Azathioprine for Crohn's Disease: A 2026 Patient Guide

If your gastroenterologist has brought up azathioprine for Crohn's disease, you probably have a lot of questions - and maybe a few worries. Azathioprine is one of the oldest immunomodulators still in active use for Crohn's, and the conversation around it has shifted meaningfully in recent years. The 2025 ACG Crohn's guideline walked back its role as a standalone therapy, the 2025 CPIC update now requires testing two enzymes before you start (not just one), and a large Danish nationwide study has given us the clearest picture yet of its long-term cancer risk. This guide walks through all of it so you can have an informed conversation with your care team.

Key Takeaways

• The 2025 ACG guideline recommends against azathioprine and 6-mercaptopurine for inducing remission in moderate-to-severe Crohn's disease, but conditionally suggests them for maintenance after steroid-induced remission (1)
• The 2025 CPIC update recommends genotyping both TPMT and NUDT15 before starting any thiopurine - not just TPMT alone - because NUDT15 variants are common in East Asian, South Asian, and Hispanic populations (2)
• A Danish nationwide cohort of 43,419 IBD patients found cancer risk rose with cumulative thiopurine exposure (adjusted hazard ratio approximately 1.36 after 5+ years), but returned to baseline after discontinuation (3)
• Combination therapy with IV infliximab plus an immunomodulator outperforms infliximab monotherapy on clinical, endoscopic, and immunogenicity outcomes, making thiopurines a key biologic partner (4)
• Thiopurines take 3 to 6 months for full effect, so they are used for long-term maintenance rather than to rapidly quiet a flare (5)

What Azathioprine and 6-Mercaptopurine Are and How They Work

Azathioprine (brand names Imuran, Azasan) is a prodrug, meaning your body converts it into its active form - 6-mercaptopurine (brand names Purinethol, Purixan) - after you take it. Both medications belong to a class called thiopurines, which are immunomodulators that reduce the activity of T and B lymphocytes and dampen the production of inflammatory cytokines driving intestinal inflammation.

Thiopurines as a drug class

Unlike biologics such as infliximab or adalimumab, thiopurines are older, oral, small-molecule drugs. They work broadly on the immune system rather than targeting a single specific protein. Despite being available for decades, they remain widely prescribed - often alongside a biologic rather than on their own. As we discuss in our IBD medications comparison guide, thiopurines occupy a distinct role in the treatment landscape.

Why they are slow-acting immunomodulators

One thing that catches many patients off guard is how long thiopurines take to work. According to the Crohn's and Colitis Foundation, it can take 3 to 6 months before you feel the full effect (5). That timeline is why gastroenterologists use them for maintenance therapy - keeping you in remission over the long haul - rather than as a fast-acting option to settle an acute flare. They never replace prescribed biologics or a physician's overall treatment plan; they are one tool in a multi-modal approach.

Where Thiopurines Fit in the 2025 ACG Crohn's Guidelines

The 2025 American College of Gastroenterology Clinical Guideline for Management of Crohn's Disease in Adults reshaped how gastroenterologists think about thiopurines. Understanding the guideline's specific recommendations can help you make sense of what your doctor is suggesting and why.

Not recommended for induction of remission

The 2025 ACG guideline recommends against using azathioprine (1.5-2.5 mg/kg/day) and 6-mercaptopurine (0.75-1.5 mg/kg/day) for induction of remission in moderately-to-severely active Crohn's disease (1). This is a strong recommendation supported by moderate-quality evidence. In plain language: thiopurines are not the right choice to get an active flare under control quickly.

Suggested for maintenance and combination therapy

The same guideline conditionally suggests azathioprine and 6-mercaptopurine for maintenance of remission in patients who achieved remission with corticosteroids (1). This is their established sweet spot - helping you stay well after steroids have done the heavy lifting.

Their most important modern role, though, may be as a combination partner with an anti-TNF biologic. The historical SONIC trial established that infliximab plus azathioprine was superior to either drug alone for Crohn's induction. A 2026 systematic review and meta-analysis by Anjie and colleagues, published in Clinical Gastroenterology and Hepatology, confirmed that IV infliximab combined with an immunomodulator outperforms IV infliximab monotherapy on clinical, endoscopic, and immunogenicity outcomes in IBD (4). As we covered in our Humira (adalimumab) patient guide, combination therapy helps reduce the formation of anti-drug antibodies that can make biologics less effective over time.

TPMT and NUDT15 Testing Before You Start

Before prescribing a thiopurine, your gastroenterologist should order genetic testing for two specific enzymes. This is not optional - it is a safety step that can prevent a potentially life-threatening reaction.

What these two enzymes do

TPMT (thiopurine methyltransferase) and NUDT15 are enzymes your body uses to break down thiopurines. If you carry genetic variants that reduce the activity of either enzyme, standard doses can accumulate to dangerous levels and cause severe bone marrow suppression - a condition where your body stops making enough blood cells. The 2025 ACG guideline recommends TPMT testing before initial use of azathioprine or 6-mercaptopurine to help guide dosing (1).

