Mercaptopurine is a medicine used to treat certain types of cancer and leukaemia.

How is mercaptopurine given?

Mercaptopurine is given by mouth in tabletor liquid form, once a day. It should be given at least one hour after food in the evening, but not with milk products as these affect how the medicine is absorbed. Please wait one further hour before eating food or drinking milk. Your child can drink clear fluids.

What are the side effects of mercaptopurine?

Bone marrow suppression

There will be a temporary reduction in how well your child’s bone marrow works. This means your child may become anaemic, bruise or bleed more easily than usual, and have a higher risk of infection. Please tell your doctor if your child seems unusually tired, has bruising or bleeding, or any signs of infection, especially a high temperature. Your child’s blood counts will be checked regularly to see how the bone marrow is working. Mercaptopurine doses may be changed weekly according to your child’s blood count. Advice will be given by your doctor, nurse or pharmacist.

Temporary effect on liver function

Mercaptopurine can cause some mild changes to your child’s liver function. This should return to normal when the treatment is finished. Blood tests may be taken to monitor your child’s liver function (LFTs).

Itchy red rash

Please tell your doctor or nurse if your child develops a rash. They will advise you on the appropriate treatment to use.

Nausea and vomiting

Anti-sickness medicines can be given to reduce or prevent these symptoms. Please tell your doctor or nurse if your child’s sickness is not controlled or persists.

Mouth sores and ulcers

You will be given advice about appropriate mouth care including a copy of our mouth care leaflet. If your child complains of having a sore mouth, please tell your doctor or nurse.

Interactions with other medicines

Some medicines can react with mercaptopurine, altering how well it works. Always check with your doctor or pharmacist before giving your child any other medicine, including medicines on prescription from your family doctor (GP), medicines bought from a pharmacy (chemist) or any herbal or complementary medicines.

Important information you should know about mercaptopurine

  • Keep all medicines in a safe place where children cannot reach them.
  • Mercaptopurine tablets or liquid should be kept in a cool, dry place away from direct sunlight or heat.
  • You should handle these medicines with care, avoiding touching them where possible. If you are pregnant or think you could be pregnant, please discuss handling instructions with your doctor, nurse or pharmacist. Please see our Special handling requirements information sheet for further details.
  • If your child is taking mercaptopurine liquid, it can only be used for eight weeks once opened. Write the date you opened the bottle on the label to remind you.
  • If your child vomits after taking the dose, inform the doctor or nurse. Do not give them another dose.
  • If you forget to give your child their dose, do not give them a double dose. Inform your doctor or nurse and keep to your child’s regular dose schedule.
  • Sometimes it is necessarily to halve tablets to get the correct dose. A tablet cutter may be used for this but you should keep it only for cutting chemotherapy tablets.
  • If your child cannot swallow tablets, you can ask the pharmacist for the liquid preparation. If this is unsuitable, you can make tablets into a mixture as follows: 
    • You will need a small container such as a medicine pot and an oral syringe.
    • If you prefer you can wear a pair of household rubber gloves to protect yourself. Do not use them for any other purpose.
    • Tip the required number of tablets into a clean container.
    • Pour a little water on the tablets and allow to disperse. This may take a few minutes. Do not try to make the tablets disperse faster by stirring or shaking the container.
    • Draw up the dose using the oral syringe and give to your child.
    • Wash the syringe and container in warm, soapy water and do not use for any other purpose.
  • If you accidentally spill the tablets or mixture, wash the area thoroughly with plenty of water.
  • If the mixture accidentally gets into your eyes, wash with plenty of running water for at least 10 minutes.
  • Used paper towels, masks, vomit and dirty disposable nappies should be placed inside two rubbish bags and disposed of along with your normal rubbish.
  • If your doctor decides to stop treatment with mercaptopurine or the medicine passes its expiry date, return any remaining medicine to the pharmacist. Do not flush it down the toilet or throw it away.
Compiled by: 
The Pharmacy department in collaboration with the Child and Family Information Group
Last review date: 
February 2017


Please read this information sheet from GOSH alongside the patient information leaflet (PIL) provided by the manufacturer. If you do not have a copy of the manufacturer’s patient information leaflet please talk to your pharmacist. A few products do not have a marketing authorisation (licence) as a medicine and therefore there is no PIL.

For children in particular, there may be conflicts of information between the manufacturer’s patient information leaflet (PIL) and guidance provided by GOSH and other healthcare providers. For example, some manufacturers may recommend, in the patient information leaflet, that a medicine is not given to children aged under 12 years. In most cases, this is because the manufacturer will recruit adults to clinical trials in the first instance and therefore the initial marketing authorisation (licence) only covers adults and older children.  

For new medicines, the manufacturer then has to recruit children and newborns into trials (unless the medicine is not going to be used in children and newborns) and subsequently amend the PIL with the approved information. Older medicines may have been used effectively for many years in children without problems but the manufacturer has not been required to collect data and amend the licence. This does not mean that it is unsafe for children and young people to be prescribed such a medicine ‘off-licence/off-label’. However, if you are concerned about any conflicts of information, please discuss with your doctor, nurse or pharmacist.