Crohn's & Colitis

Azathioprine and Mercaptopurine – Frequently asked Questions & Answers

This information has been provided with kind permission from Crohn’s and Colitis UK - a nationwide charity committed to providing information and support, funding crucial research and working to improve healthcare services for anyone affected by Inflammatory Bowel Disease.

For further information please download the full patient publication from Crohn’s and Colitis UK

How long will they take to work?

Azathioprine and mercaptopurine do not work immediately. It may take up to three months (and for some people up to six months) before you notice any benefit.

How long will I be taking azathioprine or mercaptopurine?

If you respond well to azathioprine or mercaptopurine you should be able to keep taking it for some years, provided the results of your monitoring blood tests are satisfactory and you do not develop any serious adverse reactions. Research suggests that for some people azathioprine or mercaptopurine can continue to be effective for at least five years and possibly longer.

How effective are azathioprine and mercaptopurine?

Azathioprine has been used for IBD for over 30 years. Mercaptopurine is a newer drug and tends to be prescribed less frequently in the UK.

A number of studies have found that these drugs can help to reduce symptoms and maintain remission in about six out 10 people with UC and seven out of 10 people with Crohn’s. There is also increasing evidence that, for some people with severe IBD, a combination therapy of azathioprine and one of the biologic drugs, such as infliximab or adalimumab, may be even more effective at bringing on and maintaining remission. (For more details on these drugs, see our Infliximab and Adalimumab drug treatment information sheets.)

However, azathioprine and mercaptopurine can also cause a range of side effects (see below) and one or two out of every 10 people stop taking these drugs because of an adverse reaction. The benefits of treatment with azathioprine or mercaptopurine, either alone or in combination with other drugs, will need to be balanced against such side effects, so discuss this with your doctor.

What checks will I need?

It is important to have regular blood tests while you are on azathioprine or mercaptopurine At the start of treatment you will need a Full Blood Count (FBC) and a Liver Function Test (LFT). This is because azathioprine and mercaptopurine can suppress normal bone marrow function and may occasionally cause liver problems.

These tests may be repeated weekly at first, then monthly until you are on your maintenance dose, and finally on a three monthly basis. Some doctors prefer to take a Full Blood Count every 2-4 weeks for two months, and then every 4-8 weeks.

Some centres offer a blood test before treatment to measure an enzyme called TPMT (Thiopurine Methyltransferase). This test can help to predict who is more likely to experience side effects with azathioprine or mercaptopurine. However, it cannot identify everyone who might be affected, so you will still need regular treatment monitoring and blood tests.

Your treatment monitoring may be managed by your hospital team or shared between the hospital and your GP. You may be given a booklet to record your blood test results. Take this booklet with you every time you see your GP, hospital doctor, specialist nurse or pharmacist, as it helps them to share information.

Will I need to take any special precautions while being treated with azathioprine or mercaptopurine?

Try to avoid close contact with people who have infections. Azathioprine and mercaptopurine affect the way the body’s immune system works, which can make you more prone to infections. Also, even a mild infection such as a cold or sore throat may develop into a more serious illness. Contact your doctor if you begin to feel unwell and think you may have caught an infection.

You may also be more likely to become seriously ill from the viruses that cause chickenpox, shingles, measles and pneumococcal disease. If you are not already immune, it could be a good idea to have a vaccination against these diseases before starting treatment with azathioprine or mercaptopurine. Otherwise, tell your doctor or nurse as soon as possible if you come into contact with anyone who has any of these conditions and you may be able to have a protective injection.

Avoid driving and hazardous work until you have learned how azathioprine or mercaptopurine affects you, as these drugs can occasionally cause dizziness.

Azathioprine and mercaptopurine increase the skin’s sensitivity to sunlight and the risk of developing some forms of skin cancer. Use sunblock and wear a hat and light clothing when out in strong sunshine. Do not use sunlamps or sun beds.

Does azathioprine or mercaptopurine affect fertility or pregnancy?

Tell your doctor if you are thinking of starting a pregnancy or find you are pregnant, and either you or your partner are taking azathioprine or mercaptopurine.

Azathioprine and mercaptopurine are not known to affect fertility, but there is a small amount of evidence that suggests conceiving a child with a man taking these drugs might increase the risk of miscarriage or birth defects. Because of this some doctors have advised men planning to father a child to switch to other medication. However, more recent research has not confirmed these findings and most doctors now tend to advise men to keep taking azathioprine or mercaptopurine, rather than risk a flare-up.

Most doctors also recommend women to continue with their azathioprine or mercaptopurine while pregnant as there may be a greater risk to the baby if the woman stops her treatment and becomes unwell. A large number of studies have looked at the effects on azathioprine and mercaptopurine on women with IBD in pregnancy, and the general consensus is that these drugs are safe and well tolerated. Even so, it is important to talk to your specialist about the risks and benefits for you personally, so that your decision can be based on your own health.

What are the possible side effects?

All medicines can cause unwanted side effects, although not everyone will get them. Some side effects may improve as your body adjusts to the new medication. Side effects from azathioprine and mercaptopurine may include the following:

  • Nausea (feeling sick), vomiting and loss of appetite. This reaction can be especially strong during the first few weeks of treatment. Taking your medication after eating or in two smaller doses each day instead of all at once may help reduce these side effects. A temporary reduction in dosage might also help, but talk to your doctor or nurse specialist before you make any changes to your dose or how you take it.
  • In some people, a flu-like illness with fever and general aches and pains a few days or weeks after starting treatment. Tell your doctor if you begin to feel unwell in this way. It is not usually serious but may mean the treatment has to be stopped.
  • Suppression of normal bone marrow function, which can cause anaemia and a reduction in the number of platelets and white blood cells. This can lead to weakness or breathlessness, a susceptibility to infections, and a tendency to bruise or bleed easily. Regular monitoring should pick up a reduced blood count but it is important that you contact your doctor if you develop an infection of any kind.
  • More occasionally, liver inflammation. Your treatment monitoring will include regular liver function tests (LFTs) to check your liver is working properly.
  • Diarrhoea, which may be difficult to distinguish from flare-up symptoms.
  • Pancreatitis, (inflammation of the pancreas, a digestive gland in the abdomen), which can cause acute pain in the abdomen. Contact your doctor if you experience this type of symptom.
  • An increased risk of developing lymphoma (a type of cancer affecting the lymph glands). However, research has shown that for most people the extra risk is likely to be small and is outweighed by the potential benefits gained from treatment with azathioprine or mercaptopurine.
  • An increased risk of certain types of skin cancer. This can be reduced by the careful use of sun blocks and suitable clothing when out in strong sunshine.
  • Other rare side effects can include dizziness, hair loss and skin rashes.

This information is designed to answer common questions you may have if you have been given azathioprine or mercaptopurine to treat your Crohn’s Disease or Ulcerative Colitis, the two main forms of Inflammatory Bowel Disease (IBD). It is not intended to replace specific advice from your own doctor or any other health professional. You can obtain further information from your doctor, pharmacist, the information leaflet supplied with your medication or from the website:

© Crohn’s and Colitis UK 2015

About the author

This information has been provided with kind permission from Crohn’s and Colitis UK – a nationwide charity committed to providing information and support, funding crucial research and working to improve healthcare service for anyone affected by  Inflammatory Bowel Disease.

Information and Support Line: 0300 222 5700

Visit for more information and a full list of their information sheets, booklets and guides.

To support their vital work, donate to Crohn’s and Colitis UK here

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