A thiopurine immunosuppressant
Mercaptopurine
A thiopurine used to treat some leukaemias and inflammatory bowel disease, with regular blood-count monitoring.
What is Mercaptopurine?
Mercaptopurine is a thiopurine medicine used to treat certain types of leukaemia and to keep inflammatory bowel disease, such as Crohn's disease and ulcerative colitis, under control. It works by slowing rapidly dividing cells and calming the immune system, which means it lowers the blood counts and raises the risk of infections. Regular blood tests are essential to watch the blood counts and the liver. An enzyme called TPMT is usually checked before starting, because some people break the medicine down slowly and need a lower dose, and it should not normally be taken with allopurinol unless the dose is carefully reduced.
Education and reference only. This is a plain-language guide to Mercaptopurine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Mercaptopurine is a thiopurine, a medicine that both fights some cancers and dampens the immune system. It is used in certain leukaemias, especially in children, and to maintain remission in inflammatory bowel disease such as Crohn's disease and ulcerative colitis. It is closely related to azathioprine, which the body partly turns into mercaptopurine. It is taken as tablets or a liquid by mouth, and is a specialist medicine that needs regular blood monitoring.
How it works
Mercaptopurine interferes with the building blocks that cells need to copy their DNA, which slows down fast-dividing cells such as some cancer cells and overactive immune cells. This helps control leukaemia and calms the immune-driven inflammation of bowel disease. Because it also affects normal blood-forming cells in the bone marrow, it can lower the blood counts, so these are checked regularly. An enzyme called TPMT helps break the drug down, and people with low TPMT activity build up more of it and are more prone to side effects.
Company & origin
Originated / developed by: Generic (long-established).
A long-established thiopurine used in the UK to treat certain leukaemias and inflammatory bowel disease.
Practical use
How to take Mercaptopurine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it exactly as prescribed, usually once a day, and keep to a consistent routine with food as advised.
- Make sure you have had your TPMT enzyme check before starting, as it guides whether the dose needs reducing.
- Do not take allopurinol (a gout medicine) with it unless your specialist has specifically adjusted the dose.
- Attend all blood tests, which check your blood counts and liver and are more frequent at first.
- Report fever, sore throat, unusual bruising or bleeding promptly, as these can signal low blood counts.
- Never stop it suddenly without advice, and tell every prescriber that you take it, including before any new medicine.
Weighing it up
Advantages & disadvantages of Mercaptopurine
Advantages
- An effective treatment for certain leukaemias and for keeping inflammatory bowel disease in remission.
- Allows many people with bowel disease to reduce or avoid long courses of steroids.
- Taken by mouth as tablets or liquid, with dosing tailored using the TPMT test and blood counts.
Disadvantages
- Lowers the blood counts and needs regular blood tests throughout treatment.
- Increases the risk of infections and can affect the liver.
- Has a serious interaction with allopurinol and a small increased risk of certain cancers with long-term use.
Practical use
Good to know
Before starting, the TPMT enzyme is usually tested, because people who break the medicine down slowly can have dangerously low blood counts and need a much lower dose or a different treatment. Regular blood tests to check the full blood count and the liver are essential throughout treatment and are most frequent at the start. The most important interaction is with allopurinol (used for gout): taken together they can cause severe drops in blood counts, so mercaptopurine should not normally be combined with it unless the dose is greatly reduced and closely monitored. It increases the risk of infections, so report fevers, sore throats, unusual bruising or bleeding promptly. It can affect the liver and, rarely, increases the risk of certain cancers; sun protection is sensible, and it should not be stopped suddenly without advice.
Who should not take it / use with caution
- People with very low TPMT enzyme activity may be unable to take it or need a much lower dose.
- It is generally avoided in active serious infections and significant liver problems.
- Those who cannot attend regular monitoring blood tests are not suitable without arrangements in place.
Monitoring
- Regular full blood count tests, most frequent at the start, to watch for low blood counts.
- Liver blood tests to check for liver effects during treatment.
- TPMT enzyme testing before starting, and ongoing watch for infections and other side effects.
Side effects
- Lowered blood counts, increased infection risk, nausea and changes in liver blood tests are common.
- Tiredness, loss of appetite, mouth ulcers and, sometimes, hair thinning.
- Less commonly, inflammation of the pancreas, serious liver problems and a higher risk of certain cancers.
Key interactions
- Allopurinol greatly increases its effect and can cause severe drops in blood counts, so it should not normally be combined unless the dose is reduced.
- Other medicines that affect the blood counts or the immune system can add to its effects.
- Some vaccines (live vaccines) are generally avoided while taking it, so check before any vaccination.
Available as: Tablets and oral liquid taken by mouth.
Answers
Mercaptopurine: frequently asked questions
Why do I need a TPMT test before starting?
The TPMT enzyme breaks the medicine down; people with low activity build up more of it and can have dangerously low blood counts, so the dose is adjusted accordingly.
Why can't I take allopurinol with it?
Allopurinol greatly increases the level and effect of mercaptopurine and can cause severe drops in blood counts, so they should not normally be combined unless the dose is carefully reduced.
How often will I need blood tests?
Blood tests are needed regularly throughout treatment and are most frequent at the start, to check your blood counts and liver.
Does it make infections more likely?
Yes, because it lowers the immune system and blood counts, so report fevers, sore throats or unusual bruising and bleeding promptly.
Can I stop it if I feel well?
No. Stopping suddenly can let your condition return, so never change or stop it without specialist advice.
The wider class
About Thiopurines
Mercaptopurine belongs to the thiopurines class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
Browse by body system
Authoritative sources
- BNF
- NICE CKS
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