Thiopurine: Medical Uses, Side Effects, and Key Risks

Thiopurines are a class of immunosuppressive drugs used to treat conditions ranging from inflammatory bowel disease and childhood leukemia to autoimmune hepatitis and organ transplant rejection. The three drugs in the family are mercaptopurine (6-MP), azathioprine, and thioguanine, all developed in the 1950s and still widely prescribed today. They work by disrupting the immune system’s ability to sustain inflammation, but that same suppressive power creates a set of risks that require careful monitoring, genetic screening, and regular blood work for anyone taking them long-term.

How Thiopurines Work

Once you swallow azathioprine, your liver rapidly converts it into mercaptopurine, which is then broken down into several active and inactive products.¹ The key active product is a molecule called 6-thioguanine nucleotide (6-TGN), which gets incorporated into the DNA of rapidly dividing immune cells. This alone can slow immune cell growth, but thiopurines also target a specific signaling molecule inside T cells called Rac1. One of the drug’s metabolites acts as a decoy that blocks Rac1 from doing its job, which converts what would normally be an activation signal into a self-destruct signal, pushing the T cell toward programmed death.² The net effect is fewer aggressive immune cells driving inflammation in the gut, joints, liver, or wherever the autoimmune process is happening.

Mercaptopurine doesn’t follow just one metabolic path. An enzyme called xanthine oxidase (XO) breaks some of it down into an inactive waste product, while another enzyme called thiopurine methyltransferase (TPMT) shunts some of it toward a different metabolite called 6-methylmercaptopurine (6-MMP). Only the fraction that escapes both of those routes ends up as the therapeutic 6-TGN. This three-way split is why the same dose can behave so differently from person to person, and why genetics matter so much in thiopurine prescribing.

Where Thiopurines Are Used

The broadest use of thiopurines is in inflammatory bowel disease (IBD), covering both Crohn’s disease and ulcerative colitis. They are typically prescribed as maintenance therapy to keep people in remission after an initial flare has been brought under control, often by corticosteroids. A modeling study found that azathioprine therapy in Crohn’s disease reduces the death rate from the disease itself from roughly 53 per 100,000 person-years down to about 20, a meaningful benefit that outweighs the drug’s own risks for most patients.³

In childhood acute lymphoblastic leukemia (ALL), mercaptopurine combined with methotrexate forms the backbone of maintenance chemotherapy, which typically continues for two to three years after the initial intensive treatment phases. The benefit of this prolonged maintenance period is well proven, though there is still no international consensus on exactly how to dose these drugs, because individual patients metabolize them so differently.⁴

Thiopurines also have a well-established role in autoimmune hepatitis, a condition where the immune system attacks the liver. A randomized controlled trial demonstrated that once corticosteroids have induced remission, azathioprine alone can maintain that remission in most patients, sparing them the side effects of long-term steroid use.⁵ Azathioprine is also prescribed after organ transplantation to prevent rejection, and in various other autoimmune conditions like lupus, vasculitis, and certain skin diseases such as bullous pemphigoid.

Genetic Testing Before You Start

One of the most important developments in thiopurine prescribing is the recognition that your genes can dramatically affect how you handle these drugs. Two enzymes in particular have well-documented genetic variation that matters clinically.

The first is TPMT. About one in 300 people has very low or absent TPMT activity because they carry two defective copies of the gene. If these individuals receive a standard dose of a thiopurine, the drug accumulates to dangerous levels in blood-forming cells, potentially causing fatal bone marrow suppression.⁶ A larger group, roughly one in ten people, carries one defective copy and has intermediate activity. These individuals can usually tolerate thiopurines, but often at a reduced dose. Most prescribers now test TPMT status before writing the first prescription.

The second gene, NUDT15, was identified more recently and is especially relevant for people of East Asian and South American (with Native American ancestry) descent. In East Asian populations, about 20% carry one copy of the variant (C/T), and roughly 1% carry two copies (T/T). Early severe drops in white blood cells occurred in fewer than 3% of people without the variant, around 20% of those with one copy, and in nearly all patients with two copies.⁷ Additional variants beyond the best-known one have also been linked to high rates of white blood cell suppression, with one study finding that certain variant carriers experienced white blood cell drops at rates of 58% and 100% compared with about 16% in people without the variant.⁸ NUDT15-informed preemptive dose reduction has been quickly adopted into clinical practice in many settings.⁹

The practical takeaway: if your doctor does not mention TPMT and NUDT15 testing before starting you on a thiopurine, ask about it. Getting these results first can prevent a dangerous adverse reaction within the first weeks of treatment.

Bone Marrow Suppression

The most feared short-term side effect of thiopurines is myelosuppression, a drop in your blood cell counts caused by the drug’s effect on the bone marrow. Because thiopurines target rapidly dividing cells, the bone marrow, which churns out red cells, white cells, and platelets constantly, is vulnerable. Thiopurines can induce life-threatening myelosuppression, and this risk persists throughout the entire duration of therapy, not just in the early weeks.¹⁰

Myelosuppression typically shows up as a falling white blood cell count, sometimes accompanied by drops in platelets or red cells. The risk is partly dose-dependent, meaning higher drug levels produce greater marrow suppression, but idiosyncratic reactions also occur where the bone marrow fails unpredictably at standard doses.¹¹ This is why regular blood count monitoring, usually every few months once the dose is stable, is non-negotiable for anyone on a thiopurine. If your white blood cells drop too low, you become vulnerable to serious infections that a healthy immune system would handle easily.

