Azathioprine
Azathioprine / Azathioprine Sodium
Also known as: Imuran · Azasan · Azathioprinum · BW-57322 · NSC-39084
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Inflammatory bowel disease | Initially 2 mg/kg PO once daily for 2 weeks, then tapered to 2 mg/kg PO every other day for 2-4 weeks, then 1 mg/kg PO every other day. | PO | q24h then tapered | Long-term | 🌎 NA |
| Immune-mediated anemia, colitis, immune-mediated skin disease, and acquired myasthenia gravis | 2 mg/kg PO once daily (q24h); long-term therapy 0.5-1 mg/kg PO every other day | PO | q24h then q48h | Long-term | 🌎 NA |
| Adjunctive therapy in myasthenia gravis in non-responsive patients | Initially, 1 mg/kg PO once daily. If neutrophil and platelet counts are normal after 2 weeks, dose is increased to 2 mg/kg PO once daily. | PO | q24h | Until clinical remission | 🌎 NA |
| Lymphoplasmacytic enteritis (if poor response to prednisolone) | 2 mg/kg PO once daily for 5 days, then on alternate days to prednisolone | PO | q24h then q48h | Long-term | 🌎 NA |
| Severe cases of immune-mediated hemolytic anemia (IMHA) | 2.2 mg/kg PO once daily (q24h) | PO | q24h | Long-term | 🌎 NA |
| Acute immune-mediated hemolytic anemia (IMHA) with glucocorticoids | 1-2 mg/kg PO once daily | PO | q24h | Long-term maintenance | 🌎 NA |
| Severe and refractory inflammatory bowel disease | 2.2 mg/kg PO once daily | PO | q24h | Long-term | 🌎 NA |
| Adjunctive treatment of ocular fibrous histiocytomas | 2 mg/kg PO daily for 2 weeks, reevaluate, and reduce to 1 mg/kg every other day for 2 weeks, then 1 mg/kg once weekly for 1 month | PO | Tapering schedule | 10 weeks | 🌎 NA |
| Prevention of rejection of MHCmatched renal allografts (with cyclosporine) | 1-5 mg/kg PO every other day | PO | q48h | Long-term | 🌎 NA |
| Perianal fistulas (anal furunculosis) | Initially 2 mg/kg PO once daily (q24h)... Then reduce to 2 mg/kg PO every other day (q48h) and continued for 12 weeks... After 12 weeks, reduce dose to 1 mg/kg PO every other day (q48h) | PO | Tapering schedule | 12 months | 🌎 NA |
| Glomerulonephritis | 2 mg/kg PO once daily | PO | q24h | Unspecified | 🌎 NA |
| Chronic hepatitis (to reduce inflammation) | 2.2 mg/kg PO once daily in combination with corticosteroids... Azathioprine dose is given every other day after 1-2 weeks. | PO | q24h then q48h | Long-term | 🌎 NA |
| Immune-mediated diseases (e.g., IMHA, polyarthritis) | 2 mg/kg p.o. q24h for a maximum of 2-3 weeks, then 0.5-2 mg/kg p.o. q48h | PO | q24h initially, then q48h | Max 2-3 weeks for initial q24h dosing, then maintenance | 🌍 EU |
- Inflammatory bowel disease: May take 2-6 weeks before beneficial effects are seen.
- Immune-mediated anemia, colitis, immune-mediated skin disease, and acquired myasthenia gravis: With prednisolone administered on the alternate days.
- Adjunctive therapy in myasthenia gravis in non-responsive patients: Taper to every other day when clinical remission occurs. Discontinue if WBC <4,000 cells/mcL or neutrophils <1,000 cells/mcL.
- Severe cases of immune-mediated hemolytic anemia (IMHA): In addition to prednisone. Tapered gradually.
- Acute immune-mediated hemolytic anemia (IMHA) with glucocorticoids: Used for long-term maintenance as steroid side effects become intolerable.
- Severe and refractory inflammatory bowel disease: A lag time of 3-5 weeks is expected before clinical improvement is noted.
- Perianal fistulas (anal furunculosis): Dose reduced based on clinical response or myelosuppression markers.
- Glomerulonephritis: Immunosuppressive treatment is controversial.
