Azathioprine and mercaptopurine for inflammatory bowel disease

Inflammatory bowel disease (IBD) describes a group of medical conditions that make the digestive system become inflamed (swollen and painful). The 2 main types of IBD are:

These are long-term conditions that involve inflammation of the gut.

Azathioprine and mercaptopurine are medicines used to treat IBD. They control your body’s immune system to treat the inflammation.

Azathioprine and mercaptopurine can also help other medicines work better to treat your condition, including adalimumab and infliximab. They prevent your body from making substances called antibodies in response to these medicines and reducing how effective they are.

Azathioprine and mercaptopurine work slowly. It may take up to 3 months before they start to help. The length of treatment is different for each person, but it is likely to be several years or longer in some cases. 

Unlicensed medicine

Mercaptopurine is not officially approved (licensed) to treat IBD. However, it is thought to be a safe and effective treatment. Read more about unlicensed medicines

Before you start the medicine

You need a few blood tests to make sure that it's safe for you to start taking the medicine. These may include tests to check:

  • your blood count (the types and numbers of cells in your blood)
  • how well your liver and kidneys work
  • the amount of a chemical substance (enzyme) called TPMT in your blood, which breaks down azathioprine and mercaptopurine
  • for hepatitis B (a liver infection)
  • for hepatitis C (a virus that can infect the liver)
  • for Epstein-Barr virus, which causes glandular fever (a viral infection)
  • for HIV (a virus that attacks the body's immune system)
  • for chickenpox

We strongly recommend that you have the vaccine to protect yourself from pneumonia (swelling of the tissue in 1 or both lungs). You also need to have the flu vaccine every year while taking the medicine. Your GP surgery can arrange these vaccines for you.

How to take the medicine

The amount (dose) of medicine that you take depends on your weight. Usually, you have between 25mg and 200mg each day. We might change the dose during your treatment.

Both medicines are usually available as 25mg and 50mg tablets. For children, a liquid preparation of mercaptopurine is also available.

It is best to take the medicine with or immediately after food to help reduce stomach upset. You need to swallow the medicine whole with a glass of water.

People usually take the medicine in the morning. However, it can help to take the medicine before you go to bed if you have problems with feeling sick (nausea).

In most cases, you take the full dose at the same time. Sometimes, however, we may recommend splitting the dose throughout the day.

If you forget to take a dose 

This section explains what to do if you forget to take a dose of azathioprine or mercaptopurine.

  • If you remember within 12 hours of your dose being due, take your dose as normal.
  • If you remember more than 12 hours later, take the next dose as planned. Do not double your dose.

Try not to forget any doses. Both medicines work slowly. If you regularly forget to take your medicine, you will have a lower level in your blood. This might make the medicine less effective.

Regular blood tests

When you start taking azathioprine or mercaptopurine, you need regular blood tests every few weeks. These blood tests are essential. If you miss the blood tests, your prescription might be stopped until you have them.

If your blood tests are stable after 3 months of treatment, you can continue to have your blood checked every 3 months with your GP. 

When you start the medicine, our specialist IBD pharmacists monitor you. They check if:

  • you have any side effects
  • your blood test results are normal

You can expect to have regular appointments with the pharmacists or get phone calls from them.

We also monitor the levels of azathioprine and mercaptopurine in your blood. Depending on the results, we may change your dose.

Taking azathioprine and mercaptopurine with other medicines

Both medicines can affect other medicines that you may take and change how they work.

Before you start taking azathioprine or mercaptopurine, please tell your doctor about all the medicines that you take. This includes any prescribed medicines or medicines that you buy from a pharmacy or shop (including herbal and homeopathic medicines).

Always check with your doctor or pharmacist before starting any new medicines.

Allopurinol for gout

A medicine called allopurinol, which is used to treat gout, does not mix with azathioprine or mercaptopurine. For this reason, you should not usually take the medicines together.

In some cases, however, we prescribe allopurinol to be used with a low dose of azathioprine or mercaptopurine. We do this if you get lots of side effects or have abnormal blood tests.

Other medicines

Other medicines can also react with react with azathioprine and mercaptopurine. They include:

  • co-trimoxazole (an antibiotic used to treat bacterial infections)
  • trimethoprim (an antibiotic mainly used to treat bladder infections)
  • warfarin (a medicine that stops blood clotting)


It is best to avoid ‘live’ vaccines when you take azathioprine or mercaptopurine and for at least 3 months after stopping the medicine. Live vaccines use a weakened form of the germ that causes a disease.

