Azathioprine
Pronunciation: ay-za-THYE-oh-preen.
- Generic name
- azathioprine
- Brand names
- Imuran, Azasan, Imurek, Azamun
- Drug class
- Purine antimetabolite immunosuppressant (thiopurine)
- Form
- Tablet; powder for solution for injection
- Route
- Oral (tablet); intravenous (powder for injection, where marketed)
- Legal status
- Prescription-only medicine
Supply Rules Vary
Trade names by country: United States · United Kingdom · Canada · Australia · New Zealand
Australia: Imuran; Azathioprine (Sandoz) and other generics (5) · Full directory
Boxed warning: Malignancy and pharmacogenomic myelosuppression risk
Chronic immunosuppression with azathioprine, a purine antimetabolite, increases the risk of malignancy in humans. Reports include post-transplant lymphoma and hepatosplenic T-cell lymphoma (HSTCL), a rare and often fatal type of lymphoma, in patients with inflammatory bowel disease. Patients with thiopurine S-methyltransferase (TPMT) or nucleotide diphosphatase (NUDT15) deficiency may be at increased risk of severe and life-threatening myelotoxicity from azathioprine if given conventional doses. Death associated with pancytopenia has been reported in patients with absent TPMT activity receiving azathioprine. Consider alternative therapy in patients with homozygous TPMT or NUDT15 deficiency, and reduced dosages in patients with heterozygous deficiency. Testing for TPMT and NUDT15 deficiency should be considered in patients with severe bone marrow toxicity or repeated episodes of myelosuppression; genotyping/phenotyping does not identify all patients at risk and does not replace ongoing blood-count monitoring. (1)
What azathioprine is and how it works
Azathioprine is a purine antimetabolite immunosuppressant used to help prevent rejection after kidney transplantation and to manage certain autoimmune and inflammatory conditions, including rheumatoid arthritis, inflammatory bowel disease, and systemic lupus erythematosus. (1) (2)
Because azathioprine suppresses the immune system, it carries a real risk of increased infections and, with long-term use, an increased risk of certain cancers including lymphoma and skin cancers. Some people have a genetic difference (in the TPMT or NUDT15 genes) that makes them process azathioprine much more slowly, putting them at risk of severe, potentially fatal bone-count suppression at usual doses -- genetic testing before starting is often recommended, though it does not remove the need for regular blood tests.
Mechanism of action: how azathioprine works
Azathioprine is a prodrug converted to 6-mercaptopurine and then to active thioguanine nucleotides, which interfere with purine synthesis and suppress T- and B-lymphocyte proliferation, producing an immunosuppressive effect. TPMT and NUDT15 are key enzymes in its metabolism; reduced activity leads to accumulation of active/toxic metabolites. (1)
What azathioprine is used for
Prevention of kidney transplant rejection
Azathioprine is used as an adjunct to help prevent rejection following kidney (renal) transplantation. (1)
Rheumatoid arthritis
Used to reduce the signs and symptoms of active rheumatoid arthritis. (1)
Inflammatory bowel disease and other autoimmune conditions
Used (in some countries) for moderate-to-severe inflammatory bowel disease, systemic lupus erythematosus, and other autoimmune/inflammatory conditions, often to allow a lower corticosteroid dose. (2)
Before taking azathioprine
Do not take / not appropriate for
- Do not take azathioprine if you have a known hypersensitivity to azathioprine or any ingredient in the product, or a known hypersensitivity to 6-mercaptopurine. (all products) (2)
- Azathioprine should not be used by pregnant women being treated for rheumatoid arthritis. (rheumatoid arthritis indication) (1)
- For rheumatoid arthritis, azathioprine should not be used in patients previously treated with alkylating agents (such as cyclophosphamide, chlorambucil, or melphalan) because of a substantially increased risk of malignancy. (rheumatoid arthritis indication) (1)
Tell your clinician about
- Whether you or a family member have ever been tested for TPMT or NUDT15 gene variants, or whether such testing has been recommended for you.
- Any history of cancer, especially skin cancer or lymphoma.
- All current infections or a history of serious/recurrent infections.
- Liver problems or previous hepatotoxicity with any medicine.
- If you are pregnant, planning to become pregnant, or breastfeeding.
- All other medicines you take, especially allopurinol, febuxostat, warfarin, ACE inhibitors, or ribavirin.
