Mercaptopurine is a chemotherapy drug that is usually given to treat acute leukaemias. This information should ideally be read with our general information about chemotherapy and your type of cancer.
What mercaptopurine looks like
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Mercaptopurine is available as 50mg yellow-coloured tablets.
You should swallow the tablets whole with a glass of water.
Chemotherapy is usually given as a course of several sessions (cycles) of treatment over a few months. The length of your treatment and the number of cycles you have will depend on the type of cancer you are being treated for. Your nurse or doctor will discuss your treatment plan with you.
The number of tablets you need to take may alter from one week to the next depending on how the mercaptopurine is affecting your blood cell counts. It's important to make sure you know exactly how many tablets to take.
Before you begin your treatment your doctor will arrange for you to have blood tests. You'll usually be given anti-sickness drugs before and/or during your treatment.
Each person’s reaction to chemotherapy is different. Some people have very few side effects while others may experience more. The side effects described here won't affect everyone who has mercaptopurine and may be different if you are having more than one chemotherapy drug.
We have outlined the most common side effects but haven't included those that are rare and unlikely to affect you. If you notice any effects that aren't listed below, discuss them with your doctor, chemotherapy nurse or pharmacist.
Risk of infection
Mercaptopurine can reduce the number of white blood cells, which help fight infection. White blood cells are produced by the bone marrow. If the number of your white blood cells is low you'll be more prone to infections. A low white blood cell count is called neutropenia.
Neutropenia begins seven days after treatment, and your resistance to infection is usually at its lowest 10-14 days after chemotherapy. The number of your white blood cells will then increase steadily and usually return to normal before your next cycle of chemotherapy is due.
Contact your doctor or the hospital straight away if:
your temperature goes above 38ºC (100.4ºF)
you suddenly feel unwell even with a normal temperature.
You'll have a blood test before having more chemotherapy to check the number of white blood cells. Occasionally, your treatment may need to be delayed if the number of your blood cells (blood count) is still low.
Bruising and bleeding
Mercaptopurine can reduce the production of platelets, which help the blood to clot. Tell your doctor if you have any unexplained bruising or bleeding, such as nosebleeds, bleeding gums, blood spots or rashes on the skin. You may need to have a platelet transfusion if your platelet count is low.
Feeling tired is a common side effect of chemotherapy, especially towards the end of treatment and for some weeks after it’s over. It’s important to try to pace yourself and get as much rest as you need. Try to balance this with some gentle exercise, such as short walks, which will help. If tiredness is making you feel sleepy, don’t drive or operate machinery.
Mercaptopurine can cause changes in the way your liver works, although your liver will return to normal when the treatment has finished. Your skin and the whites of your eyes may become yellow (jaundiced). The doctor will take regular blood samples to check your liver is working properly.
Less common side effects
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Feeling sick (nausea) and being sick (vomiting)
This is usually mild. If you do feel sick, it may begin a few hours after treatment and last for a few days. Your doctor can prescribe anti-sickness (anti-emetic) drugs to prevent or greatly reduce nausea and vomiting.
If the sickness isn't controlled, or if it continues, tell your doctor; they can prescribe other anti-sickness drugs that may work better for you.
Some anti-sickness drugs can cause constipation. Let your doctor or nurse know if this is a problem.
Sore mouth and ulcers
Your mouth may become sore or dry, or you may notice small ulcers during this treatment. Drinking plenty of fluids, and cleaning your teeth regularly and gently with a soft toothbrush, can help reduce the risk of this happening. Some people may find sucking on ice soothing.
Tell your nurse or doctor if you have any of these problems, as they can prescribe mouthwashes and medicine to prevent or clear mouth infections.
You may notice that food tastes different. Normal taste usually comes back after treatment finishes. A dietitian or specialist nurse at your hospital can give you advice about ways of coping with this side effect. You may also like to read our section about eating well.