Liver Damage and Other Acute Side Effects

Thiopurines can damage the liver, though estimates of how common this is vary widely. Reported rates of azathioprine-induced liver toxicity range from less than 1% to 10%, depending on the study and patient population. One prospective study found the rate to be about 10%, with a typical onset around six months into therapy, while a systematic review of IBD patients put the figure closer to 1% per year of treatment.¹² The discrepancy likely reflects differences in how liver injury was defined and how actively it was looked for.

Beyond liver problems, common side effects in the first weeks include nausea, vomiting, and general malaise. Some people develop pancreatitis, an inflammation of the pancreas that causes severe abdominal pain. Pancreatitis is typically an allergic-type reaction unrelated to drug levels, and it means the drug has to be stopped permanently; rechallenge almost always triggers the same reaction. Flu-like symptoms, fever, and rash can also occur and usually signal intolerance rather than a dose-dependent problem.

Skin Cancer and Lymphoma Risks

Long-term thiopurine use is associated with a higher risk of certain cancers, which is perhaps the most anxiety-inducing aspect of these drugs for patients. The best-documented link is with non-melanoma skin cancer. In a cohort of IBD patients, thiopurine exposure was associated with a fivefold increase in non-melanoma skin cancer risk overall, and in Caucasian patients specifically, the risk was more than twelve times higher.¹³ This is thought to be related to the drug’s ability to make skin cells more sensitive to ultraviolet radiation. Patients on thiopurines are typically advised to use sunscreen rigorously, wear protective clothing, and get annual skin checks.

Lymphoma risk is also elevated but remains small in absolute terms. Modeling for Crohn’s disease estimated that azathioprine increases lymphoma incidence from about 11 per 100,000 person-years to about 30 per 100,000 person-years.³ To put that in perspective, the same analysis found that the reduction in Crohn’s-related death more than compensated for the added lymphoma risk, producing a net gain in quality-adjusted life expectancy. The risk-benefit math is less clear for conditions where the underlying disease is less dangerous, and this is one reason thiopurines tend to be reserved for conditions where the immune-driven damage is significant.

Infection Risks

Because thiopurines suppress immune function, they increase vulnerability to certain infections, particularly viral infections that a fully functioning immune system keeps in check. Cytomegalovirus (CMV) is one concern; in immunosuppressed patients, a CMV infection can become systemic, affecting the gut, the central nervous system, and blood cell production.¹⁴

Epstein-Barr virus (EBV), the virus behind mononucleosis, is another area of vigilance, especially in children and young adults who may not have encountered EBV before starting the drug. When new EBV infection occurs in a patient on thiopurines, the usual practice is to suspend the drug until the viral load clears.¹⁵ Checking a patient’s EBV status before starting thiopurines has become increasingly common, particularly in pediatric IBD clinics, because EBV-naïve patients on thiopurines face a higher risk of a rare but serious condition called EBV-driven lymphoproliferative disease.

Therapeutic Drug Monitoring

Unlike many medications where you simply take a fixed dose and hope for the best, thiopurines lend themselves to measurement-based dose adjustment. Doctors can measure the levels of the active metabolite (6-TGN) and the potentially hepatotoxic metabolite (6-MMP) in red blood cells to guide dosing. Studies have established that 6-TGN levels in a certain therapeutic range are associated with a higher chance of clinical response, while levels below that range suggest the drug isn’t working and levels above it raise the risk of marrow suppression.¹⁶ High 6-MMP levels, meanwhile, signal a risk of liver toxicity.

Therapeutic drug monitoring is especially useful in two situations. First, when a patient does not seem to be responding to the drug, metabolite levels can distinguish between genuinely not working (adequate 6-TGN but no improvement), under-dosing (low 6-TGN suggesting the dose needs to go up), and non-adherence (undetectable metabolites suggesting the patient isn’t taking the drug).¹⁷ Second, serial monitoring over time with dose adjustments aimed at maintaining adequate 6-TGN levels may improve long-term outcomes in IBD patients.¹⁸

There is a specific metabolic pattern that frustrates clinicians: some patients shunt most of the drug toward 6-MMP rather than 6-TGN, producing high hepatotoxic metabolites and low therapeutic ones. These “hypermethylators” get all the side effects and none of the benefit at standard doses, and simple dose increases just make the imbalance worse. This pattern is where the allopurinol combination strategy becomes relevant.

The Allopurinol Combination

Allopurinol is a drug most people associate with gout treatment. It works by blocking xanthine oxidase, one of the enzymes that breaks down mercaptopurine. When given alongside a thiopurine, allopurinol redirects the drug’s metabolism away from the 6-MMP pathway and toward the therapeutic 6-TGN pathway. This can rescue patients who are hypermethylators, turning a failing regimen into a successful one.

