Idelalisib (Zydelig®)

Idelalisib (Zydelig®) is a targeted therapy drug. It is used to treat chronic lymphocytic leukaemia and follicular lymphoma.

What is idelalisib (Zydelig®)?

Idelalisib (Zydelig®) is a targeted therapy drug used to treat chronic lymphocytic leukaemia and follicular lymphoma. It may sometimes be used to treat other cancers. Some people may be given idelalisib in a clinical trial.

It is best to read this information with our general information about the type of cancer you have.

Idelalisib belongs to a group of targeted therapy drugs known as cancer growth inhibitors. It blocks the signals from proteins called kinases that make the leukaemia or cancer cells grow and divide. Blocking these signals can cause cancer cells to stop growing or die.

Your doctor will talk to you about this treatment and its possible side effects before you agree (consent) to have treatment.

How idelalisib is given

Idelalisib can be given on its own, or in combination with other targeted therapy drugs such as rituximab or ofatumumab.

Idelalisib comes as tablets. The nurse or pharmacist will give you the tablets to take home. Always take them exactly as explained. This is important to make sure they work as well as possible for you.

During treatment you will usually see a blood cancer doctor (haematologist) or a cancer doctor, a cancer nurse and a specialist pharmacist. This is who we mean when we mention doctor, nurse or pharmacist in this information.

You will have regular blood tests during treatment to check your blood cells are at a safe level and to see how well your liver is working.

You should keep taking idelalisib for as long as your doctor tells you. Do not stop taking it without talking to your doctor.

Taking idelalisib tablets

You usually take idelalisib twice a day. They must be swallowed whole with a glass of water. They should not be chewed, opened or crushed. Take them at the same times every day.

If you forget to take the tablets, take the missed dose as soon as possible within 6 hours. If more than 6 hours have gone by, do not take your missed dose. Leave it and take your next dose at the usual time.

Other things to remember about your tablets:

  • Keep them in the original package and at room temperature, away from heat and direct sunlight.
  • Keep them safe and out of sight and reach of children.
  • If you are sick just after taking the tablets, contact the hospital. Do not take another dose.
  • If your treatment is stopped, return any unused tablets to the pharmacist.

About side effects

We explain the most common side effects of this treatment here. We also include some less common side effects.

You may get some of the side effects we mention, but you are unlikely to get all of them. If you are also having treatment with other cancer drugs, you may have some side effects that we have not listed here. Always tell your doctor, nurse or pharmacist about any side effects you have.

Your doctor can give you drugs to help control some side effects. It is important to take them exactly as your nurse or pharmacist explains. This means they will be more likely to work for you. Your nurse will give you advice about managing your side effects. After your treatment is over, most side effects start to improve.

Serious and life-threatening side effects

Some cancer treatments can cause severe side effects. Rarely, these may be life-threatening. Your cancer doctor or nurse can explain the risk of these side effects to you.

Contact the hospital

Your nurse will give you telephone numbers for the hospital. If you feel unwell or need advice, you can call them at any time of the day or night. Save these numbers in your phone or keep them somewhere safe.

More information

We cannot list every side effect for this treatment. There are some rare side effects that are not listed. You can visit the electronic Medicines Compendium (eMC) for more detailed information.

Common side effects

Risk of infection

This treatment can reduce the number of white blood cells in your blood. These cells fight infection. If the number of white blood cells is low, you are more likely to get an infection. A low white blood cell count is sometimes called neutropenia.

Your doctor may give you antibiotics and other drugs to try to stop you getting an infection. These are called prophylactic medicines.

An infection can be very serious when the number of white blood cells is low. It is important to get any infection treated as soon as possible. Contact the hospital straight away on the 24-hour contact number you have been given if:

  • your temperature goes over 37.5°C (99.5°F)
  • you suddenly feel unwell, even with a normal temperature
  • you have symptoms of an infection
  • your temperature goes below 36°C (96.8°F).

Symptoms of an infection include:

  • feeling shivery and shaking
  • a sore throat
  • a cough
  • breathlessness
  • diarrhoea
  • needing to pass urine (pee) a lot, or discomfort when you pass urine.

It is important to follow any specific advice your cancer treatment team gives you.

The number of white blood cells will usually return to normal before your next treatment. You will have a blood test before having more treatment. If your white blood cell count is low, your doctor may delay your treatment for a short time, until your cell count increases.

Chest infection

Idelalisib can increase the risk of getting a chest infection (pneumonia). To reduce this risk, you will be given antibiotics to take during your treatment and for a few months after treatment has finished. Contact your doctor straightaway if you have any of the following symptoms:

  • a cough
  • breathlessness
  • coughing up mucus (phlegm)
  • a fever, with a temperature over 37.5°C (99.5°F).