- Immune-mediated diseases (e.g., IMHA, polyarthritis): Often used in conjunction with corticosteroids. Clinical response may take up to 6 weeks.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Severe and refractory inflammatory bowel disease | 0.3 mg/kg PO once every other day | PO | q48h | Long-term | 🌎 NA |
| Immune-mediated diseases | Not recommended | PO | Not recommended | Not recommended | 🌍 EU |
- Severe and refractory inflammatory bowel disease: Must be used with extreme caution due to myelotoxicity. May take 3-5 weeks for beneficial effects.
- Immune-mediated diseases: Cats often develop severe, non-responsive fatal leucopenia and thrombocytopenia.
Ferrets
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Inflammatory bowel disease | 0.9 mg/kg PO q24-72h | PO | q24-72h | Unspecified | 🌎 NA |
- Inflammatory bowel disease: Used alongside prednisone and dietary management.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Immunosuppressive | 3 mg/kg PO q24h | PO | q24h | Unspecified | 🌎 NA |
| Autoimmune skin diseases (e.g., pemphigus foliaceous) | 13 mg/kg PO q24h for 1 month, then every other day (q48h) | PO | q24h then q48h | Long-term | 🌎 NA |
- Immunosuppressive: Appears to be a relatively safe immunosuppressive treatment in the horse.
- Autoimmune skin diseases (e.g., pemphigus foliaceous): Used as a steroid-sparing drug. May cause thrombocytopenia.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Azathioprine is a potent purine antagonist immunosuppressive agent widely used in veterinary medicine, primarily in dogs, to treat a variety of immune-mediated and autoimmune diseases (e.g., immune-mediated hemolytic anemia [IMHA], inflammatory bowel disease [IBD], myasthenia gravis, and pemphigus).
Key Clinical Pearls:
- Steroid-Sparing Effect: It is frequently co-administered with corticosteroids (like prednisone or prednisolone). This combination allows for a gradual reduction in the corticosteroid dose, minimizing long-term steroid adverse effects while maintaining disease remission.
- Delayed Onset: Azathioprine is not fast-acting. It typically requires 2 to 6 weeks to achieve full immunosuppressive clinical efficacy. Therefore, it is not suitable as a sole agent for acute, life-threatening autoimmune crises.
- Species Sensitivity: Cats are exquisitely sensitive to azathioprine-induced bone marrow suppression due to naturally low levels of the enzyme thiopurine methyltransferase (TPMT). Its use in felines is highly controversial and generally avoided.
- Toxicity Risks: The most significant adverse effect is myelosuppression (leukopenia, anemia, thrombocytopenia). Hepatotoxicity and acute pancreatitis are also notable risks in dogs.
Mechanism of action
Azathioprine is a prodrug that must be metabolized to exert its effects.
- Pathway: Azathioprine is rapidly converted in the body (primarily in erythrocytes and the liver) to 6-mercaptopurine (6-MP).
- 6-MP is further metabolized into active thioguanine nucleotides (TGNs).
- Mechanism: These active metabolites act as purine antagonists. They are falsely incorporated into cellular DNA and RNA, which inhibits purine metabolism, RNA/DNA synthesis, and cellular mitosis.
- Immunological Effect: Because lymphocytes (T-cells and B-cells) lack a salvage pathway for purine synthesis and rely heavily on de novo synthesis, they are profoundly affected. This leads to a marked suppression of delayed hypersensitivity and cellular immunity, with a lesser effect on humoral antibody responses.
Safety & warnings
Contraindications
- Known hypersensitivity to azathioprine
- Cats (generally contraindicated due to severe, potentially fatal myelotoxicity)
Adverse effects
- Bone marrow suppression (leukopenia, anemia, thrombocytopenia)
- Gastrointestinal distress (vomiting, diarrhea, anorexia)
- Acute pancreatitis
- Hepatotoxicity
- Poor hair growth
- Increased susceptibility to secondary infections
- Increased risk of neoplastic illnesses with long-term use
Precautions
Black Box Warning Equivalent: Azathioprine is a known mutagen and teratogen. Pregnant women or those trying to conceive should avoid handling this medication. If handling is necessary, strict use of protective gloves is required.
- Hepatic Dysfunction: Use with extreme caution in patients with pre-existing liver disease, as hepatotoxicity is a known adverse effect.
