Azathioprine — NZ Medication Guide
📋 Reviewed by a Registered Pharmacist NZ — This information is for general educational purposes only and does not replace professional medical advice. Always consult your doctor or pharmacist before starting, stopping, or changing any medication.
What is Azathioprine?
Azathioprine is a thiopurine immunosuppressant used for transplantation, autoimmune conditions, and inflammatory bowel disease. This medication is funded by Pharmac for IBD, transplantation, and autoimmune conditions under Special Authority.
What is Azathioprine Used For?
Azathioprine is used for prevention of organ transplant rejection, rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease (Crohn’s disease and ulcerative colitis), dermatomyositis, and other autoimmune conditions.
How Does Azathioprine Work?
Azathioprine is a prodrug converted to 6-mercaptopurine (6-MP) and then to active thioguanine nucleotides. These are incorporated into DNA, disrupting DNA synthesis in rapidly dividing cells — particularly lymphocytes. This inhibits T and B lymphocyte proliferation and the immune response.
How to Take Azathioprine
Usually taken once daily, with or without food (food may reduce GI upset). Doses are weight-based (1–3 mg/kg/day), adjusted based on tolerance and response. Full immunosuppressive effect takes 2–3 months. Full blood count and liver function monitoring every 1–3 months is mandatory.
Common Side Effects of Azathioprine
- Nausea and vomiting (take with food)
- Bone marrow suppression — leucopenia, thrombocytopenia, anaemia
- Elevated liver enzymes
- Fatigue
- Infections
Serious Side Effects — Seek Medical Attention
Contact your doctor or call 111 immediately if you experience any of the following:
- Serious bone marrow suppression (dose-dependent — TPMT enzyme deficiency increases risk dramatically)
- Myelosuppression — profound pancytopenia (if xanthine oxidase inhibitor such as allopurinol is added without dose reduction — DANGEROUS)
- Lymphoma and skin malignancy (long-term immunosuppression)
- Hepatotoxicity including venoocclusive disease (rare)
- Severe opportunistic infections
Drug Interactions
Azathioprine may interact with other medicines. Always inform your doctor and pharmacist of all medications you are taking. Key interactions include:
- Allopurinol (CONTRAINDICATED combination without dose reduction — allopurinol inhibits xanthine oxidase, which is required to metabolise azathioprine — causes life-threatening myelosuppression; reduce azathioprine dose by 75% if allopurinol unavoidable)
- Warfarin (reduced anticoagulant effect)
- ACE inhibitors (enhanced leucopenia)
- NSAIDs
- Live vaccines (contraindicated)
New Zealand Prescribing Information
Azathioprine (25 mg, 50 mg tablets — Imuran® and generics) is funded by Pharmac. TPMT (thiopurine methyltransferase) enzyme activity testing is recommended before starting — patients with low or absent TPMT are at very high risk of myelosuppression. The DANGEROUS interaction with allopurinol must be communicated clearly to all patients and prescribers. Regular FBC and LFT monitoring is mandatory.
Frequently Asked Questions
Why is allopurinol dangerous with azathioprine?
Allopurinol inhibits xanthine oxidase, the enzyme that breaks down azathioprine. If azathioprine and allopurinol are given together at normal doses, azathioprine accumulates to toxic levels — causing life-threatening bone marrow failure (myelosuppression). If both must be used together, the azathioprine dose must be reduced to 25% of normal and blood counts monitored very carefully.
What is TPMT testing?
TPMT (thiopurine methyltransferase) is an enzyme that metabolises azathioprine. Some patients have low or absent TPMT activity due to genetic variation — these patients are at very high risk of severe myelosuppression even at standard doses. Testing TPMT activity before starting azathioprine identifies these high-risk patients, allowing dose adjustment or use of an alternative.
⚠️ Disclaimer: This page is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or a qualified health provider. In New Zealand, medication availability and funding may vary — check with Pharmac or your pharmacist for current information.
Reviewed by a Registered Pharmacist NZ
References & Further Information
The following New Zealand and international resources were used to inform this page: