Medically reviewed by: [Mahmood Rehan]
Last updated: May 2026
Reading time: ~12 minutes
⚠️ FDA BLACK BOX WARNING: Azathioprine carries a black box warning — the FDA’s most serious advisory — for increased risk of malignancy (including lymphoma and skin cancer) due to chronic immunosuppression, and for serious infections from immune suppression. Physicians prescribing this drug must be fully familiar with these risks before use.
Azathioprine (brand names: Imuran, Azasan) is an immunosuppressive drug used primarily to prevent organ rejection after kidney transplantation and to treat active rheumatoid arthritis. It belongs to the thiopurine class of drugs and works by suppressing the immune system’s ability to attack the body’s own tissues or a transplanted organ.
It was the first immunosuppressive agent used in organ transplantation and remains one of the most prescribed drugs in autoimmune and transplant medicine.
1. What Is Azathioprine Used For?
FDA-Approved Uses
Azathioprine has two FDA-approved indications:
- Kidney transplant rejection prevention — adjunctive therapy to prevent rejection after renal transplantation
- Rheumatoid arthritis (RA) — symptomatic treatment of active RA that has not responded adequately to conventional management
Off-Label Uses
Physicians commonly prescribe azathioprine off-label for a range of autoimmune and inflammatory conditions, including:
- Inflammatory bowel disease (Crohn’s disease and ulcerative colitis)
- Lupus nephritis
- Multiple sclerosis
- Myasthenia gravis
- Dermatomyositis and polymyositis
- Autoimmune hepatitis
- Behçet’s disease
- Pemphigus vulgaris
- Bullous pemphigoid
- Pyoderma gangrenosum
- Lichen planus
- Psoriasis (refractory)
- Relapsing polychondritis
- Chronic immune thrombocytopenic purpura (ITP)
- Recurrent pericarditis
- Atopic dermatitis (severe, refractory)
2. Drug Class
- Drug class: Immunosuppressant / Antimetabolite
- Subclass: Thiopurine (purine analogue)
- Category: Disease-modifying antirheumatic drug (DMARD)
Azathioprine is a prodrug — not active on its own. It must be metabolized by the body into active compounds (6-mercaptopurine and thioguanine nucleotides) to produce its immunosuppressive effects.
3. How Azathioprine Works: Mechanism of Action
Azathioprine is a prodrug of 6-mercaptopurine (6-MP). After oral or intravenous administration, it is rapidly converted in the body to 6-MP, which is then further metabolized into two key active compounds:
- 6-Thioguanine nucleotides (6-TGN) — the primary immunosuppressive metabolites
- 6-Methylmercaptopurine ribonucleotides (6-MMPR)
The enzymes responsible for this conversion are:
- Hypoxanthine-guanine phosphoribosyltransferase (HPRT)
- Thiopurine methyltransferase (TPMT)
6-TGN metabolites are incorporated into the DNA of rapidly dividing immune cells (particularly T and B lymphocytes), blocking their replication. This inhibits purine synthesis and prevents immune cell proliferation — the mechanism responsible for both its immunosuppressive benefits and many of its side effects.
In practical terms: azathioprine reduces the immune system’s ability to mount an attack — whether against a transplanted kidney, a patient’s own joints, or other tissues in autoimmune disease.
4. Azathioprine Dosage
Azathioprine is available in oral tablet form (50 mg, 75 mg, 100 mg) and as an intravenous injection (for hospital use).
