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Trade/other name(s): 6-MP, Purinethol, Purixan

Why would this drug be used?

Mercaptopurine is used to treat leukemia. It may also be used to treat certain non-cancer illnesses.

How does this drug work?

Mercaptopurine is a member of the general group of chemotherapy drugs known as anti-metabolites. It interferes with the making of DNA and RNA. Without those building blocks, cells stop growing and die.

Before taking this medicine

Tell your doctor…

  • If you are allergic to anything, including medicines, dyes, additives, or foods.
  • If you have any medical conditions such as kidney disease, liver disease (including hepatitis), heart disease, congestive heart failure, diabetes, gout, infections, or if you have had kidney stones. These conditions may require that your medicine dose, regimen, or timing be changed.
  • If you are pregnant, trying to get pregnant, or if there is any chance of pregnancy. There's a chance this drug may cause birth defects if either the male or female is taking it at the time of conception or during pregnancy. Check with your doctor about what kinds of birth control can be used with this medicine.
  • If you are breastfeeding. This drug passes into breast milk, and could affect the baby.
  • If you think you might want to have children in the future. This drug might cause sterility. Talk with your doctor about the possible risk with this drug and the options that may preserve your ability to have children.
  • About any other prescription or over-the-counter medicines you are taking, including vitamins and herbs. In fact, keeping a written list of each of these medicines (including the doses of each and when you take them) with you in case of emergency may help prevent complications if you get sick.

Interactions with other drugs

The drug allopurinol (which may be used for gout or to prevent kidney stones) slows the breakdown of mercaptopurine and can cause severe side effects when the drugs are used together. If you must take allopurinol, your doctor will reduce your mercaptopurine dose.

The antibiotic trimethoprim-sulfa (Bactrim, Septra) may worsen mercaptopurine's effects on blood counts.

Certain drugs that are used to treat bowel inflammation and ulcerative colitis, such as olsalazine (Dipentum), sulfasalazine (Azulfidine), mesalamine (Asacol, Canasa) may raise the blood levels of mercaptopurine and cause worse side effects. Your doctor may need to watch you more closely.

Any drugs or supplements that interfere with blood clotting can raise the risk of bleeding during treatment with mercaptopurine. These include:

  • Vitamin E
  • Non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen (Advil, Motrin), naproxen (Aleve, Naprosyn), and many others
  • Warfarin (Coumadin)
  • Ticlopidine (Ticlid)
  • Clopidogrel (Plavix)

Note that many cold, flu, fever, and headache remedies contain aspirin or ibuprofen. Ask your pharmacist if you aren't sure what's in the medicines you take.

The blood thinner warfarin (Coumadin) may not work as well when you take mercaptopurine. Your doctor may need to check your blood more often.

Check with your doctor, nurse, or pharmacist about other medicines, vitamins, herbs, and supplements, and whether alcohol can cause problems with this medicine.

Interactions with foods

No serious interactions with food are known at this time. Check with your doctor, nurse, or pharmacist about whether foods may be a problem.

Tell all the doctors, dentists, nurses, and pharmacists you visit that you are taking this drug.

How is this drug taken or given?

Mercaptopurine is taken by mouth once a day, usually for 5 days, with or without food. It may be taken much longer in some cases. Your dose is based on your weight and the reason you are taking the drug.

Be sure you understand the full instructions before you take it. If you are unsure, call your doctor.

Store the medicine in a tightly closed container away from heat and moisture and out of the reach of children and pets.


This drug can kill large numbers of cancer cells within the first 24 hours of treatment, spilling the cells' contents into the blood. This can lead to electrolyte imbalances and tumor lysis syndrome, which can result in serious kidney damage and other problems. If your doctor thinks you may be at risk, he or she will give you medicines and/or fluids to help prevent it. Drink plenty of fluids while taking this medicine.

Do not get any immunizations (vaccines), either during or after treatment with this drug, without your doctor's OK. Mercaptopurine may affect your immune system. This could make vaccinations ineffective, or could even lead to serious infections if you get a live virus vaccine during or soon after treatment. Try to avoid contact with people who have recently received a live virus vaccine, such as the oral polio vaccine or smallpox vaccine. Check with your doctor about this.

Your doctor will likely test your blood throughout your treatment, looking for possible effects of the drug on blood counts (described below) or on other body organs. Based on the test results, you may be given medicines to help treat any effects. Your doctor may also need to reduce or delay your next dose of this drug, or even stop it completely. Be sure to keep all your appointments for lab tests and doctor visits.

This drug can lower your white blood cell count, especially in the weeks after the drug is given. A low white blood cell count increases your chance of getting an infection. Be sure to let your doctor or nurse know right away if you have any signs of infection, such as fever (100.5° or higher), chills, pain when passing urine, a new cough, or bringing up sputum.

This drug may lower your platelet count in the weeks after it is given, which can increase your risk of bleeding. Speak with your doctor before taking any drugs or supplements that might affect your body's ability to stop bleeding, such as aspirin or aspirin-containing medicines, warfarin (Coumadin), or vitamin E. Tell your doctor right away if you have unusual bruising, or bleeding such as nosebleeds, bleeding gums when you brush your teeth, or black, tarry stools.

This drug may lower your red blood cell count. If this occurs, it is usually a few months after starting treatment. A low red blood cell count (known as anemia) can cause shortness of breath, or make you to feel weak or tired all the time. Your doctor may give you medicines to help prevent or treat this condition, or you may need to get blood transfusions.

Patients who have very low levels of the enzyme needed to break down this drug (called TPMT) can have severe low blood cell counts if this drug is given. Your doctor may suspect that you have low levels of this enzyme if your blood counts get very low after you are given this drug and take a long time to recover. Your doctor can then check your level of TPMT and adjust your future doses of mercaptopurine.

Call your doctor if you have sores in your mouth or throat that make it hard to eat, if you notice pain in your belly, or if you have diarrhea.

This drug can cause severe liver damage in a few people. Call your doctor right away if you notice nausea, vomiting, fatigue, poor appetite, dark urine, yellowing skin or eyes, or tenderness under the right side of the rib cage.

Avoid pregnancy during treatment and for some months afterward. This drug may harm a growing fetus. Talk with your doctor about this.

Possible side effects

You will probably not have most of the following side effects, but if you have any talk to your doctor or nurse. They can help you understand the side effects and cope with them.


  • Low white blood cell count with increased risk of infection*
  • Low blood platelet count with increased risk of bleeding*
  • Low red blood cell count (anemia) with symptoms like tiredness, paleness, shortness of breath*

Less common

  • Rash
  • Darkening of skin


  • Nausea
  • Vomiting
  • Loss of appetite
  • Sores on lips, in mouth, or ulcers in the intestine*
  • Liver damage, which may cause poor appetite, swollen belly, or jaundice (skin or whites of eyes turn yellow)*
  • Death due to liver damage or other causes

*See “Precautions” section for more detailed information.

There are some other side effects not listed above that can also occur in some patients. Tell your doctor or nurse if you develop these or any other problems.

FDA approval

Yes – first approved before 1984 (FDA cannot verify dates of drugs approved before 1984.)

Disclaimer: This information does not cover all possible uses, actions, precautions, side effects, or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for talking with your doctor, who is familiar with your medical needs.

Last Medical Review: 02/05/2010
Last Revised: 05/01/2014