How to take azathioprine
- Azathioprine is taken by mouth, usually once daily or in divided doses, exactly as prescribed for your specific condition.
- Your prescriber sets your individual dose based on your body weight, your condition, and -- where tested -- your TPMT/NUDT15 status; this is never a fixed, self-selected dose.
- Take with food if it causes stomach upset, unless your clinician advises otherwise.
- You will need regular blood tests throughout treatment, especially when starting or changing your dose.
Procedures and treatment changes
Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (2) (3) (4) (5) (6)
Missed dose
If you miss a dose, take it as soon as you remember unless it is almost time for your next dose -- do not double up. Contact your clinician or pharmacist if you are unsure, especially if you have missed several doses.
Overdose
Overdose can cause severe bone marrow suppression (low blood counts), increasing infection and bleeding risk, sometimes appearing days after ingestion. Seek emergency medical care immediately for a suspected overdose; blood counts will need close monitoring. (1)
Side effects
Azathioprine commonly affects blood counts and increases infection risk. Report anything that concerns you to your clinician.
Low white blood cell count (leukopenia)
What to notice: Increased infections, fever, sore throat.
What to do: Regular blood tests will monitor this; contact your clinician promptly if you develop signs of infection. (1)
Nausea and vomiting
What to notice: Stomach upset, especially when starting treatment.
What to do: Taking with food may help; tell your clinician if severe or persistent. (1)
Diarrhea
What to notice: Loose stools.
What to do: Tell your clinician if severe or persistent. (1)
Skin rash
What to notice: New or worsening rash.
What to do: Tell your clinician; may require dose review. (1)
Signs of severe myelosuppression
What to notice: Unusual bruising or bleeding, persistent fever, severe fatigue, signs of infection that won't go away.
What to do: Contact your clinician promptly; this may indicate severe bone marrow suppression, which can be life-threatening, particularly in TPMT/NUDT15-deficient patients. (1)
Possible liver problems
What to notice: Yellowing of skin or eyes, dark urine, severe abdominal pain, unusual tiredness.
What to do: Contact your clinician promptly for liver function assessment. (1)
Possible new cancer
What to notice: A new lump, unusual skin lesion or mole, unexplained weight loss, persistent swollen glands.
What to do: Contact your clinician promptly for evaluation; long-term immunosuppression carries a real increased malignancy risk. (1) (2)
Serious infection
What to notice: High fever, chills, productive cough, confusion, or any infection that is severe or not improving.
What to do: Seek prompt medical care; immunosuppression increases the risk of serious, including opportunistic, infections. (1)
Serious warnings
Malignancy (lymphoma, hepatosplenic T-cell lymphoma, skin cancers) — boxed_warning
Symptoms: New lump, unusual skin lesion, unexplained weight loss, persistent swollen glands.
Action: Chronic immunosuppression with azathioprine carries a real, boxed-warning-level increased risk of malignancy. Routine skin checks and prompt reporting of new symptoms are important; discuss your individual risk with your clinician. (1)
TPMT/NUDT15 deficiency-related severe myelosuppression — boxed_warning
Symptoms: Severe infections, unusual bruising/bleeding, profound fatigue -- can occur even at usual doses in genetically susceptible individuals.
Action: Testing for TPMT/NUDT15 deficiency should be considered before starting or after severe/repeated myelosuppression. Homozygous deficiency should prompt consideration of alternative therapy; heterozygous deficiency should prompt substantial dose reduction. Testing does not replace ongoing blood-count monitoring -- deaths from pancytopenia have occurred in patients with absent TPMT activity. (1)
Life-threatening interaction with allopurinol or febuxostat — emergency
Symptoms: Severe bone marrow suppression from azathioprine overdose-like toxicity if the interaction is not managed.
Action: Allopurinol (and febuxostat) block the enzyme that breaks down azathioprine. If allopurinol cannot be avoided, azathioprine dose must be reduced to about 25% of the original dose, with close blood-count monitoring; avoid combining with febuxostat. Real, documented cases of serious harm have occurred when this was not managed. (1) (2) (6)
Serious infections — emergency
Symptoms: High fever, severe or worsening infection, signs of an opportunistic infection.
