Mercaptopurine is a chemotherapy drug that is given as a treatment for some types of cancer. It is most commonly used to treat acute leukaemias. This information describes mercaptopurine, how it is given and some of its specific side effects. It should ideally be read with our general information about chemotherapy and about your type of leukaemia, which give further information and advice.

If you have any further questions you can ask your doctor or nurse at the hospital where you are having your treatment, or speak to one of our cancer support service nurses.

Mercaptopurine is available as 50mg yellow-coloured tablets.

The tablets should be swallowed whole with a glass of water.

Chemotherapy is usually given as a course of several sessions (or 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 for which you are being treated. Your nurse or doctor will discuss your treatment plan with you.

The number of tablets that you need to take may alter from one week to the next, depending on how the mercaptopurine is affecting your blood cell counts (see possible side effects). It is important to make sure that you know exactly how many tablets to take.

Each person’s reaction to chemotherapy is unique. Some people have very few side effects, while others may experience more. The side effects described in this information will not affect everyone who is given mercaptopurine and may be different if you are having more than one chemotherapy drug.

We have outlined the most common side effects as well as rarer ones, so that you can be aware of them if they occur. However, we have not included those that are very rare and therefore extremely unlikely to affect you. If you notice any effects which you think may be due to the drug, but which are not listed in this information, please discuss them with your doctor or chemotherapy nurse.

Lowered resistance to infection Mercaptopurine can reduce the production of white blood cells by the bone marrow, making you more prone to infection. This effect can begin seven days after treatment starts and will often continue throughout your treatment.

Contact your doctor or the hospital straightaway if:

• your temperature goes above 38ºC (100.5ºF)
• you suddenly feel unwell (even with a normal temperature).

You will have a blood test before having more chemotherapy to make sure that your cells have recovered. Occasionally it may be necessary to delay your treatment if the number of blood cells (the blood count) is still low.

Bruising or bleeding Mercaptopurine can reduce the production of platelets (which help the blood to clot). Let your doctor know if you have any unexplained bruising or bleeding, such as nosebleeds, blood spots or rashes on the skin, or bleeding gums.

Tiredness and feeling weak You may feel very tired. It is important to allow yourself plenty of time to rest.

Your liver may be temporarily affected Mercaptopurine may cause changes in the way that your liver works. Your liver will return to normal when the treatment is finished. This is very unlikely to cause you any harm, but your doctor will monitor this carefully. Samples of your blood will be taken from time to time to check your liver is working properly.

Feeling sick (nausea) and being sick (vomiting) If you do feel sick this may begin after the treatment is given. This is usually mild and your doctor can prescribe very effective anti-sickness (anti-emetic) drugs to prevent, or greatly reduce, nausea and vomiting. If the sickness is not controlled, or continues, tell your doctor; they can prescribe other anti-sickness drugs which may be more effective. 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 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 to reduce the risk of this happening. Tell your nurse or doctor if you have any of these problems, as special mouthwashes and medicines to prevent or clear any mouth infection can be prescribed.

Taste changes You may notice that your food tastes different. Normal taste will usually come back after the treatment finishes.

Diarrhoea This can usually be easily controlled with medicine, but let your doctor know if it is severe or if it continues. It is important to drink plenty of fluids if you have diarrhoea.

Skin changes Mercaptopurine can cause a rash, which may be itchy. Your doctor can prescribe medicine to help with this.

A high temperature and chills These may occur, but they do not usually last long. Your doctor may prescribe medicines to reduce these side effects.

Mercaptopurine interacts with a number of drugs, including allopurinol, which is a medicine that is commonly given to people having chemotherapy. Allopurinol is used to help reduce an increase of uric acid in the blood. To prevent the interaction you will usually be told to stop taking allopurinol before you start taking mercaptopurine. Your doctor can give you further advice about this. It is important to follow the instructions given to you by the doctor.

Risk of blood clots Cancer can increase your risk of developing a blood clot (thrombosis), and having 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 is important to tell your doctor straightaway if you have any of these symptoms. However, most clots can usually be successfully treated with drugs to thin the blood. Your doctor or nurse can give you more information.

Other medicines Some medicines can be harmful to take when you are having chemotherapy. Let your doctor know about any medications you are taking, including non-prescribed drugs such as complementary therapies and herbal drugs.

Fertility Your ability to become pregnant or father a child may be affected by taking this drug. It is important to discuss fertility with your doctor before starting treatment.

Contraception It is not advisable to become pregnant or father a child while taking mercaptopurine, as the developing foetus may be harmed. It is important to use effective contraception while taking this drug, and for at least a few months afterwards. Again, discuss this with your doctor.

• The tablets should be stored in a dry place at room temperature away from direct sunlight.
• It is important to take your tablets at the right times. You must take them as directed by your doctor.
• Keep the tablets in a safe place where children cannot reach them, as mercaptopurine could harm them.
• If your doctor decides to stop the treatment, return any remaining tablets to the pharmacist. Do not flush them down the toilet or throw them away.
• If you are sick just after taking the tablet tell your doctor immediately, as you may need to take another dose. Do not take another tablet without first telling your doctor.
• If you forget to take a tablet do not take a double dose. Let your doctor know and keep to your regular dose schedule.

This section is based upon our Mercaptopurine factsheet which has been compiled using information from a number of reliable sources, including:

• Martindale: The Complete Drug Reference (35th edition). Eds. Sweetman et al. Pharmaceutical Press, 2007.
• British National Formulary (54th edition). British Medical Association and Royal Pharmaceutical Society of Great Britain, September 2007.
• The Medicines.org.uk website – www.medicines.org.uk (November 2007).
• The Chemotherapy Source Book (3rd edition). Ed. Perry. Lippincott Williams and Wilkins, 2001.

For further references, please see general bibliography.