The catch is that blocking xanthine oxidase also slows down overall drug clearance, so the thiopurine dose must be cut drastically — typically to about 25 to 33% of the original dose — when allopurinol is added. Allopurinol is usually started at a low dose of 100 mg daily.¹⁹ This combination has been shown to help patients bypass many adverse drug reactions, not just liver-related ones, and represents the largest reported experience of combination thiopurine-allopurinol therapy in IBD.²⁰ If you have been told your thiopurine metabolites show a skewed pattern, asking about allopurinol co-therapy is reasonable.

Combination With Biologic Drugs

In IBD treatment, thiopurines are increasingly used not alone but alongside biologic medications like infliximab, a drug that blocks a key inflammatory molecule called TNF. The landmark SONIC trial in Crohn’s disease found that the combination of infliximab plus azathioprine produced corticosteroid-free remission in about 57% of patients at 26 weeks, compared with roughly 44% on infliximab alone and 30% on azathioprine alone.²¹ Mucosal healing rates followed a similar pattern. Serious infection rates were comparable across all three groups, with the combination group actually trending lowest.

Broader population data have reinforced these findings, showing that combination therapy reduces the risk of treatment failure compared with anti-TNF therapy alone in both Crohn’s disease and ulcerative colitis.²² One reason for the benefit is that thiopurines help reduce the body’s tendency to develop antibodies against biologic drugs, which is a common reason biologics stop working over time. In practice, many gastroenterologists start both drugs together in patients with moderate-to-severe disease, then consider dropping the thiopurine after a year or two once the biologic has established deep remission.

Stopping Thiopurines

After years of steady remission, it is natural to wonder whether you still need the drug. The answer depends on the disease and individual risk factors. A study of 237 IBD patients who stopped thiopurines during sustained remission found that moderate or severe relapse occurred in about 23% of Crohn’s patients and 12% of ulcerative colitis patients within the first year, rising to 39% and 26%, respectively, by two years.²³ Crohn’s patients relapsed faster than those with ulcerative colitis. Elevated inflammatory markers at the time of withdrawal predicted relapse, suggesting that stopping is safest when inflammation is truly quiet by blood tests and not just by symptoms.

Reassuringly, most patients who relapsed after stopping were able to recapture remission by restarting the drug. Still, the relapse rates are high enough that most doctors approach withdrawal carefully, weighing the cumulative cancer risk of continued therapy against the chance of a flare and the patient’s individual disease course.

Pregnancy and Breastfeeding

Whether to continue thiopurines during pregnancy and breastfeeding is a question that comes up frequently for women with IBD or autoimmune hepatitis. Major gastroenterology guidelines generally consider azathioprine and mercaptopurine acceptable during pregnancy, on the logic that uncontrolled disease flares pose a greater risk to the pregnancy than the drug itself. However, every pregnancy involves a discussion of individual risk.

Breastfeeding while on azathioprine has been studied in small groups, and the findings are reassuring though limited. In one study monitoring breastfed infants of mothers taking azathioprine, blood counts in the babies remained within normal ranges for the most part. One infant showed a slightly low neutrophil count at six weeks that subsequently recovered, and another had a marginally low total white cell count, but all babies remained clinically well with no increased infection rates.²⁴ These data are encouraging, but the sample sizes are small enough that careful infant monitoring is still recommended during breastfeeding on thiopurines.

Vaccinations on Thiopurine Therapy

A common worry is that immunosuppressive drugs will render vaccines useless. For inactivated (non-live) vaccines, the concern appears to be overblown. A study of IBD patients treated with thiopurines found that they mounted a significant antibody response to the Haemophilus influenzae type b (Hib) vaccine, with no evidence of blunted immune response even at higher drug doses.²⁵ Patients who had been on thiopurines for months responded just as well as those who were treatment-naïve.

Live vaccines are a different story. Because thiopurines suppress the immune system, live vaccines such as the measles-mumps-rubella (MMR) and varicella vaccines carry a theoretical risk of causing the very infections they are meant to prevent. Current guidelines recommend avoiding live vaccines while on thiopurines and, ideally, making sure live vaccine schedules are up to date before starting therapy. For flu shots, COVID vaccines, pneumococcal vaccines, and other inactivated vaccines, patients on thiopurines should follow the standard schedules and can expect a functional immune response.

The Origin of the Drug Class

Thiopurines have an unusually distinguished pedigree for a class of drugs that many patients have never heard of. Mercaptopurine was discovered in 1951 by Gertrude Elion as a treatment for acute leukemia, building on earlier work with thioguanine in 1950. Azathioprine followed in 1957 as a prodrug designed to improve on mercaptopurine’s pharmacokinetics. In 1988, Elion was awarded the Nobel Prize in Physiology or Medicine, shared with George Hitchings and James Black, for her work on thiopurines and other antimetabolite drugs.²⁶ Seven decades later, these drugs remain first-line or second-line treatments across multiple specialties, a testament to how fundamental the underlying biology is, even as newer biologic therapies continue to expand the treatment landscape.