Mercaptopurine is a chemotherapy medication used primarily to treat acute lymphoblastic leukemia (ALL), the most common childhood cancer. It works by mimicking one of the building blocks cells need to copy their DNA, effectively starving fast-growing cancer cells of the raw materials they need to multiply. Beyond cancer treatment, mercaptopurine is also widely used to manage inflammatory bowel diseases like Crohn’s disease and ulcerative colitis.
Primary Use: Acute Lymphoblastic Leukemia
Mercaptopurine’s main approved use is as part of maintenance chemotherapy for ALL. After initial rounds of intensive treatment push the leukemia into remission, mercaptopurine is taken daily by mouth, often for two to three years, to keep the cancer from returning. It’s used in both children and adults as part of a combination regimen with other chemotherapy drugs, not as a standalone treatment.
The standard starting dose is weight-based, typically 1.5 to 2.5 mg per kilogram of body weight taken once daily. Throughout treatment, your care team will adjust the dose based on blood test results, specifically keeping your white blood cell counts in a target range that balances cancer suppression against the risk of dropping too low.
Use in Inflammatory Bowel Disease
Mercaptopurine has been used to treat ulcerative colitis and Crohn’s disease since the early 1960s, though this remains technically off-label. It serves as a steroid-sparing therapy, meaning it helps people who needed corticosteroids to control a flare stay in remission without staying on steroids long-term. This matters because long-term steroid use carries its own serious side effects.
A randomized trial published in the Journal of Crohn’s & Colitis found that optimized mercaptopurine treatment was superior to placebo in achieving clinical remission, endoscopic healing, and tissue-level improvement at one year in ulcerative colitis patients who hadn’t responded to first-line medications. The researchers concluded that thiopurines remain a valid treatment option even as newer, more expensive biologic therapies have entered the market. When used for IBD, doctors often monitor drug metabolite levels in the blood to fine-tune the dose for each patient, an approach called therapeutic drug monitoring.
How Mercaptopurine Works
Your cells need purines, a type of molecular building block, to make new DNA every time they divide. Mercaptopurine is a synthetic imposter that closely resembles these natural purines. Once you swallow it, your body converts it into active compounds that get incorporated into DNA and RNA in place of the real thing. The result is defective genetic material that cells can’t use, which halts cell division and eventually kills the cell.
This mechanism hits fast-dividing cells hardest, which is why it’s effective against leukemia cells and overactive immune cells. But it also explains most of its side effects: healthy cells that divide quickly, like those in bone marrow and the gut lining, get caught in the crossfire.
Genetic Testing Before Starting Treatment
Not everyone processes mercaptopurine at the same speed. Two enzymes in particular, called TPMT and NUDT15, play a major role in how your body breaks down the drug. Some people carry gene variants that make these enzymes less active or completely nonfunctional. For those individuals, a standard dose can build up to dangerously high levels, causing severe bone marrow suppression.
Clinical guidelines from the Clinical Pharmacogenetics Implementation Consortium (CPIC) recommend testing for TPMT and NUDT15 gene variants before starting mercaptopurine. People with reduced enzyme activity need lower doses, and those with very low activity may need an alternative drug entirely. If genetic testing isn’t available, your doctor will rely on close blood count monitoring to catch toxicity early.
Common and Serious Side Effects
The most frequent side effect, occurring in more than 20% of patients, is myelosuppression: a drop in the blood cells produced by your bone marrow. This can show up as anemia (low red blood cells, causing fatigue), low white blood cells (raising infection risk), or low platelets (increasing bruising and bleeding). Between 5% and 20% of patients experience nausea, vomiting, diarrhea, loss of appetite, general malaise, or rash.
Liver damage is the other major concern. Mercaptopurine is processed through the liver, and hepatotoxicity can occur at any dose, though it becomes more likely at higher doses. The FDA label notes reports of fatal liver injury associated with the drug. Liver function blood tests are recommended weekly when first starting treatment, then monthly once levels are stable.
There are rarer but serious risks as well. Long-term use carries a small increased risk of certain cancers, including an aggressive type called hepatosplenic T-cell lymphoma. Mercaptopurine can also cause fetal harm and is considered a risk during pregnancy.
A Critical Drug Interaction: Allopurinol
One interaction stands out for its severity. Allopurinol, a common medication for gout, blocks one of the pathways your body uses to clear mercaptopurine. Taking both together can cause active drug levels to spike dramatically, leading to life-threatening bone marrow suppression. New Zealand’s medicines safety authority and other regulators have flagged this combination as potentially fatal.
If both drugs are absolutely necessary, the mercaptopurine dose must be cut to just 25% of the usual amount, with very close blood count monitoring. But in general, the combination is avoided whenever possible. If you’re prescribed mercaptopurine, make sure every provider you see knows about it before prescribing anything new, especially gout medications.
Monitoring During Treatment
Mercaptopurine requires regular blood work for as long as you take it. Complete blood counts track your red cells, white cells, and platelets to catch myelosuppression before it becomes dangerous. Liver function panels (checking enzymes and bilirubin) watch for early signs of liver injury. Early in treatment, these tests happen weekly. Once your dose is stable and your levels are predictable, monthly monitoring is typical, though your doctor may check more often if you have pre-existing liver disease or are taking other medications that stress the liver.
For patients with kidney or liver impairment, doctors generally start at the lowest recommended dose or space doses further apart to reduce the risk of drug accumulation.
Pregnancy and Breastfeeding
Mercaptopurine can harm a developing fetus, so it’s generally avoided during pregnancy when possible. For breastfeeding, the picture is more nuanced. Most professional gastroenterology guidelines consider breastfeeding acceptable for mothers taking mercaptopurine for IBD, since the drug passes into breast milk in relatively small amounts. Waiting four hours after a dose before nursing can further reduce the amount an infant is exposed to. Some doctors recommend monitoring breastfed infants with periodic blood counts and liver tests, though not all experts consider this necessary.
When mercaptopurine is being used as a cancer treatment, most sources recommend against breastfeeding during therapy, reflecting the higher doses and more aggressive treatment context involved.