Azathioprine
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Dosing and administration[3]
Oral. Check TPMT (and where available NUDT15) activity before starting; low or absent activity markedly increases myelosuppression risk and requires dose reduction or avoidance.
| Indication | Starting dose | Titration and maximum |
|---|---|---|
| Immunosuppression (lupus, vasculitis, steroid-sparing) | Start low (for example 50 mg/day) and titrate to a usual dose of 1 to 3 mg/kg/day (commonly 2 to 2.5 mg/kg/day). | Adjust to response and tolerability; reduce in renal impairment and per TPMT status. |
Renal adjustment. Reduce the dose in renal impairment.
Check TPMT (and NUDT15 where available) before starting; and never co-prescribe with allopurinol or febuxostat without a major dose reduction or avoidance, as fatal myelosuppression can result.
Monitoring
- CBC frequently at initiation (for example weekly for the first weeks) then at least monthly; liver function periodically.
- Watch for myelosuppression, hepatotoxicity, and infection; long-term risk of certain malignancies.
Contraindications
- Hypersensitivity to azathioprine.
- Concurrent allopurinol or febuxostat without a major azathioprine dose reduction (see interactions).
- Serious active infection.
Key interactions
- Allopurinol and febuxostat inhibit xanthine oxidase and markedly increase azathioprine toxicity: reduce azathioprine to about 25% of the dose (with allopurinol) or avoid the combination (febuxostat).
- ACE inhibitors: additive anemia and leucopenia.
- Warfarin: effect may be altered.
- Avoid live vaccines.
Clinical notes
How it is used in lupus
Used across SLE manifestations as a steroid-sparing immunosuppressive, and preferred when pregnancy is planned. In lupus nephritis it is pregnancy-compatible and used when the disease is in remission but ongoing treatment is required.[1, 2]
Common side effects
Gastrointestinal intolerance, cytopenias, and infection risk occur; transaminase elevation is less common. Described from background clinical knowledge; reconcile with the product monograph.
Tests and monitoring
TPMT genotyping, and NUDT15 if available, prior to initiation. CBC with differential, AST, and ALT every 2 weeks for 8 weeks, then every 2 months. Use caution with allopurinol, which increases azathioprine levels.[1]
Important cautions
Pregnancy-compatible when ongoing treatment is required, and generally continued under specialist care; full reproductive-health guidance is maintained on the pregnancy record. Avoid co-administration issues with allopurinol.[2]
References
- Sammaritano LR, Askanase A, Bermas BL, et al. 2025 American College of Rheumatology (ACR) Guideline for the Treatment of Systemic Lupus Erythematosus. Arthritis Care Res (Hoboken). 2025. doi:10.1002/acr.25690 https://doi.org/10.1002/acr.25690
- Sammaritano LR, Askanase A, Bermas BL, et al. 2024 American College of Rheumatology (ACR) Guideline for the Screening, Treatment, and Management of Lupus Nephritis. Arthritis Care Res (Hoboken). 2025;77(9):1045-1065. doi:10.1002/acr.25528 https://doi.org/10.1002/acr.25528
- IMURAN (azathioprine) Product Monograph. Health Canada authorized product monograph. https://health-products.canada.ca/dpd-bdpp/