Dosage by Indication
| Indication | Starting Dose | Maintenance Dose | Notes |
|---|---|---|---|
| Kidney transplant rejection prevention | 3–5 mg/kg/day IV or orally on day of transplant | 1–3 mg/kg/day orally | Begin taper as tolerated |
| Rheumatoid arthritis | 1 mg/kg/day (50–100 mg) orally | Increase by 0.5 mg/kg every 4 weeks; max 2.5 mg/kg/day | Reduce to lowest effective dose |
| Inflammatory bowel disease (off-label) | 2–2.5 mg/kg/day orally | Individualized | Monitor CBC closely |
| Lupus nephritis (off-label) | 2 mg/kg/day orally | With or without low-dose corticosteroids | Per ACR 2012 guidelines |
Dose Adjustments
- Renal impairment: Reduce the dose by 25–50%
- Hepatic impairment: Use with caution; monitor liver function tests closely
- Allopurinol co-administration: Reduce azathioprine dose by 60–75% — critical interaction (see Drug Interactions)
- TPMT-deficient patients: Significant dose reductions required (see TPMT section)
Best Time to Take Azathioprine
Azathioprine should ideally be taken with food or after a meal to reduce gastrointestinal side effects such as nausea and stomach pain. If a once-daily dose causes persistent nausea, dividing the dose (morning and evening, both with food) can help. Taking it at the same time each day ensures consistent blood levels.
5. Azathioprine Side Effects
About 70% of patients tolerate azathioprine well without significant problems. However, side effects can occur and vary in severity.
Very Common Side Effects (10% or more)
- Nausea and vomiting (especially in the first weeks of treatment)
- Bone marrow suppression, leading to:
- Leukopenia (low white blood cell count) — most common cause of dose reduction
- Thrombocytopenia (low platelet count)
- Anemia (low red blood cell count)
Common Side Effects (1–10%)
- Loss of appetite
- Fatigue
- Diarrhea
- Abdominal discomfort
- Hair loss (alopecia) — may resolve spontaneously even with continued use
- Mouth sores or ulcers
- Elevated liver enzymes (hepatotoxicity)
- Increased susceptibility to infections
Serious Side Effects — Require Immediate Medical Attention
| Side Effect | Signs to Watch For | Action |
|---|---|---|
| Severe leukopenia | Unusual bruising, repeated infections, fever | Stop azathioprine; contact doctor immediately |
| Hepatotoxicity | Jaundice, dark urine, upper right abdominal pain | Liver function tests; consider dose reduction or discontinuation |
| Pancreatitis | Severe abdominal pain radiating to the back, nausea, vomiting | Discontinue immediately |
| Hypersensitivity reaction | Severe nausea/vomiting, diarrhea, fever, malaise within first weeks | Discontinue; do not re-challenge |
| PML (Progressive Multifocal Leukoencephalopathy) | Confusion, weakness, vision changes, personality changes | Emergency evaluation — rare but potentially fatal brain infection |
| Lymphoma / cancer | Unusual lumps, night sweats, unexplained weight loss | Full oncology evaluation |
6. Long-Term Side Effects of Azathioprine
Long-term use carries additional risks that require ongoing monitoring:
- Increased cancer risk — chronic immunosuppression increases the risk of:
- Lymphoma (including hepatosplenic T-cell lymphoma — primarily in young males with IBD)
- Leukemia
- Skin cancers (particularly squamous cell carcinoma)
- Post-transplant lymphoma
- Cumulative bone marrow suppression
- Chronic liver toxicity (rare at therapeutic doses)
- Increased long-term infection susceptibility, including opportunistic infections
7. TPMT Testing and Azathioprine
Thiopurine methyltransferase (TPMT) is an enzyme that metabolizes azathioprine. Genetic variation in TPMT activity determines how quickly a patient breaks down the drug:
| TPMT Activity Level | Frequency in Population | Risk | Recommended Action |
|---|---|---|---|
| Normal (high activity) | ~90% | Standard | Use standard dosing |
| Intermediate activity | ~10% | Moderate accumulation of active metabolites | Reduce dose by 30–50% |
| Deficient / absent activity | ~0.3% | Severe, potentially fatal bone marrow suppression | Avoid azathioprine or use very low doses with intensive monitoring |
Most guidelines now recommend TPMT testing before starting azathioprine, especially for IBD and autoimmune conditions. Many clinicians also use the NUDT15 gene test (particularly relevant in Asian populations) as a complementary predictor of thiopurine toxicity.
If your doctor has not discussed TPMT testing before starting azathioprine, raise this with them.
8. Drug Interactions
Azathioprine has serious interactions with several drugs. Always inform your prescribing physician and pharmacist of all medications, supplements, and herbal remedies you take.
