Azathioprine is a type of medicine called an immunosuppressant. Immunosuppressants help "calm" (or control) your body's immune system.

This medicine helps treat inflammatory conditions such as:

• rheumatoid arthritis
• Crohn's disease and ulcerative colitis
• severe inflammation of the liver, skin or arteries
• some blood disorders

If you have had a transplant, taking azathioprine can prevent your body from rejecting your new organ.

Azathioprine is sometimes also used to treat conditions that affect the skin, such as lupus or atopic eczema.

Azathioprine is available on prescription only. You will usually be prescribed this medicine by a specialist doctor.

It comes as tablets. It is also available as an injection, but this is usually only given in hospital.

• You'll have regular blood tests before and during your treatment.
• Depending on why you're taking it, you'll usually notice an improvement in your condition after a few weeks.
• Men and women taking azathioprine need to use suitable contraceptives to prevent pregnancy.
• Ask your doctor for urgent advice if you have been in contact with anyone who has chickenpox or shingles while taking azathioprine.
• Use a sunscreen while taking azathioprine, as this medicine can make your skin more sensitive to sunlight.

Most adults and children can take azathioprine. You will have a blood test before you start taking this medicine to make sure it is safe for you.

This medicine is not suitable for some people. Tell your doctor before taking this medicine if you:

• have had an allergic reaction to azathioprine or any other medicines (including mercaptopurine, a medicine for treating blood cancer)
• have an infection or a high temperature, or generally feel unwell
• have any unusual bleeding or bruising
• have ever had any liver problems
• have ever had cancer
• have a condition that affects your bone marrow
• are due to have surgery involving muscle relaxants
• have Lesch Nyhan syndrome, or a rare inherited condition affecting your NUDT15 gene
• have ever been told that your body produces too little thiopurine methyltransferase (TMT, an enzyme)
• are pregnant, trying to get pregnant or could make someone pregnant

Always follow your doctor's instructions for how to take azathioprine.

Swallow the tablets whole, with a drink of water. Do not chew them.

You'll usually take your tablets once or twice a day. You can take them with or without food.

Dosage

Your dose depends on your body weight and why you need to take azathioprine. Your doctor will tell you how much to take.

After a transplant

The usual dose on the 1st day is up to 5mg for each kilogram you weigh.

From day 2 onwards, you'll take 1mg to 4mg for each kilogram you weigh. You'll usually need to take this medicine long term, probably for the rest of your life.

For other conditions

The usual starting dose is 1mg to 3mg each day, for each kilogram you weigh.

Your doctor may lower your dose as your condition gets better.

However, it takes a while for azathioprine to work. You may have to wait a few months to see an improvement.

Regular blood tests during treatment

Taking azathioprine can sometimes affect your liver, kidneys or bone marrow.

You will have blood tests to check your liver function, kidney function and blood count before you start taking this medicine.

From week 1 to week 8 of your treatment you'll have blood tests every week. This is particularly important if you're taking a high dose, or you have kidney or liver problems.

From week 9 onwards you will have blood tests less often. Your doctor will decide how often you need them. You may only need them every few months.

It's important to have ongoing monitoring for as long as you're taking this medicine.

What if I forget to take a dose?

Check with your doctor or a pharmacist if you miss 2 doses or more.

If you forget to take 1 dose, take it as soon as you remember, unless it's almost time for the next dose. In this case, skip the missed dose and take the next one at the usual time.

Do not take a double dose to make up for a missed dose.

If you forget doses often, it may help to set an alarm to remind you. You could also ask a pharmacist for advice on other ways to help you remember to take your medicine.

What if I take too much?

If you need to go to hospital, take the medicine packet or leaflet inside it, plus any remaining medicine, with you.

Like all medicines, azathioprine can cause side effects, but not everyone gets them.

Common side effects

These common side effects happen in more than 1 in 10 people (when you first start treatment or when your dose is increased).

Tell your doctor if these side effects bother you or do not go away:

• feeling sick (mild nausea)
• headaches

Serious side effects

Some people can have serious side effects when taking azathioprine.

Stop taking your medicine and contact your specialist or a doctor immediately if:

• you feel tired all the time, dizzy or sick, or you are vomiting or have diarrhoea
• you have a high temperature with shivering or chills, cough or a sore throat
• you get a rash or notice any changes to your skin, such as blisters or peeling
• your joints or muscles are hurting
• your pee changes colour or you start peeing more or less than usual (this can be a sign of kidney problems)
• you feel confused, lightheaded or weak (these can be signs of low blood pressure)
• you're bleeding or bruising more easily than usual
• you notice lumps anywhere on your body
• you have severe stomach ache (abdominal pain) and back pain

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to azathioprine.

These are not all the side effects of azathioprine. For a full list, see the leaflet inside your medicine packet.

What to do about:

• feeling sick – stick to simple meals and do not eat rich or spicy food while you're taking this medicine. It might help to take azathioprine after you have had a meal or snack.
• headaches – make sure you rest and drink plenty of fluids. Do not drink too much alcohol. Ask a pharmacist to recommend a painkiller.

These side effects are more likely when you first start taking azathioprine or when your dose is increased. You will usually feel better after a week or so.

Talk to your doctor if these side effects do not go away or they get worse.

Azathioprine is not usually recommended in pregnancy or while breastfeeding. However, your doctor may want to prescribe azathioprine if this is the best treatment for you.

If you become pregnant while taking azathioprine, speak to your doctor immediately about the risks and benefits of continuing to take this medicine.

If your doctor says it's OK for you to take azathioprine during pregnancy, your baby can be more likely to get an infection. They may need to have blood tests and extra monitoring after they're born. Your doctor or midwife may want to delay giving your child vaccines until they are 6 months old.

Read more about how azathioprine can affect you and your baby from Best Use of Medicines in Pregnancy (BUMPs).

Azathioprine and breastfeeding

This medicine is not usually recommended when breastfeeding. Small amounts of azathioprine may pass into your breast milk.

Speak to your doctor if you want to breastfeed. If they say it's OK for you to take azathioprine while breastfeeding, your baby will probably need extra monitoring or blood tests. This is to make sure they do not have any side effects.

Contact a doctor or your health visitor if your baby is more sleepy than usual or is not feeding properly.

Azathioprine can affect the way some medicines work. Other medicines can also affect the way azathioprine works.

Tell your doctor or a pharmacist if you:

• take allopurinol (a medicine mainly used for gout)
• take ciclosporin or tacrolimus (immunosuppressant medicines)
• take warfarin (used to prevent blood clots)
• are having chemotherapy (used to treat cancer)
• are going to have any type of surgery – tell your doctor or anaesthetist that you take azathioprine beforehand
• have recently had or are due to have any vaccinations (especially a "live" vaccine)

Children taking azathioprine must not have a "live" flu vaccine (this usually applies to children aged 2 to 17 years). Adults are given an "inactivated" flu vaccine and this usually causes no problems with azathioprine.

Mixing azathioprine with herbal remedies and supplements

There's very little information about taking herbal remedies and supplements with azathioprine.