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: Its U.S. prescribing information contraindicates use with azathioprine. 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; febuxostat is contraindicated with azathioprine
  • Regular blood monitoring (CBC, LFTs) is required for the duration of treatment

Questions about your diagnosis? Contact our office

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

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 activity/genotype and NUDT15 genotype testing

Testing can identify patients at high risk of severe bone marrow suppression. TPMT can be assessed by enzyme activity or genotype; NUDT15 is generally assessed by genotype:

  • 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; reduced function increases the risk of severe myelosuppression.

The FDA label recommends considering TPMT and NUDT15 evaluation - particularly in patients who develop severe myelosuppression or in those at higher risk by ancestry. Many specialists arrange testing before starting. CPIC explains how available genotype results should guide dosing; its guideline does not determine whether testing should be ordered. 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 with platelets: the label recommends weekly for the first month, twice monthly during months 2 and 3, then monthly, or more often after dose or treatment changes
  • Liver function tests
  • Contact the prescribing team promptly for fever, sore throat, new infection, unusual bruising, or bleeding; blood counts may need checking before the next routine visit

Pregnancy and Breastfeeding

Discuss pregnancy plans with your prescribing specialist and obstetric clinician before treatment. The label warns of fetal harm, and azathioprine or its metabolites can enter breast milk. The decision to continue treatment or breastfeed needs an individual assessment of disease control and infant risk; do not stop or change the dose on your own.

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 an antimetabolite used to suppress the immune system. For your eye or neurologic condition, the goal is controlling harmful inflammation and reducing steroid exposure. It can still lower blood counts and increase infection and cancer risk, so laboratory monitoring matters.

References