Mercaptopurine is a chemotherapy drug used primarily to treat acute lymphoblastic leukemia (ALL), the most common childhood cancer. It is also widely prescribed off-label to manage inflammatory bowel diseases like Crohn’s disease and ulcerative colitis. The drug works by interfering with the body’s ability to build new DNA, which slows the growth of rapidly dividing cells, whether cancerous or immune-related.
Leukemia Maintenance Therapy
The FDA-approved use of mercaptopurine is as part of a combination chemotherapy maintenance regimen for ALL. After the initial, more intensive phases of treatment bring leukemia into remission, maintenance therapy keeps it there. This phase typically lasts two to three years and involves taking mercaptopurine daily alongside weekly low-dose methotrexate, with occasional additions of other chemotherapy agents.
Maintenance therapy is a critical piece of ALL treatment. Skipping doses or taking mercaptopurine inconsistently during this phase is associated with a significant risk of relapse, according to the National Cancer Institute. For children with ALL, survival rates have improved dramatically over the past several decades, and mercaptopurine-based maintenance is a core reason why. In one Dutch study, extending the maintenance period by a full year for a specific high-risk genetic subgroup improved the two-year event-free survival rate from about 74% to 91%.
Inflammatory Bowel Disease
Outside of cancer treatment, mercaptopurine is one of the most commonly prescribed drugs for ulcerative colitis and Crohn’s disease. In many countries, it remains a preferred first-line option for patients who need something stronger than standard anti-inflammatory medications to keep their disease in check. It belongs to a class of drugs called thiopurines, which dampen the overactive immune response that drives IBD.
A Cochrane review of five placebo-controlled studies found that thiopurines like mercaptopurine reduce the risk of relapse in ulcerative colitis. Among patients taking a thiopurine, 45% experienced a return of symptoms, compared to 67% of those on placebo. One small trial also suggested mercaptopurine outperformed a standard anti-inflammatory (5-aminosalicylate) at maintaining remission, though the study was too small to draw firm conclusions. The evidence is categorized as low-certainty, but decades of clinical use support its role when other treatments fall short.
How Mercaptopurine Works
Your body constantly builds purines, which are essential building blocks of DNA and RNA. Mercaptopurine mimics these natural molecules closely enough to sneak into the production process, but once inside, it jams the machinery. Specifically, it blocks an early step in purine synthesis, preventing cells from assembling the raw materials they need to copy their genetic code and divide. Cancer cells and overactive immune cells divide rapidly, making them especially vulnerable to this disruption.
Genetic Testing Before Starting Treatment
Not everyone processes mercaptopurine the same way, and the differences can be dangerous. Two enzymes, known as TPMT and NUDT15, play major roles in how your body breaks down the drug. Variations in the genes coding for these enzymes can cause the active drug to build up to toxic levels, leading to severe drops in blood cell counts.
TPMT gene variants are the primary cause of mercaptopurine intolerance in people of European and African descent. In Asian and Hispanic populations, NUDT15 variants are more common and more clinically significant. About 1 in every 50 people of East Asian descent carries a NUDT15 variant that causes poor metabolism of the drug, a rate actually higher than TPMT-related poor metabolism in Europeans. Clinical guidelines from the Clinical Pharmacogenetics Implementation Consortium recommend genetic testing for both TPMT and NUDT15 before starting therapy, with dose reductions for patients who carry risk variants.
Patients with certain TPMT variants may actually have better cancer outcomes because the drug stays active longer in their bodies. But that same trait increases the risk of serious side effects, including infections and secondary cancers. This is why testing matters: it allows doctors to find the dose that maximizes benefit while keeping toxicity manageable.
Side Effects and Monitoring
The most significant risks of mercaptopurine are bone marrow suppression and liver damage. Because the drug slows cell division broadly, it can reduce production of white blood cells, red blood cells, and platelets. This makes infections, anemia, and bleeding more likely. Liver toxicity can also develop, sometimes without obvious symptoms early on.
For these reasons, regular blood testing is essential throughout treatment. Once a patient is on a stable dose, guidelines recommend a complete blood count and liver function tests at least every three months. During the early weeks of treatment or after dose changes, monitoring is typically more frequent.
A Critical Drug Interaction
One of the most dangerous drug interactions with mercaptopurine involves allopurinol, a common medication for gout. Allopurinol blocks an enzyme called xanthine oxidase, which is responsible for breaking mercaptopurine down into inactive forms. When both drugs are taken together, active mercaptopurine accumulates in the body and can cause life-threatening bone marrow suppression, including dangerously low white blood cell and platelet counts.
If the combination is absolutely necessary, the mercaptopurine dose must be cut to 25% of the usual amount, with close blood count monitoring. But in general, the two drugs are not recommended together.
How to Take It
Mercaptopurine comes as a tablet or an oral suspension. Food reduces how much of the drug your body absorbs, so consistency matters more than the specific choice. If you take it with food, always take it with food. If you take it on an empty stomach, stick with that. Most treatment protocols call for a daily dose, often taken at bedtime, with dosing based on body surface area. The typical range for leukemia maintenance falls between 25 and 75 mg per square meter of body surface, given in cycles that can last anywhere from a week to nearly three months depending on the treatment plan.