Action: Seek prompt medical care; immunosuppression increases susceptibility to bacterial, viral, fungal, and opportunistic infections, including rare cases of progressive multifocal leukoencephalopathy. (1)
Interactions
| Medicine or test | Effect | Action |
|---|---|---|
| Allopurinol and febuxostat (xanthine oxidase inhibitors) | Block the breakdown of azathioprine, causing dangerous accumulation of active metabolites and life-threatening bone marrow suppression. | If allopurinol cannot be avoided, azathioprine dose must be reduced to approximately 25% of the original dose with close blood-count monitoring. Febuxostat combination should be avoided. (1) (2) (6) |
| Aminosalicylates (e.g., mesalazine, sulfasalazine) | May inhibit TPMT activity, increasing azathioprine toxicity risk. | Use cautiously with closer blood-count monitoring if combined. (1) |
| ACE inhibitors | Increased risk of anemia and severe leukopenia when combined with azathioprine. | Your clinician will monitor blood counts more closely. (1) |
| Ribavirin | Increased myelotoxicity risk. | Weekly complete blood count monitoring is recommended if combined. (1) |
| Warfarin | Azathioprine may reduce the anticoagulant effect of warfarin. | Your clinician will monitor your INR more closely if you take both. (1) |
Pregnancy, breastfeeding, kidney/liver function, age
TPMT or NUDT15 gene variants
If you are known to have reduced TPMT or NUDT15 enzyme activity (or a family history of severe reaction to a thiopurine medicine), tell your clinician before starting azathioprine -- your dose may need to be substantially reduced, or an alternative medicine considered. (1) (4)
Pregnancy
Azathioprine should not be used by pregnant women being treated for rheumatoid arthritis. For other indications, it should only be used in pregnancy after careful assessment of risk versus benefit, since it is teratogenic in animal studies; leukopenia and thrombocytopenia have been reported in newborns of treated mothers. (1) (2)
Breastfeeding
Breastfeeding is not generally recommended during azathioprine treatment because the drug and its metabolites can transfer into breast milk. Discuss with your clinician. (1)
Liver impairment
Azathioprine can affect liver function; your clinician will monitor liver blood tests and may adjust your dose. (1)
Children
Azathioprine is used in children for specific indications under specialist supervision; dosing and monitoring are individualized by a pediatric specialist. (2)
Monitoring
Before starting
- Consider TPMT and NUDT15 testing (genotyping or phenotyping), especially where locally available -- but remember testing does not identify every at-risk patient.
- Baseline complete blood count and liver function tests.
- Screen for current infections, history of cancer, and pregnancy status.
- Review all current medicines for interactions, especially allopurinol/febuxostat.
During treatment
- Complete blood count: typically weekly for the first month, twice monthly for months two and three, then monthly thereafter, or more frequently if the dose changes or with abnormal results.
- Periodic liver function tests.
- Ongoing skin checks and cancer-risk awareness with long-term use.
- Watch for and promptly report signs of infection, bleeding/bruising, or jaundice.
Storage
Store at room temperature, away from moisture, heat, and direct light, as directed on your product's label. Keep out of reach of children. Follow local guidance for safe handling and disposal, since azathioprine is a cytotoxic-class medicine.
Professional information
Clinician-facing context only. Always refer to the current local product information (prescribing information/SmPC) and institutional protocols. TPMT/NUDT15 status and indication govern dosing; no universal calculator applies.
All regions (general structure; consult local label for exact figures)
Indication: Renal transplantation (adjunct)
Dose summary: Initial 3-5 mg/kg/day starting at transplantation; maintenance 1-3 mg/kg/day, individualized.
Renal context: Dose individualized to graft function and blood counts; consult local label. (1)
All regions (general structure; consult local label for exact figures)
Indication: Rheumatoid arthritis
Dose summary: Initial approximately 1.0 mg/kg (50-100 mg) daily; may increase after 6-8 weeks by 0.5 mg/kg increments every 4 weeks; maximum 2.5 mg/kg/day; adequate trial at least 12 weeks.
Renal context: Dose individualized; consult local label. (1)
TPMT/NUDT15-guided adjustment
Indication: All indications
Dose summary: Homozygous TPMT or NUDT15 deficiency: consider alternative therapy. Heterozygous deficiency: substantially reduced starting dose with close monitoring. Concomitant allopurinol: reduce azathioprine to approximately 25% of the original dose.
Renal context: Not a substitute for renal-specific dose adjustment; individualize per patient. (1) (2)
Hepatic guidance
Hepatotoxicity, including elevated liver enzymes and rare veno-occlusive disease, has been reported; monitor liver function and adjust dose as needed.
