent TPMT activity break the drug down poorly and are at high risk of severe bone-marrow suppression, so the dose is reduced or the drug avoided. During treatment, regular full blood counts and liver tests are essential because marrow suppression and hepatotoxicity can develop at any time. A genuinely dangerous interaction is with allopurinol (used for gout): it blocks azathioprine's breakdown and can cause life-threatening toxicity, so the combination is avoided or the azathioprine dose drastically reduced under specialist guidance. Patients are counselled to report signs of infection, unusual bruising or bleeding, and there is a recognised small increase in some cancers (including skin), so sun protection and skin awareness are advised.
Examples
Practical use
How to take it & use it well
- Take azathioprine regularly as prescribed, usually with or after food to reduce stomach upset.
- Before starting, your clinician usually checks an enzyme called TPMT, which shows how your body handles the medicine and helps set a safe dose.
- Attend all your blood-test appointments, as regular monitoring of your blood count and liver is essential to catch problems early.
- Report any signs of infection, unusual bruising or bleeding, sore throat or fever promptly, as the medicine can lower your blood cells.
- Protect your skin from the sun with sunscreen and clothing, and attend skin checks, as long-term use raises skin-cancer risk.
Common uses
- Autoimmune and inflammatory disease (steroid-sparing)
- Inflammatory bowel disease
- Prevention of transplant rejection
Monitoring
- TPMT before starting; regular full blood count and liver function
- Signs of infection, bruising or bleeding
- Skin surveillance and sun protection
Weighing it up
Advantages & disadvantages
Advantages
- Calms an overactive immune system, helping control conditions such as inflammatory bowel disease and some rheumatic and skin diseases.
- Can reduce the need for steroids, lowering their long-term side effects.
- Helps prevent rejection in people who have had an organ transplant.
- Long experience of use means its monitoring and risks are well established.
Disadvantages
- Can lower the blood cells that fight infection and help clotting, so regular monitoring is required.
- Increases the risk of infections and, over the long term, of some cancers including skin cancer.
- May cause nausea, and occasionally affects the liver or, rarely, the pancreas.
- Has a very dangerous interaction with allopurinol that can cause severe, life-threatening blood problems.
Key safety principles
What to watch for
- Bone-marrow suppression — check TPMT before starting and monitor full blood counts throughout.
- Dangerous interaction with allopurinol — avoid or greatly reduce the dose under specialist care.
- Liver toxicity and a small increased risk of infections and certain cancers (including skin) — advise sun protection.
Key interactions
What to avoid or check alongside
- Allopurinol (and febuxostat) for gout: dangerously raise azathioprine levels and can cause severe bone-marrow suppression, so the combination needs major dose reduction or avoidance.
- Other medicines that suppress the bone marrow (such as some cancer drugs and co-trimoxazole): add to the risk of low blood counts.
- Warfarin: azathioprine may reduce its blood-thinning effect, so monitoring is needed.
- Live vaccines: generally avoided because the suppressed immune system may not handle them safely.
- Certain blood-pressure medicines (ACE inhibitors): can add to the risk of low blood counts.
Patient & carer advice
- Attend your blood tests reliably — they keep you safe
- Report a sore throat, fever, unusual bruising or bleeding
- Protect your skin from the sun and tell us if you take allopurinol for gout
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Azathioprine: frequently asked questions
Why is allopurinol so dangerous with azathioprine?
Allopurinol blocks an enzyme that breaks azathioprine down, causing its level to rise sharply. This can severely lower your blood counts and be life-threatening. The combination is avoided or used only with a much reduced, carefully monitored dose.
What is the TPMT test before starting?
TPMT is an enzyme that breaks down azathioprine. Testing it before treatment shows whether you process the medicine normally; very low activity means a much higher risk of harmful blood effects, so the dose is adjusted or another drug chosen.
Why do I need regular blood tests?
Azathioprine can lower your white cells, red cells and platelets and occasionally affect the liver. Regular blood tests catch these changes early so the dose can be adjusted before you come to harm.
Why should I protect my skin from the sun?
Long-term immune-suppressing treatment increases the risk of skin cancer. Using sunscreen, covering up, avoiding sunbeds and attending skin checks helps reduce that risk and catch any problems early.
Can I have vaccines while taking azathioprine?
Live vaccines are usually avoided because your immune system is suppressed. Non-live vaccines, including recommended seasonal ones, are generally encouraged. Check with your clinician before any vaccination.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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