# Azathioprine: veterinary dose and monograph

> Azathioprine is a veterinary drug. A purine-analogue IMMUNOSUPPRESSANT used in DOGS as a steroid-sparing second agent for immune-mediated and inflammatory diseases — immune-mediated haemolytic anaemia (IMHA), immune-mediated thrombocytopenia, inflammatory bowel disease… Indications include immune-mediated haemolytic anaemia (IMHA) — steroid-sparing adjunct; immune-mediated thrombocytopenia and polyarthritis; inflammatory bowel disease (refractory). This monograph lists 4 reference doses for dogs, cats, horses and ferrets, each with route, frequency and source.

Source: https://vetmasterji.com/drugs/azathioprine · Last reviewed: 2026-10-01

## 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

*Azathioprine doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Immune-mediated disease (steroid-sparing) | 1–2 mg/kg | PO | q24h initially, then q48h | — |

- 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

*Azathioprine doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Refractory immune-mediated disease (generally avoided) | 0.3 mg/kg | PO | q24h → q48h (Plumb’s: q48h) | — |

- 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

*Azathioprine doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Immune-mediated disease (steroid-sparing) | 1–3 mg/kg | PO | q24h for 1 month, then q48h | — |

- 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

*Azathioprine doses for ferrets*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Inflammatory bowel disease | 0.9 mg/kg | PO | q24–72h | — |

- 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

> **Caution:** 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

> **Caution:** 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.

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## Frequently asked questions

### What is the dose of Azathioprine for dogs?

Reference doses for Azathioprine in dogs: 1–2 mg/kg PO q24h initially, then q48h (Immune-mediated disease (steroid-sparing)). Confirm against the label and the patient before use.

### What is the dose of Azathioprine for cats?

Reference doses for Azathioprine in cats: 0.3 mg/kg PO q24h → q48h (Plumb’s: q48h) (Refractory immune-mediated disease (generally avoided)). Confirm against the label and the patient before use.

### What is the dose of Azathioprine for horses?

Reference doses for Azathioprine in horses: 1–3 mg/kg PO q24h for 1 month, then q48h (Immune-mediated disease (steroid-sparing)). Confirm against the label and the patient before use.

### When should Azathioprine not be used?

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).

### What are the side effects of Azathioprine in animals?

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.

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.