Overdose note
Overdose principally causes myelosuppression, which may be delayed in onset; manage with supportive care and close hematologic monitoring. (1)
Frequently asked questions
Why do I need genetic testing before starting azathioprine?
Some people have a genetic difference in the TPMT or NUDT15 genes that makes them break down azathioprine much more slowly than usual. In these people, a normal dose can cause severe, and sometimes fatal, suppression of bone marrow function. Testing before starting can identify many (but not all) at-risk patients, so regular blood tests remain necessary throughout treatment regardless of the test result. (1)
Does azathioprine increase my cancer risk?
Yes -- this is a real, FDA boxed-warning risk. Long-term immunosuppression with azathioprine increases the risk of certain cancers, including lymphoma (in rare cases a specific and often fatal type called hepatosplenic T-cell lymphoma) and skin cancers. Your clinician will discuss your individual risk and the importance of routine skin checks. (1) (2)
Can I take allopurinol (for gout) with azathioprine?
This combination is dangerous unless carefully managed. Allopurinol blocks an enzyme that normally breaks down azathioprine, which can cause a life-threatening drop in blood cell counts. If the combination cannot be avoided, your azathioprine dose must be reduced to about a quarter of the usual dose with close blood monitoring. Never combine these medicines without your clinician's explicit direction. (1) (6)
Is azathioprine available everywhere?
Azathioprine is a long-established, widely marketed medicine, currently authorised in the United States, United Kingdom, European Union (as Imurek, nationally authorised per member state), Canada, and Australia based on the regulator sources checked. It is also marketed in New Zealand, confirmed via an active Medsafe safety communication, though this session could not independently extract the full current New Zealand data-sheet text. (1) (2) (3) (4) (5) (6)
What is the mechanism of action of azathioprine?
Azathioprine is a prodrug converted to 6-mercaptopurine and then to active thioguanine nucleotides, which interfere with purine synthesis and suppress T- and B-lymphocyte proliferation, producing an immunosuppressive effect. TPMT and NUDT15 are key enzymes in its metabolism; reduced activity leads to accumulation of active/toxic metabolites. (1)
References
- IMURAN (azathioprine) tablet - prescribing informationU.S. Food and Drug Administration (via DailyMed, National Library of Medicine) · United States · dated 2026 · accessed 2026-07-30 · source 1
- Azathioprine tablets 50 mg - Summary of Product Characteristics (SmPC)electronic Medicines Compendium (UK), MHRA-approved product information · United Kingdom · dated 2026 · accessed 2026-07-30 · source 2
- Azathioprine - list of nationally authorised medicinal products (PSUSA/00000275/201812)European Medicines Agency (CMDh/PSUSA nationally-authorised-product procedure) · European Union · dated 2026 · accessed 2026-07-30 · source 3
- IMURAN (azathioprine tablets USP, 50 mg) - Product MonographHealth Canada (product monograph) · Canada · dated 2026 · accessed 2026-07-30 · source 4
- IMURAN azathioprine 50mg and 25mg tablets, 50mg powder for injection - ARTG 31398/31393/11102Therapeutic Goods Administration · Australia · dated 1992-06-12 · accessed 2026-07-30 · source 5
- Allopurinol - Life-threatening interaction with azathioprine or mercaptopurine (Prescriber Update safety article)Medsafe (New Zealand Medicines and Medical Devices Safety Authority) · New Zealand · dated 2020-09 · accessed 2026-07-30 · source 6
Brand names by country
Single-ingredient azathioprine names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.
Priority countries
United States
- Azasan75 mg, 100 mg · salix pharmaceuticals
- IMURAN50 mg · sebela pharmaceuticals inc.
United Kingdom
- Imuran; generic azathioprine tablets (multiple manufacturers) (2)current licensed product · 25 mg, 50 mg · Multiple, product-dependent
Canada
- IMURAN
Australia
- Imuran; Azathioprine (Sandoz) and other generics (5)current register product · 25 mg, 50 mg, 50 mg powder for injection · Aspen Pharmacare Australia Pty Ltd (Imuran); other sponsors for generics
New Zealand
- Imuran; Azamun (6)current marketed product confirmed via safety communication · product-dependent · Not independently confirmed this session; product identity confirmed live via Medsafe
View brands in additional countries.
European Union
Imurek (3)