2025 CPIC dosing recommendations

The 2025 update to the Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline, published in Clinical Pharmacology and Therapeutics in January 2026, takes this a step further by recommending genotyping of both TPMT and NUDT15 before starting any thiopurine (2). This matters because NUDT15 variants are particularly common in East Asian, South Asian, and Hispanic populations - testing TPMT alone would miss a meaningful proportion of patients at risk.

Here is how the CPIC dosing guidance breaks down in practical terms:

• Normal metabolizers receive the standard dose
• Intermediate metabolizers start at 30 to 80 percent of the standard dose
• Poor metabolizers require approximately a tenfold reduced dose, or their doctor may choose an alternative therapy entirely (2)

If your doctor has not mentioned TPMT and NUDT15 testing, it is entirely reasonable to ask about it before starting.

Monitoring While You Are on Thiopurines

Starting a thiopurine is not a set-it-and-forget-it situation. Ongoing monitoring is essential to catch problems early and to make sure the drug is working effectively.

Baseline and ongoing blood tests

Before your first dose, your care team will typically order a baseline workup that includes a complete blood count (CBC), liver function tests (LFTs), TPMT and NUDT15 genotyping, a review of your vaccination status, and TB screening. Once you start the medication, the standard monitoring schedule at most centers looks something like this:

• Weeks 1 to 4: CBC and LFTs weekly
• Weeks 5 to 12: CBC and LFTs every two weeks
• After 3 months: CBC and LFTs every 3 months, ongoing

Schedules may vary by center and by your individual risk factors, so follow whatever your gastroenterologist recommends. The Crohn's and Colitis Foundation has a plain-language overview of what this monitoring involves (5).

Metabolite testing (6-TGN and 6-MMP)

Your doctor may also check thiopurine metabolite levels. The two metabolites that matter are 6-thioguanine nucleotides (6-TGN), which reflect the active immunosuppressive effect of the drug, and 6-methylmercaptopurine (6-MMP), which at high levels can signal a risk of liver toxicity or that your body is shunting the drug down a less useful metabolic pathway.

Metabolite testing is not something most patients need routinely. It is most useful in specific situations: loss of response after a period of benefit, suspected non-adherence, or when your clinician is deciding whether to adjust the dose, add allopurinol to redirect metabolism, or switch to a different drug.

Side Effects, Infection Risk, and Cancer Risk

This is the section many patients are most anxious about, and rightly so. Thiopurines carry real risks, and understanding them clearly is the best way to make a shared decision with your care team.

Common and less serious side effects

The most frequently reported side effects include nausea, fatigue, headache, hair thinning, mouth sores, and mild elevations in liver enzymes (5). Many patients find that taking azathioprine with food helps with the nausea. These side effects are usually manageable and often improve over time.

Serious risks: pancreatitis, myelosuppression, infections, and cancer

Pancreatitis affects roughly 3 to 5 percent of people who start a thiopurine and usually appears within the first few weeks of treatment. Symptoms include severe upper abdominal pain, nausea, and vomiting. If this happens, the drug must be stopped - and thiopurines should not be retried.

Myelosuppression (bone marrow suppression) is the reason for all those blood tests. It can happen even with normal TPMT and NUDT15 results, which is why ongoing CBC monitoring matters. Catching a dropping white blood cell count early allows your doctor to adjust or stop the medication before you are at serious risk.

Infection risk increases because thiopurines suppress your immune system. Before starting, your vaccination schedule should be reviewed and brought up to date. Live vaccines - such as MMR, varicella, and the live shingles vaccine - are generally avoided while you are on the drug. As we covered in our vaccines guide for Crohn's disease patients, planning ahead on vaccines makes a real difference.

Cancer risk is the concern that generates the most anxiety. A Danish nationwide cohort study of 43,419 IBD patients followed from 1996 to 2018 found that thiopurine use was associated with an increased risk of cancer compared with unexposed IBD patients (3). The adjusted hazard ratio was positively associated with cumulative exposure and reached approximately 1.36 in patients with more than 5 years of use (3). There is a reassuring finding, however: in patients who discontinued thiopurines, the adjusted hazard ratios returned to the level of unexposed patients, suggesting the risk is at least partly reversible (3).

Non-melanoma skin cancer risk is consistently elevated with thiopurine use. Consistent sun protection - broad-spectrum SPF 30 or higher and sun-protective clothing - and regular dermatologic check-ups are recommended for anyone on long-term thiopurines.

Hepatosplenic T-cell lymphoma (HSTCL) deserves specific mention. This is a rare but often fatal type of lymphoma that has been seen mostly in young men on prolonged thiopurine therapy, particularly in combination with an anti-TNF biologic. While the absolute risk is very low, it does shape clinical decision-making, especially around whether to use thiopurines versus methotrexate as the combination partner with a biologic in younger male patients.

Putting risk in context: For most patients, the consequences of uncontrolled Crohn's disease - hospitalizations, surgeries, nutritional complications, lost quality of life - are substantial. The absolute risk of lymphoma on thiopurines remains low, and your gastroenterologist weighs these risks against the benefit of disease control when recommending a treatment plan.

