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[Pharmacology](https://durgical.com/en/pharmacology/) · [Antiproliferative immunosuppressants](https://durgical.com/en/pharmacology/classes/antiproliferative-immunosuppressants/)

# Azathioprine

An older immunosuppressant tablet that becomes 6-mercaptopurine and stops immune cells multiplying. Blood counts are checked; allopurinol is a dangerous mix.

Updated 2026-11-01

- Class: [Antiproliferative immunosuppressants](https://durgical.com/en/pharmacology/classes/antiproliferative-immunosuppressants/)
- Target: Purine synthesis, through thioguanine nucleotides
- ATC code: L04AX01
- WHO Model List of Essential Medicines

## Mechanism of action

Azathioprine is a prodrug of 6-mercaptopurine. It is converted to thioguanine nucleotides, which inhibit purine synthesis and are incorporated into DNA, so rapidly dividing lymphocytes cannot proliferate. 6-Mercaptopurine is inactivated by TPMT (methylation) and by xanthine oxidase (oxidation); low TPMT or NUDT15 activity, or blocking xanthine oxidase, lets active metabolites build up.

- Converted to 6-mercaptopurine
- Then to thioguanine nucleotides (active)
- Purine synthesis falls and the nucleotides enter DNA
- Lymphocytes cannot proliferate

## Pharmacokinetics

**Absorption**
Well absorbed by mouth

**Metabolism**
Converted to 6-mercaptopurine; inactivated by TPMT and xanthine oxidase; toxicity is more likely with kidney impairment

## Indications

- Prevention of kidney transplant rejection, as an adjunct
- Active rheumatoid arthritis [Rheumatoid arthritis →](https://durgical.com/en/conditions/rheumatoid-arthritis/)
- Crohn disease (off label) [Crohn disease →](https://durgical.com/en/conditions/crohn-disease/)
- Ulcerative colitis (off label) [Ulcerative colitis →](https://durgical.com/en/conditions/ulcerative-colitis/)

## Adverse effects

### Common

- Nausea and vomiting
- Leukopenia
- Raised liver enzymes
- Rash

### Serious (rare, important)

- Severe bone marrow suppression, especially with TPMT or NUDT15 deficiency
- Malignancy: post-transplant lymphoma, hepatosplenic T-cell lymphoma in inflammatory bowel disease, skin cancer
- Serious infections, including progressive multifocal leukoencephalopathy
- Hypersensitivity reaction with severe nausea, vomiting and fever; pancreatitis
- Hepatotoxicity

## Contraindications

- Hypersensitivity to azathioprine
- Rheumatoid arthritis in pregnancy
- Rheumatoid arthritis previously treated with alkylating agents (high risk of malignancy)

## Key interactions

**Allopurinol and febuxostat**
Block xanthine oxidase: active metabolites accumulate and marrow toxicity can be severe; febuxostat is not used with it.

**Aminosalicylates (sulfasalazine, mesalazine)**
Inhibit TPMT; used with caution.

**Warfarin**
May weaken its anticoagulant effect.

**Ribavirin and other marrow-suppressing drugs**
Severe pancytopenia.

## What is monitored

- Complete blood count with platelets, often at first and then regularly
- TPMT and NUDT15 testing before starting
- Liver tests
- Skin checks for cancer

## Administration and safety

- Oral tablet, usually after meals; taking it after meals or in divided doses can lessen nausea.
- Taken at about the same times every day.

## Nursing care

- Ask about allopurinol or febuxostat before the first dose.
- Check that TPMT and NUDT15 results are available.
- Teach the signs of infection and bleeding: fever, sore throat, bruising.
- Teach sun protection and skin checks.
- Report severe nausea, vomiting, fever or abdominal pain soon after starting.

## Pregnancy and breastfeeding

Can harm the fetus; it is contraindicated for rheumatoid arthritis in pregnancy, and in transplant recipients the treating team weighs it case by case.

## Memory aid

AzaTHIOprine runs on THIOpurine: TPMT and xanthine oxidase are its brakes. Cut a brake (low TPMT, allopurinol) and the marrow crashes.

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme test is done before azathioprine, and why?

TPMT (and NUDT15) activity or genotype: deficient patients build up thioguanine nucleotides and develop severe myelotoxicity at usual doses.

## A patient on azathioprine is given allopurinol for gout. What is the risk?

Allopurinol blocks xanthine oxidase, one of the routes that inactivate 6-mercaptopurine, so active metabolites accumulate and bone marrow suppression can be severe.

## Which lymphoma is linked to azathioprine in young patients with inflammatory bowel disease?

Hepatosplenic T-cell lymphoma, a rare and aggressive lymphoma reported with azathioprine, some cases in patients who also received a TNF blocker.

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## Related drugs

- [Mycophenolate mofetil](https://durgical.com/en/pharmacology/mycophenolate-mofetil/)
- [Ciclosporin](https://durgical.com/en/pharmacology/ciclosporin/)
- [Tacrolimus](https://durgical.com/en/pharmacology/tacrolimus/)

## Related conditions

- [Rheumatoid arthritis](https://durgical.com/en/conditions/rheumatoid-arthritis/)
- [Crohn disease](https://durgical.com/en/conditions/crohn-disease/)
- [Ulcerative colitis](https://durgical.com/en/conditions/ulcerative-colitis/)

[← Back to the class: Antiproliferative immunosuppressants](https://durgical.com/en/pharmacology/classes/antiproliferative-immunosuppressants/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Azathioprine: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682167.html) (MedlinePlus (U.S. National Library of Medicine), 2023); [DailyMed - AZATHIOPRINE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ceab8e8b-d022-4d0c-a552-cc5782446248) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=L04AX01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/azathioprine/