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

Mercaptopurine is given by mouth in tablet or 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 giving your child food or milk after the dose. Your child can drink clear fluids.

What are the side effects?

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 (reduced red blood cells), bruise or bleed more easily than usual, and have a higher risk of infection.

Your child’s blood counts will be checked regularly to see how the bone marrow is working. Please tell your doctor if your child seems unusually tired, has bruising or bleeding, or any signs of infection, especially a high temperature.

Temporary effect on liver function

Mercaptopurine can sometimes cause some changes to your child’s liver function. This should return to normal when the treatment is finished. Blood tests (LFTs) will be taken to monitor your child’s liver function. Please contact your doctor immediately if your child complains of pain in their right side or the whites of their eyes or their skin develops a yellow tinge.

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. This often happens when the mercaptopurine is stopped. It is temporary and will improve in time.

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 very bad or continues for more than a few days.

Mouth sores and ulcers

Your child may get painful or bleeding gums, ulcers or a sore mouth. You will be given advice about appropriate mouth care including a copy of our 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.

Giving mercaptopurine at home

If your child cannot swallow tablets, you can ask the pharmacist for the liquid preparation.

Accidental spillages

  • If the liquid gets onto your skin, you must wash the area immediately, using plenty of water. If the skin is sore you should contact your GP (family doctor) for advice.
  • If the liquid accidentally gets into your eyes, wash with plenty of running water for at least 10 minutes. If your eyes are sore after this, you should go to your nearest Accident and Emergency (A&E) department.
  • If the mixture is spilt on clothing, the spill should be blotted dry with kitchen paper. Clothing should be removed immediately and washed separately from other items. Used kitchen paper should be disposed of as above.
  • If you accidentally spill the tablets or liquid on the work surface or floor, wearing gloves, cover the spillage with kitchen paper. Wipe the area with water then clean with household cleaner and water. 
  • 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 any type of spillage occurs you should contact GOSH for advice immediately.


  • 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 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.
  • Sometimes, a blood test called TGN is carried out to check whether your child is receiving enough mercaptopurine. You will be advised by the medical team if this is due. 
Compiled by:
The Pharmacy department in collaboration with the Child and Family Information Group
Last review date:
May 2020