Mercaptopurine for Crohn's disease and ulcerative colitis

What is mercaptopurine?

Mercaptopurine is part of a group of medications known as thiopurines which are immunosuppressants. This means they work to reduce the activity of the immune system. Mercaptopurine is chemically very similar to azathioprine, though in the UK azathioprine is used more widely than mercaptopurine.

It is used in the treatment of Crohn’s disease and ulcerative colitis but is also used to treat acute lymphocytic leukemia, a type of cancer that begins in the white blood cells. It is not usually the first medication given to people with inflammatory bowel disease (IBD) but is used to help people come off of steroids and to maintain remission. For this reason it is often referred to as ‘maintenance therapy’ and can be taken long-term. It is also used to help in the treatment of fistulas.

How is mercaptopurine taken?

It is taken in tablet form, usually once a day. You may be told to take it on an empty stomach one hour before food or three hours after eating. You should avoid taking mercaptopurine at with cow’s milk as it contains high levels of xanthine oxidase which can inactivate mercaptopurine.

The dosage you will be given will depend on your weight and disease severity.

Mercaptopurine is a slow-acting medication and can take between 3-6 months to work. This means you may need to take another medication, such as steroids, at the same time to control your symptoms.


What are the different types of mercaptopurine available?

Mercaptopurine is often referred to as 6-mercaptopurine, 6-MP or Purinethol.

Can mercaptopurine be taken with other medication?

There are some medications which can interact with mercaptopurine. These include:

  • Allopurinol (although this may be prescribed by your doctor to help with side effects)
  • Warfarin
  • 5-aminosalicylates
  • Anti bacterials
  • Febuxosat

You should inform your doctor of any medications or supplements you are taking to ensure they do not react with your new medication.

Mercaptopurine is sometimes prescribed at the same time as biologic medications - such as adalimumab or infliximab. This is known as combination therapy.

Before starting your treatment you will receive a number of tests. One of these will look at the level of enzymes in your body. If you have a high level of enzymes then you may be prescribed allopurinol to be taken at the same time as mercaptopurine as these enzymes can cause side effects of the medication.

Are there any potential side effects of mercaptopurine?

As with all medications some people experience side effects when taking mercaptopurine. Some of the more common side effects reported include:

  • Nausea
  • Vomiting
  • Diarrhoea
  • Darkening of the skin
  • Hair loss
  • Rash or itchy skin

More serious side effects include:

  • Easy bruising or bleeding
  • Dizziness
  • Fainting
  • Joint pain or swelling
  • Tongue or mouth sores or pain
  • Unusual tiredness
  • Symptoms of liver disease (which can include persistent nausea/vomiting, stomach/abdominal pain, dark urine, yellowing eyes/skin)

You should tell your doctor if you experience any side effects.

Is there anything I should know before taking mercaptopurine?

  • You will have screening carried out before starting mercaptopurine. This will include checking for levels of enzymes in your blood and being screened for immunity to the chickenpox virus and for other infections. You may also be screen for tuberculosis (TB)
  • You will need regular blood tests and monitoring. As mercaptopurine reduces the number of white and red blood cells, as well as platelets, you will need blood tests to check levels of these cells and also to check your liver function. You kidney function should also be tested every six months
  • You will be more sensitive to sunlight. You should avoid sunbeds, strong sunlight and use sun cream with a high SPF
  • You are at a slightly increased risk of developing certain cancers, such as skin cancer and lymphoma
  • You may get ill more often. As mercaptopurine suppresses the immune system you may find that you pick up more colds, coughs, infections and other illnesses and find it hard to get rid of them or they may develop into more serious illnesses. You should also avoid contact with people who have illnesses such as chicken pox
  • If you are male or female taking mercaptopurine and want to have a baby you should discuss this with your doctor before getting pregnant or if you find out you or your partner are pregnant. Manufacturers recommend patients, or their partners, who are pregnant or likely to get pregnant should not take the drug. However, experts believe it can be taken where the benefits outway the risks
  • If you have liver or kidney problems you should inform your doctor and you may need to be monitored while taking the medication or be given different medication
  • You can be allergic to mercaptopurine
  • You shouldn’t have any live vaccines while taking mercaptopurine or within six months of stopping. These include vaccines for polio, yellow fever, tuberculosis, German measles and MMR (measles, mumps and rubella). You can have a flu vaccine as long as it is the inactivated version. If you come into contact with someone who has recently had a live vaccine there is a chance the infection could be passed onto you

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