Practical Questions Patients Ask

These are the everyday questions that come up after you leave the clinic.

Pregnancy, breastfeeding, and fertility

Thiopurines are generally considered compatible with pregnancy and breastfeeding, and they are often continued through pregnancy in patients with IBD. Active, uncontrolled disease during pregnancy carries its own risks - including preterm birth and low birth weight - so the decision to continue treatment is usually made together with a gastroenterologist and, when possible, a maternal-fetal medicine specialist.

Combination therapy, de-escalation, and switching to methotrexate

Some patients in stable deep remission on a biologic plus thiopurine combination may be able to drop the thiopurine after 1 to 2 years. This decision is individualized and based on relapse risk, biologic drug levels, and endoscopic status. It is not a one-size-fits-all call.

Switching from azathioprine to methotrexate makes sense in several situations: significant TPMT or NUDT15 variants, a prior severe reaction to a thiopurine, concern about HSTCL (particularly in young men), or a personal or family history of lymphoma.

Practical daily tips

• Take with food to reduce nausea
• Stay well hydrated throughout the day
• Use daily sun protection - broad-spectrum SPF 30 or higher plus sun-protective clothing
• Avoid live vaccines while on the drug (discuss alternatives with your care team)
• Never stop or change your dose without talking to your gastroenterologist first

Shared decision-making with your care team is what ties all of this together. Every patient's risk profile, disease severity, prior treatment history, and personal priorities are different. The role of azathioprine in your treatment plan should reflect your individual situation - not a generic recommendation.

Frequently Asked Questions

Is azathioprine used to treat a Crohn's flare?

No. The 2025 ACG guideline specifically recommends against azathioprine for induction of remission in moderate-to-severe Crohn's disease (1). It takes 3 to 6 months to reach full effect, so it is used for maintenance - keeping you in remission - rather than settling an active flare.

What is the difference between azathioprine and 6-mercaptopurine?

Azathioprine is a prodrug that your body converts into 6-mercaptopurine (6-MP). They are closely related, and both are thiopurines. Some patients who cannot tolerate azathioprine may do better on 6-MP directly, so your gastroenterologist may try switching if side effects are an issue.

Why do I need both TPMT and NUDT15 testing?

Both enzymes help your body break down thiopurines. If either enzyme's activity is reduced due to genetic variants, standard doses can cause dangerous bone marrow suppression. NUDT15 variants are particularly common in East Asian, South Asian, and Hispanic populations, which is why testing both enzymes - not just TPMT - is recommended by the 2025 CPIC guideline (2).

Does azathioprine cause cancer?

A large Danish cohort study found that cancer risk increased with cumulative thiopurine exposure, reaching an adjusted hazard ratio of about 1.36 after more than 5 years (3). However, the same study showed that risk returned to baseline levels after discontinuation. Your gastroenterologist weighs this risk against the benefit of controlling your Crohn's disease, and ongoing monitoring helps manage the balance.

Can I take azathioprine during pregnancy?

Thiopurines are generally considered compatible with pregnancy and breastfeeding and are often continued in patients with IBD to keep disease under control. Uncontrolled Crohn's during pregnancy also carries risks. This decision should be made with your gastroenterologist and, ideally, a maternal-fetal medicine specialist.

How often do I need blood tests while on azathioprine?

Most centers check a complete blood count and liver function tests weekly for the first month, every two weeks for the next two months, and then every three months thereafter. Your individual schedule may vary based on your risk profile and your doctor's preference.

Should I use azathioprine alone or with a biologic?

In modern practice, thiopurines are most commonly used alongside an anti-TNF biologic like infliximab rather than as monotherapy. A 2026 meta-analysis confirmed that combination therapy outperforms infliximab alone on clinical, endoscopic, and immunogenicity outcomes (4). Whether combination therapy is right for you depends on your disease severity and treatment history.

References

1. Lichtenstein GR, Loftus EV, Afzali A, et al. ACG Clinical Guideline: Management of Crohn's Disease in Adults. American Journal of Gastroenterology, 2025. Read guideline
2. Maillard M, Schwab M, Whirl-Carrillo M, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for Thiopurine Dosing Based on TPMT and NUDT15 Genotypes: 2025 Update. Clinical Pharmacology and Therapeutics, 2026. View on PubMed
3. Wewer MD, Letnar G, Andersen KK, et al. Thiopurines and the Risk of Cancer in Patients With Inflammatory Bowel Disease. Clinical Gastroenterology and Hepatology, 2025. View on PubMed
4. Anjie SI, Gecse KB, Meloni CM, et al. Immunogenicity and Efficacy of Subcutaneous Infliximab Monotherapy vs Combination Therapy in Inflammatory Bowel Disease: A Systematic Review and Meta-analysis. Clinical Gastroenterology and Hepatology, 2026. View on PubMed
5. Crohn's and Colitis Foundation. Mercaptopurine (6-MP) - IBD Education Tool. Read article