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How does this medication work? What will it do for me?

Mercaptopurine belongs to the group of cancer-fighting medications known as antineoplastics, and specifically to the group of antineoplastics known as antimetabolites. Mercaptopurine fights cancer by preventing the growth of cancer cells, which eventually results in their destruction. Mercaptopurine is usually used to treat leukemia. Sometimes mercaptopurine is used along with other cancer medications.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

How should I use this medication?

The recommended dose of mercaptopurine varies according to the specific disease being treated, the response to therapy, the other medications used, the stage of the disease, and the person's body size or weight. A typical starting dose for adults and children is 2.5 mg per kg of body weight per day, or 50 mg to 75 mg per metre squared of body surface area per day. The appropriate dose of mercaptopurine is usually taken by mouth as a single dose on a daily basis.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

Mercaptopurine can irritate the eyes and skin if they come into contact with the medication. Wash your hands with soap and water immediately after taking a dose of this medication to prevent particles of the tablet from spreading to other areas of the skin.

Your doctor or pharmacist may suggest that you take this medication at a specific time of the day. Some studies suggest that the time of day this medication is taken affects the success of the treatment.

It is important to take this medication exactly as prescribed by your doctor. If you vomit shortly after taking the medicine, contact your doctor for instruction on whether to take more medication or not. If you miss a dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Your doctor may want you to drink extra fluids while taking this medication in order to help you pass more fluid and protect your kidneys.

Store this medication at room temperature, and protect it from excessive heat, direct light, and moisture. Keep it out of the reach of children.

What form(s) does this medication come in?

Each pale yellow-to-buff, scored tablet, imprinted with "PURINETHOL" and "O4A", contains mercaptopurine 50 mg. Nonmedicinal ingredients: corn starch, lactose, magnesium stearate, potato starch, and stearic acid.

Who should NOT take this medication?

Do not take mercaptopurine if you:

  • are allergic to mercaptopurine or any ingredients of this medication
  • have been resistant to the effects of mercaptopurine or thioguanine in the past
What side effects are possible with this medication?

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

  • diarrhea
  • loss of appetite
  • nausea
  • skin rash and itching
  • vomiting
  • weakness

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

  • signs of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
  • signs of bleeding (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
  • symptoms of infection (fever or chills, severe diarrhea, shortness of breath, prolonged dizziness, headache, stiff neck, weight loss, or listlessness)
  • signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)
  • sores in mouth or on lips

Stop taking the medication and seek immediate medical attention if any of the following occur:

  • signs of bleeding in the stomach (e.g., bloody, black, or tarry stools, spitting up of blood, vomiting blood or material that looks like coffee grounds)
  • signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Are there any other precautions or warnings for this medication?

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.


March 27, 2014

Health Canada has issued new restrictions concerning the use of Purinethol (mercaptopurine). To read the full Health Canada Advisory, visit Health Canada's web site at www.hc-sc.gc.ca.

Anemia: Mercaptopurine may cause low levels of red blood cells. If you experience symptoms of reduced red blood cell count (anemia) such as shortness of breath, feeling unusually tired, or pale skin, contact your doctor as soon as possible.

Birth control: Effective birth control should be practiced if either partner is using this medication as this medication may harm the baby if used during pregnancy. If you become pregnant while taking this medication, contact your doctor immediately.

Bleeding: This medication can reduce the number of platelet cells in the blood. Platelets help the blood to clot, and a shortage could make you bleed more easily. Tell your doctor of any signs that your blood is not clotting as quickly. Such symptoms may include black and tarry stools, blood in the urine, easy bruising, or cuts that won't stop bleeding.

Infection and vaccines: As well as killing cancer cells, this medication can reduce the number of cells that fight infection in the body (white blood cells). Avoid contact with people with contagious infections and tell your doctor if you begin to notice signs of an infection, such as fever or chills, severe diarrhea, shortness of breath, prolonged dizziness, headache, stiff neck, weight loss, or listlessness. Also tell your doctor if you have been vaccinated, or are planning to be vaccinated with a live vaccine.

Kidney function: Kidney disease or reduced kidney function may cause this medication to build up in the body, causing side effects. If you have reduced kidney function or kidney disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Liver function: Mercaptopurine can reduce liver function and can cause liver failure. If you have liver problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed. Your doctor will test your liver function regularly with blood tests while you are taking this medication.

If you experience symptoms of liver problems such as fatigue, feeling unwell, loss of appetite, nausea, yellowing of the skin or whites of the eyes, dark urine, pale stools, abdominal pain or swelling, and itchy skin, contact your doctor immediately.

Second cancers: Mercaptopurine can cause damage to other normal genes and cells. Rarely there have been reports of certain types of leukemia and lymphoma developing in people who have been treated with mercaptopurine. Discuss any concerns you may have with your doctor.

Tumour lysis syndrome: Mercaptopurine, like many other cancer medications, causes many cancer cells to be suddenly killed when treatment is first started. This can overwhelm the body with waste products from the cells. As a result, the body may not be able to keep up with getting rid of all the waste. When this happens, you may experience nausea, shortness of breath, notice cloudy urine or joint pain. This is called tumour lysis syndrome. Your doctor may prescribe some medications to help your body get rid of the waste products. Make sure you understand how to use these medications and report any of these signs or symptoms to your doctor immediately.

Pregnancy: There is a possibility of birth defect if either the man or woman is using mercaptopurine at the time of conception, or if it is taken during pregnancy. Effective birth control should be practiced while using this medication. This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while you or your partner are taking this medication, contact your doctor immediately.

Breast-feeding: This medication passes into breast milk. Because of the risks associated with mercaptopurine, a decision should be made to cease breast-feeding or discontinue the medication, taking into account the importance of the medication to the mother. Women who are taking this medication should not breast-feed.

Seniors: Seniors may be more likely to experience side effects from this medication. Discuss any concerns you have with your doctor.

What other drugs could interact with this medication?

There may be an interaction between mercaptopurine and any of the following:

  • allopurinol
  • amphotericin B
  • azathioprine
  • BCG
  • clozapine
  • denosumab
  • immunosuppressants (e.g., prednisone, dexamethasone, tacrolimus, cyclosporine)
  • echinacea
  • febuxostat
  • golimumab
  • infliximab
  • leflunomide
  • mesalamine
  • natalizumab
  • ocrelizumab
  • olsalazine
  • other cancer medications (e.g., cyclophosphamide, doxorubicin, etoposide)
  • pimecrolimus
  • sphingosine 1-phosphate (S1P) receptor modulators (e.g., fingolimod, ponesimod, siponimod)
  • sulfamethoxazole
  • sulfasalazine
  • tofacitinib
  • trimethoprim
  • vaccines (e.g., yellow fever, BCG, cholera, typhoid, varicella, meningococcal, diphtheria)
  • upadacitinib
  • warfarin

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

  • stop taking one of the medications,
  • change one of the medications to another,
  • change how you are taking one or both of the medications, or
  • leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.

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