Mercaptopurine can cause diarrhoea. This can usually be easily controlled with medicine, but tell your doctor if it is severe or continues. It's important to drink plenty of fluids if you have diarrhoea.
A skin rash can sometimes occur while you're having treatment with mercaptopurine. It’s important to let your doctor know if this happens. Your doctor can prescribe medicine to help with this.
Fever and chills
These may occur from the time your treatment is given, but they don’t usually last long. Your doctor may prescribe medicines to reduce these effects.
It’s important to let your doctor know straight away if you feel unwell or have any severe side effects, even if they’re not mentioned above.
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Mercaptopurine interacts with a number of drugs including allopurinol (Zyloric®), which is commonly given to people having chemotherapy. Allopurinol helps reduce an increase of uric acid in the blood. To prevent the interaction you'll usually be told to stop taking allopurinol before you start taking mercaptopurine. Your doctor can tell you more about this. It's important to follow their instructions.
Some other medicines, including those you can buy in a shop or chemist, can be harmful to take when you are having chemotherapy. Tell your doctor about any medicines you are taking, including over-the-counter drugs, complementary therapies and herbal drugs.
Risk of developing a blood clot
Cancer can increase the risk of developing a blood clot (thrombosis), and chemotherapy may increase this risk further. A blood clot may cause symptoms such as pain, redness and swelling in a leg, or breathlessness and chest pain. Blood clots can be very serious so it’s important to tell your doctor straight away if you notice any of these symptoms. Most clots can be treated with drugs that thin the blood. The doctor or nurse can give you more information.
Your ability to become pregnant or father a child may be affected by having this treatment. It's important to discuss fertility with your doctor before starting treatment.
It's not advisable to become pregnant or father a child while having mercaptopurine as it may harm the developing baby. It’s important to use effective contraception while having this drug and for at least a few months afterwards. You can discuss this with your doctor.
It’s not known whether chemotherapy drugs can be present in semen or vaginal fluids. To protect your partner, it’s safest to either avoid sex or use a barrier form of contraception for about 48 hours after chemotherapy.
There's a potential risk that chemotherapy drugs may be present in breast milk. Women are advised not to breastfeed during chemotherapy and for a few months afterwards.
If you’re admitted to hospital for a reason not related to the cancer, it’s important to tell the doctors and nurses looking after you that you're having chemotherapy treatment. You should tell them the name of your cancer specialist so that they can ask for advice.
It’s a good idea to know who you should contact if you have any problems or troublesome side effects when you’re at home. Your chemotherapy nurse or doctor will give you details of who to contact for advice. This should include ‘out-of hours’ contact details if you need to call someone at evenings, overnight or at the weekend.
Things to remember about mercaptopurine tablets
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It is important to take your tablets at the right times as directed by your doctor.
Always tell any doctors treating you for non-cancerous conditions that you're taking a course of chemotherapy tablets that should not be stopped or restarted without advice from your cancer specialist.
Keep the tablets in their original packaging, and store them at room temperature away from heat and direct sunlight.
Keep the tablets in a safe place and out of the reach of children.
If your doctor decides to stop the treatment, return any remaining tablets to the pharmacist. Don't flush them down the toilet or throw them away.
If you're sick just after taking the tablets tell your doctor as you may need to take another dose. Don't take another tablet without telling your doctor first.
If you forget to take a tablet, don't take a double dose. Let your doctor know and keep to your regular dose schedule.
Swallow your tablets whole with a glass of water.
This section is based upon our mercaptopurine fact sheet, which has been compiled using information from a number of reliable sources, including:
Sweetman, et al. Martindale: The Complete Drug Reference. 37th edition. 2011. Pharmaceutical Press.
British National Formulary. 62nd edition. 2011. British Medical Association and Royal Pharmaceutical Society of Great Britain.
electronic Medicines Compendium (eMC). www.medicines.org.uk (accessed September 2011).
Perry MC. The Chemotherapy Source Book. 4th edition. 2007. Lippincott Williams and Wilkins.