Most Important Interaction: Allopurinol
Allopurinol + Azathioprine is a potentially dangerous combination. Allopurinol inhibits xanthine oxidase, the enzyme that breaks down azathioprine’s active metabolites. This dramatically increases azathioprine blood levels, leading to severe toxicity — including life-threatening bone marrow suppression.
If allopurinol must be used: Reduce azathioprine dose by 60–75% and monitor blood counts closely.
Other Notable Interactions
| Drug / Drug Class | Interaction | Management |
|---|---|---|
| Allopurinol | Increased azathioprine toxicity (major) | Reduce AZA dose by 60–75% |
| ACE inhibitors | Exaggerated leukopenia (especially post-kidney transplant) | Monitor CBC closely |
| Co-trimoxazole (trimethoprim-sulfamethoxazole) | Additive leukopenia | Avoid combination if possible |
| Warfarin | Azathioprine may reduce anticoagulant effect | Monitor INR closely |
| Live vaccines | Risk of vaccine-strain infection due to immunosuppression | Avoid live vaccines during treatment |
| Cyclophosphamide / Chlorambucil | Increased neoplasia risk | Avoid concurrent or sequential use |
| Ribavirin | May enhance thiopurine myelotoxicity | Use with caution |
| Febuxostat | Similar mechanism to allopurinol — similar interaction risk | Reduce dose or avoid |
9. Contraindications
Azathioprine is contraindicated (must not be used) in:
- Known hypersensitivity to azathioprine or any excipient
- Pregnancy when used for rheumatoid arthritis — it can harm the unborn baby
- Clinically active serious infection — immunosuppression will worsen it
- Known malignancy — in most cases (discuss risk-benefit with specialist)
- Patients previously treated with alkylating agents (e.g., cyclophosphamide, chlorambucil, melphalan) — significantly increased neoplasia risk
10. Azathioprine and Pregnancy
The relationship between azathioprine and pregnancy is nuanced and must be managed on a case-by-case basis.
Risks
- Spontaneous miscarriage
- Premature delivery
- Low birth weight
- Potential neonatal immunosuppression
- Teratogenic risk (FDA Category D)
Clinical Nuance
Despite the risks, azathioprine is considered one of the safer immunosuppressants for use during pregnancy in specific clinical contexts. Data from SLE and kidney transplant patients show that, when the maternal disease poses a greater risk than the medication, the benefit may outweigh the risk.
Key 2026 guidance:
- Rheumatoid arthritis: Avoid during pregnancy — FDA specifically warns against this use
- Kidney transplant / SLE / severe autoimmune disease: Decision made jointly by specialist and patient based on individual risk-benefit
- Effective contraception is strongly advised for all women of reproductive age on azathioprine
- Breastfeeding: Generally discouraged
11. Monitoring Requirements
Before Starting Azathioprine
- Complete blood count (CBC) with differential
- Liver function tests (LFTs)
- Renal function (creatinine, eGFR)
- TPMT gene testing or enzyme activity testing
- Baseline chest X-ray (in some settings)
- Exclude active infection and tuberculosis
While on Azathioprine
| Test | Frequency |
|---|---|
| CBC with differential | Weekly for first 8 weeks; monthly thereafter |
| Liver function tests | Monthly for first 3 months; then every 3 months |
| Renal function | Every 3 months |
| Skin surveillance (skin cancer) | Annual dermatology review for long-term use |
When to Stop Azathioprine
Treatment should be discontinued or dose reduced in cases of:
- Severe leukopenia (WBC below 3,000/mm³)
- Severe thrombocytopenia
- Clinical signs of hepatotoxicity
- Confirmed malignancy (in most cases)
- Severe hypersensitivity reaction
- Pregnancy (RA indication)
Azathioprine Toxicity and Treatment
There is no specific antidote for azathioprine toxicity. Management is supportive:
- Acute toxicity (within 2 hours): Activated charcoal may reduce absorption
- Severe toxicity: Azathioprine is dialyzable — hemodialysis can be used
- Bone marrow suppression: Supportive care; G-CSF in severe cases
12. Frequently Asked Questions
What is azathioprine used for?