If you are thinking about having vaccinations, check with your doctor or pharmacist first.

Drinking alcohol

We recommend that you keep well within the national recommended limits of alcohol.

Men and women are advised not to drink more than 14 units of alcohol a week, which should be spread over 3 days or more. 14 units are about the same as 6 pints of average strength beer or 6 medium (175ml) glasses of average strength wine.

Any amount of alcohol can damage the liver, especially if you have a condition like autoimmune hepatitis. This is when your immune system attacks the liver and makes it inflamed. Depending on the state of your liver, your doctor may ask you to avoid alcohol completely.

Pregnancy and breastfeeding

It is important that your IBD is controlled as well as possible during pregnancy.

Please tell us if you are pregnant or planning to get pregnant. We can then review all of your medicines and monitor you more closely.

Our team can explain the risks and benefits if you keep taking azathioprine or mercaptopurine when you are pregnant. Many people continue to take these medicines safely throughout their pregnancy.

Side effects

Azathioprine and mercaptopurine work by "calming" or controlling your immune system and making it less active. This means that you might be more likely to get an infection. Contact your GP if you start to feel unwell and think that you might have an infection.

Everyone responds differently to the medicine, but possible side effects include:

  • feeling sick (nausea), especially at the start of treatment. We may need to change how and when you take the medicine
  • flu-like symptoms, including headache, muscle ache and feeling generally unwell. You might have these symptoms when you start the treatment, but they often go away within a few weeks
  • headache
  • diarrhoea
  • dizziness
  • hair loss, although this can often get better while you continue treatment
  • rashes

More serious side effects

If you get any more serious side effects, we will need to monitor them closely. In some cases, we may decide to stop the medicine.

When you have your regular blood tests, we look for signs of the following side effects.

  • The medicine can make your bone marrow (a spongy tissue found in the centre of some bones) less active. This reduces the number of red blood cells, white blood cells and platelets (cells to control bleeding) that your bone marrow makes.
  • The medicine can make you more likely to get an infection. We check for any serious infection that needs to be treated with antibiotics.
  • About 1 in 20 people have abnormal liver function tests (which suggest that their liver is not working well).
  • About 1 in 30 people get pancreatitis (inflammation of the pancreas).

Call a GP, our team or 111 straight away if:

  • you notice any unexplained signs of bruising or bleeding 
  • you have a high temperature (fever)
  • you notice yellowing of the eyes or skin (jaundice) 
  • you have severe pain in the upper part of your stomach
  • you notice a rash after starting the medicine

Go straight to the emergency department (A&E) if you feel very unwell.

Sun protection

Azathioprine and mercaptopurine make your skin more sensitive to the sun. This increases your chance of getting some types of skin cancer.

It is important to wear protective clothing in the sun. You also need to use a high sun protection factor (SPF 50) sun cream.


Lymphoma is a type of cancer that affects the lymph glands, which help your body to fight infections. When you take azathioprine or mercaptopurine, you have a higher chance of getting lymphoma than the general population. However, this is still very rare (about 1 in 2,500 people get lymphoma).

Getting a repeat prescription

After your hospital specialist has started you on azathioprine or mercaptopurine, they advise your GP what dose to prescribe.

We give you your first prescriptions at the hospital until your GP agrees to prescribe the medicine for you. If your GP does not agree to prescribe the medicine, you will continue to get your prescriptions from our hospital.

If we need to make any changes to your dose, we will tell your GP.

Useful information

Crohn’s and Colitis UK has more information about IBD. You can visit their website at: www.crohnsandcolitis.org.uk

Resource number: 4279/VER3
Last reviewed: May 2022
Next review: October 2023

Contact us

If you have any questions or concerns about azathioprine or mercaptopruine, you can:

In your email, please give your name, date of birth, hospital number and details of your enquiry. We respond to messages from 9am to 5pm, Monday to Friday.

If you are concerned outside these hours, please contact your GP, go to 111.nhs.uk or call 111.

Do you have any comments or concerns about your care?

Contact our Patient Advice and Liaison Service (PALS)

Is this health information page useful?