- Thiopurine Methyltransferase (TPMT) Deficiency: Dogs have variable TPMT activity (similar to humans). Dogs with low TPMT activity are at a significantly higher risk for severe bone marrow suppression.
- Tapering: In recovering patients (e.g., IMHA), withdrawal of the drug must be tapered very slowly over several months. Rapid withdrawal can trigger a rebound hyperimmune response and fatal relapse.
- Pregnancy/Nursing: FDA Category D (human). Distributed into milk; use milk replacer if the dam is treated.
Drug interactions
Increased potential for hematologic toxicity
Decreases hepatic metabolism of azathioprine; significantly increases toxicity risk. Dose of azathioprine must be reduced to 1/4 to 1/3 of the usual dose if used concurrently.
Increased risk for azathioprine toxicity
Neuromuscular blocking activity may be inhibited or reversed by azathioprine
Often used together intentionally, but carries a greater potential risk for overall toxicity development
Increased potential for hematologic toxicity (additive bone marrow suppression)
Potential for reduced anticoagulant effect
Increased risk of azathioprine toxicity.
May increase the potential for haematological adverse events (e.g., severe anaemia or leucopenia).
Monitoring
- Hemograms (CBC including platelets): Monitor closely; initially every 1-2 weeks, then every 1-2 months on maintenance therapy. Reduce dose by 25% if WBC drops to 5,000-7,000 cells/mcL. Discontinue if WBC < 5,000 cells/mcL until resolved.
- Liver function tests (ALT, AST, ALP, Bilirubin)
- Serum amylase/lipase (if pancreatitis is suspected)
- Clinical efficacy and signs of secondary infection
Pharmacokinetics
Absorption
Poorly absorbed from the gastrointestinal tract.
Distribution
Rapidly taken up by lymphocytes and erythrocytes. Distributed into milk.
Metabolism
Rapidly metabolized to mercaptopurine (6-MP). Mercaptopurine is further metabolized to several other compounds via pathways including thiopurine methyltransferase (TPMT). Cats have naturally low TPMT activity, making them highly susceptible to toxicity.
Elimination
Metabolites are excreted primarily by the kidneys. Only minimal amounts of azathioprine or mercaptopurine are excreted unchanged.
Overdose
There is limited specific information regarding acute overdose of azathioprine in veterinary patients.
- Decontamination: If ingestion was recent, use standard protocols to empty the GI tract (emesis induction, activated charcoal).
- Treatment: Provide aggressive supportive care. Monitor hematologic parameters closely for delayed bone marrow suppression.
- Consultation: Contact an animal poison control center for the most up-to-date guidance.
Available products
Formulations
- Tablets
- Injection (powder for reconstitution)
- Compounded oral suspension
Human-labeled
- Azathioprine Tablets: 50 mg, 75 mg & 100 mg (Azasan®, Imuran®, generic)
- Azathioprine Sodium Injection: 100 mg (as sodium)/vial in 20 mL vials (generic)
Regulatory status
POM (Prescription Only Medicine). Handled under cascade using human formulations.
POM-V / POM. Handled under the prescribing cascade.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets should be stored at room temperature in well-closed containers and protected from light. Sodium powder for injection should be stored at room temperature and protected from light; after reconstitution, it must be used within 24 hours as it contains no preservatives. Compounded oral suspensions have optimal stability between pH 5.5 and 6.5 and should be protected from light.
Client information
Important Information for Pet Owners:
- Patience is Key: This medication takes time to work. It may take several weeks before you see an improvement in your pet's condition. Do not stop giving the medication without consulting your veterinarian.
- Safety & Handling: Azathioprine is a potent drug that can affect human DNA. Wash your hands thoroughly after handling the tablets. If you are pregnant, trying to become pregnant, or nursing, do not handle this medication or wear protective gloves if you must.
- Watch for Side Effects: Because this drug suppresses the immune system and can affect the bone marrow, your pet is at a higher risk for infections and bleeding.
- When to Call the Vet: Contact your veterinarian immediately if your pet shows signs of abnormal bleeding, unexplained bruising, pale gums, fever, loss of appetite, vomiting, diarrhea, or lethargy.
- Strict Monitoring: Your pet will need frequent blood tests (especially in the first few months) to ensure the drug is not damaging their bone marrow or liver. Keep all scheduled recheck appointments.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.