Azathioprine is FDA-approved for preventing kidney transplant rejection and treating active rheumatoid arthritis. It is also widely used off-label for inflammatory bowel disease, lupus, myasthenia gravis, and many other autoimmune conditions.
What is a high dose of azathioprine?
For rheumatoid arthritis, doses above 2.5 mg/kg/day are considered high and should not be exceeded. For kidney transplant, initial doses of 3–5 mg/kg/day may be used under close supervision, with maintenance doses typically reduced to 1–3 mg/kg/day. Any increase must be guided by your physician.
Is 100 mg of azathioprine a high dose?
Not necessarily. For a person weighing 70 kg (154 lbs), 100 mg/day equals approximately 1.4 mg/kg/day — within the standard therapeutic range for rheumatoid arthritis. Dosing is always individualized based on weight, TPMT status, kidney function, and clinical response.
What is the best time of day to take azathioprine?
There is no strict requirement, but most clinicians recommend taking it with food (at breakfast or dinner) to minimize nausea. Consistency matters more than the specific time. If nausea persists, dividing the daily dose (morning and evening, both with food) can help.
Is azathioprine a chemotherapy drug?
Azathioprine shares chemical origins with cancer chemotherapy drugs but is not used as chemotherapy in standard practice. It is used at much lower doses as an immunosuppressant. It does, however, carry a cancer risk with long-term use.
Does azathioprine cause weight gain?
No — azathioprine is not associated with weight gain. Weight loss has occasionally been reported as a side effect. Unexplained weight changes while on this medication should be discussed with your doctor.
What are the long-term side effects of azathioprine?
Long-term use is associated with increased cancer risk (lymphoma, skin cancer, leukemia), cumulative bone marrow suppression, and increased infection susceptibility. Regular monitoring and sun protection are important for patients on long-term therapy.
Is azathioprine safe during pregnancy?
For rheumatoid arthritis, avoid during pregnancy. For severe conditions like lupus or kidney transplant, a specialist must assess the individual risk-benefit balance. Effective contraception is recommended for all women of reproductive age on azathioprine.
What is the azathioprine and TPMT connection?
TPMT metabolizes azathioprine. Patients with low or absent TPMT activity cannot clear active metabolites efficiently, leading to dangerously high levels and potentially fatal bone marrow suppression. Pre-treatment TPMT testing is strongly recommended.
Is azathioprine a steroid?
No. It is a thiopurine, not a steroid. However, it is often used as a steroid-sparing agent — allowing doctors to reduce or eliminate long-term corticosteroid use in autoimmune patients.
Common Brand Names
| Brand Name | Country of Use |
|---|---|
| Imuran | US, UK, Australia |
| Azasan | US |
| Imurel | France |
| Imurek | Germany |
| Azamun | Various |
| Azanin | Japan |
The generic form (azathioprine) is widely available and bioequivalent to branded formulations.
References
- Mohammadi O, Kassim TA. Azathioprine. StatPearls [Internet]. 2026. https://www.ncbi.nlm.nih.gov/books/NBK542190/
- Drugs.com. Azathioprine side effects. Medically reviewed Dec 24, 2025.
- Mayo Clinic. Azathioprine (oral route). Updated March 2026.
- Cleveland Clinic. Azathioprine: Side Effects & Treatment. Updated Feb 2026.
- WebMD. Azathioprine (Imuran, Azasan): Uses, Side Effects, Interactions.
- RxList. Azathioprine. Updated Oct 2024.
- FDA. Imuran Prescribing Information (via DailyMed).
- NHS. Azathioprine. https://www.nhs.uk/medicines/azathioprine/
- Lippincott’s Illustrated Reviews: Pharmacology, 4th Edition.
- DrugBank. Azathioprine (DB00993). https://go.drugbank.com/drugs/DB00993
- MedlinePlus. Azathioprine. https://medlineplus.gov/druginfo/meds/a682167.html
This article is for informational purposes only and does not constitute medical advice. Always consult a licensed healthcare provider before starting, stopping, or changing any medication.