Azathioprine: veterinary dose and monograph

At a glance

Drug class

Veterinary drug

Also known as

Imuran, Azoran, Azapure

Species with doses

Dogs, Cats, Horses and Ferrets

General dose range

1–2 mg/kg PO q24h then taper to q48h — check the species tables for the indication

Routes

PO

Last reviewed

1 October 2026

Indications

• Immune-mediated haemolytic anaemia (IMHA) — steroid-sparing adjunct
• Immune-mediated thrombocytopenia and polyarthritis
• Inflammatory bowel disease (refractory)
• Myasthenia gravis, immune-mediated dermatoses, and other immune-mediated diseases

Azathioprine dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Immune-mediated disease (steroid-sparing): Papich 5e: 2 mg/kg q24h PO initially, then 0.5–1 mg/kg q48h (up to 1.5 mg/kg q48h with prednisolone). Plumb’s p.135: 2 mg/kg once daily then alternate days with prednisolone (Papich 2001); IBD 2 mg/kg daily for 2 weeks → every other day → 1 mg/kg every other day; effect may take 2–6 weeks. CBC every 1–2 weeks initially, then monthly; stop if WBC <4000/µL or neutrophils <1000/µL. Hepatotoxicity possible.

Cats

• Refractory immune-mediated disease (generally avoided): Cats are very sensitive to fatal myelosuppression — most clinicians avoid azathioprine in cats. Papich 5e: start 0.3 mg/kg q24h, moving to q48h with careful monitoring. Plumb’s p.135 (Willard 2002): 0.3 mg/kg every other day for refractory IBD; compound accurately; check temperature and WBC immediately if the cat becomes ill.

Horses

• Immune-mediated disease (steroid-sparing): Papich 5e and Plumb’s p.135 (Divers 2010): 3 mg/kg PO q24h. Autoimmune skin disease 1–3 mg/kg q24h for a month then every other day (White 2006). May cause thrombocytopenia.

Ferrets

• Inflammatory bowel disease: Plumb’s p.135 (Johnson 2006): 0.9 mg/kg PO q24–72h with prednisone 1 mg/kg q12–24h and dietary management.

Contraindications

CATS — avoid (fatal myelosuppression; use chlorambucil instead). Pre-existing bone-marrow suppression or significant hepatic disease. Concurrent live vaccines. Pregnancy (teratogenic). Known hypersensitivity. Concurrent allopurinol (blocks metabolism → severe toxicity — major interaction).

Species-specific warnings

Cats: CONTRAINDICATED — cats have very low TPMT activity and develop severe, frequently fatal bone-marrow suppression. Use chlorambucil for feline immunosuppression instead.

Adverse effects

• Bone-marrow suppression (leukopenia, thrombocytopenia, anaemia) — dose-related; catastrophic in cats
• Hepatotoxicity
• GI upset (vomiting, diarrhoea, anorexia)
• Pancreatitis
• Increased infection risk (immunosuppression); delayed onset of effect

Drug interactions

• Allopurinol (xanthine-oxidase inhibitor): blocks azathioprine metabolism → severe, potentially fatal myelosuppression — avoid or drastically reduce dose
• Other myelosuppressive/immunosuppressive drugs: additive marrow suppression/infection risk
• ACE inhibitors: additive leukopenia (reported)
• Live vaccines: avoid during immunosuppression

Pregnancy and lactation

Teratogenic — avoid in pregnancy.

Monitoring

• CBC every 1–2 weeks initially, then periodically (marrow suppression — may be delayed)
• Liver enzymes and signs of pancreatitis
• Clinical response (allow weeks for onset) and infection surveillance
• NEVER co-administer with allopurinol without major dose reduction

Toxicity and overdose

The dose-limiting toxicities are bone-marrow suppression (can be delayed and severe), hepatotoxicity and pancreatitis. CATS (and dogs with low TPMT activity, e.g. some Giant Schnauzers) suffer profound, often fatal myelosuppression — hence the feline contraindication and the need for CBC monitoring in dogs. The allopurinol interaction can turn a normal dose into a lethal one.

Management: No specific antidote. Discontinue; intensive supportive care for myelosuppression (antibiotics for neutropenic sepsis, transfusion, ± G-CSF), and hepatoprotection for hepatotoxicity. Monitor CBC closely (nadir may be delayed). Decontaminate large recent ingestions.

Clinical pearls

• A steroid-sparing immunosuppressant for canine immune-mediated disease — but it works slowly (weeks), so it is added to glucocorticoids, not used alone acutely
• ABSOLUTELY avoid in cats (fatal myelosuppression — use chlorambucil instead)
• The allopurinol interaction is a classic, dangerous trap — together they can cause lethal marrow suppression
• Monitor the CBC religiously (delayed neutropenia) and the liver; handle tablets with gloves
• Once remission is achieved, taper to every-other-day and reduce the concurrent steroid

Mechanism of action

A pro-drug converted to 6-mercaptopurine and then to thioguanine nucleotides that are incorporated into DNA/RNA and inhibit purine synthesis — preferentially suppressing rapidly proliferating LYMPHOCYTES (especially T-cell-mediated responses). This produces delayed but durable immunosuppression. Metabolism via thiopurine methyltransferase (TPMT) and xanthine oxidase determines toxicity — low TPMT (as in cats and some dogs) causes severe myelotoxicity.

Formulations and products

• 25 mg tablet
• 50 mg tablet
• Imuran — 25 / 50 mg tablet, Tablet (human-label)
• Azoran — 25 / 50 mg tablet, Tablet (human-label)

Storage

Tablets: 15–30 °C, dry, protect from light; handle with gloves (cytotoxic/immunosuppressant) and avoid crushing.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other hormones, emergency drugs, supplements and other drugs

• Amitriptyline
• Atipamezole
• Atracurium
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• B-Complex Injection (B1 + B6 + B12)
• Bethanechol
• Bromhexine
• Budesonide

Frequently asked questions