Azathioprine (Imuran)

A steroid-sparing immunosuppressant for autoimmune eye disease; carries an FDA Boxed Warning for malignancy. Many prescribers test TPMT and NUDT15 before starting to flag patients at high risk of severe myelosuppression.

Drug Class: Immunosuppressants

4 min read

Azathioprine (brand name Imuran) is an immunosuppressant medication that reduces immune system activity. It is often used as a steroid-sparing agent, allowing patients to reduce their steroid dose while maintaining disease control.

FDA Boxed Warning - malignancy

Long-term immunosuppression with azathioprine increases the risk of malignancy, including lymphoma, skin cancers, and (in young patients with inflammatory bowel disease combined with anti-TNF therapy) hepatosplenic T-cell lymphoma. Use the lowest effective dose for the shortest time appropriate, and follow these precautions:

Sun protection and annual skin checks for non-melanoma skin cancer
Age-appropriate cancer screening as recommended by your primary care clinician
Report any persistent swollen lymph nodes, unexplained weight loss, fevers, or night sweats

Critical drug interaction - allopurinol / febuxostat

Allopurinol and febuxostat inhibit a pathway that breaks down azathioprine and can cause life-threatening bone marrow suppression when combined with it. Their prescribing precautions differ:

Allopurinol: If the combination is necessary, the prescriber typically reduces azathioprine to approximately one-quarter to one-third of the usual dose, with close blood-count monitoring. Some patients need a further reduction or an alternative treatment.
Febuxostat: Concomitant use with azathioprine is not recommended. The allopurinol dose-reduction approach should not be applied to febuxostat.

Tell every clinician and pharmacist about all your medicines. If both drugs have been prescribed, contact the prescriber promptly for a medication review; do not adjust doses yourself.

Key takeaways

Steroid-sparing immunosuppressant that takes 3-6 months for full effect
FDA boxed warning for malignancy (lymphoma, skin cancer)
TPMT and NUDT15 testing is commonly done before starting to flag high-risk patients; the FDA label recommends considering it (and a known deficient genotype mandates dose change). It does not replace ongoing CBC monitoring.
Allopurinol requires a prescriber-directed azathioprine dose reduction and close monitoring; concomitant febuxostat is not recommended
Regular blood monitoring (CBC, LFTs) is required for the duration of treatment

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How It Works

Azathioprine is converted to active metabolites that interfere with DNA synthesis, particularly in rapidly dividing cells like immune cells. This reduces immune activity and inflammation.

Common Uses

Myasthenia gravis
NMO and MOGAD
Giant cell arteritis: steroid-sparing
Other autoimmune conditions

Dosing

Usually 1-3 mg/kg daily
Start low, increase gradually
Takes 3-6 months for full effect
TPMT testing recommended before starting

TPMT and NUDT15 Testing

TPMT and NUDT15 enzyme testing

Two genetic tests are commonly used to flag patients at high risk of severe bone marrow suppression on azathioprine:

TPMT (thiopurine S-methyltransferase) - low or absent activity is more common in patients of European ancestry; can cause severe bone marrow suppression at standard doses.
NUDT15 - variants are more common in patients of East Asian, South Asian, and Hispanic ancestry; predicts severe myelosuppression and is a CPIC Tier 1 actionable variant.

The FDA label recommends considering TPMT and NUDT15 evaluation - particularly in patients who develop severe myelosuppression or in those at higher risk by ancestry. CPIC and many specialists recommend testing before starting. Patients known to be deficient in either enzyme need substantial dose reduction or an alternative drug. Genetic testing does not replace ongoing CBC monitoring.

Side Effects

Common

Nausea, vomiting
Diarrhea
Fatigue
Increased infection risk

Serious (Monitor For)

Bone marrow suppression (low blood counts)
Liver problems
Increased cancer risk (long-term)
Severe infections

Monitoring

Complete blood count-regularly (weekly initially, then monthly)
Liver function tests
Watch for signs of infection
Report unusual bruising or bleeding

Frequently Asked Questions

Why does it take so long to work?

Azathioprine works by gradually reducing the immune cells that cause your condition. This process takes time: usually 3 to 6 months for significant effect.

Can I stop my steroids once I start this?

Not immediately. Azathioprine takes time to work. Your doctor will gradually taper steroids once azathioprine is effective.

Is this the same as chemotherapy?

Azathioprine is used in cancer treatment at higher doses, but for autoimmune disease, doses are lower. It still requires careful monitoring.

References

DailyMed. Azathioprine Tablets, USP: prescribing information. See Precautions: Drug Interactions for allopurinol and febuxostat.

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