Feeling tired is a common side effect. Try to pace yourself and plan your day so you have time to rest. Gentle exercise, like short walks, can give you more energy. If you feel sleepy, do not drive or operate machinery.

Feeling sick

This is usually mild. Your doctor can prescribe anti-sickness (anti-emetic) drugs to help prevent or control sickness. If you still feel sick, tell your doctor as they can prescribe different anti-sickness drugs that may work better for you.


This treatment may cause diarrhoea or bowel inflammation (colitis). Your nurse or doctor may give you anti-diarrhoea drugs to take at home. More serious diarrhoea or colitis can sometimes happen months after treatment has started and may lead to treatment being stopped.

If you have diarrhoea:

  • follow any advice you have been given about taking anti-diarrhoea drugs
  • drink at least 2 litres (3½ pints) of fluids each day
  • avoid alcohol, caffeine, milk products, high-fat foods and high-fibre foods.

Contact the hospital straight away if:

  • you have diarrhoea at night
  • you have diarrhoea more than 4 times in a day
  • the anti-diarrhoea drugs do not work within 24 hours.

Some people may need to go to hospital to have fluids through a drip. You may also need to take drugs called steroids.

Pain in the tummy (abdomen)

You may get pain or discomfort in your tummy. Your doctor can prescribe drugs to help. Tell them if the pain does not improve, or gets worse.

Very rarely, idelalisib can cause a hole (perforation) in the bowel. Tell your doctor immediately if you:

  • have severe pain in your tummy
  • feel sick or are vomiting
  •  have a high temperature or feel cold and shaky (chills).

Skin changes

This treatment may affect your skin. It can cause a rash, which may be itchy. Your doctor or nurse can tell you what to expect. If your skin feels dry, try using an unperfumed moisturising cream every day.

Always tell your doctor or nurse about any skin changes or if they get worse. They can give you advice and may prescribe creams or medicines to help. Any changes to your skin are usually temporary and improve when treatment finishes.

Rarely, a much more serious skin condition can develop. You may have a skin rash which then blisters, and your skin can peel. Contact the hospital straightaway if this happens.

Effects on the liver

This treatment can affect how your liver works. You will have regular blood tests to check this. Your doctor may give you steroids if the blood tests show liver changes.

Sometimes liver changes can be serious. Contact the hospital straight away if you have any of these symptoms during treatment or after it finishes:

  • yellow skin or eyes
  • feeling very sleepy
  • dark urine (pee)
  • unexplained bleeding or bruising
  • loss of appetite.

You may need steroids or other treatments, and may be monitored until your liver recovers.

Increased cholesterol levels

The drug can increase the cholesterol level in your blood. Your doctor will monitor this with blood tests.

Less common side effects

Effects on the lungs

Always tell your doctor if you notice wheezing or a cough, or if you feel short of breath.

If you suddenly become breathless or your breathing gets worse, possibly with a cough or high temperature, contact the hospital immediately. You may have an inflammation of the lungs. This is a rare side effect. Your doctor can tell you more about this.

Effects on the nervous system

Rarely, this treatment can affect the nervous system. It can cause a condition called PML (progressive multifocal leukoencephalopathy). Symptoms include:

  • difficulty speaking
  • difficulty walking
  • memory loss
  • confusion
  • changes in your eyesight.

Contact the hospital straight away if you have any of these symptoms. Or if someone with you notices you have these symptoms.

It is important not to drive or operate machinery if you notice these effects.

Other information

Other medicines

Some medicines can affect how this treatment works or be harmful when you are having it. Always tell your cancer doctor about any drugs you are taking or planning to take, such as:

  • medicines you have been prescribed
  • medicines you buy in a shop or chemist
  • vitamins, herbal drugs and complementary therapies.

Tell other doctors, pharmacists or dentists who prescribe or give you medicines that you are having this cancer treatment.

You can visit the electronic Medicines Compendium (eMC) for more detailed information about your treatment.


Vaccinations can reduce your risk of getting certain infections. Your doctor or nurse may talk to you about having vaccinations.

Doctors usually recommend that people with cancer have a flu vaccination and a coronavirus vaccination. These are both inactivated vaccinations that can help reduce the risk of infection. People with weak immune systems can have these, as they are not live vaccinations.

If your immune system is weak, you need to avoid live vaccinations. This is because they can make you unwell. Live vaccines, such as shingles, contain a very weak version of the illness they are vaccinating you against. Your cancer doctor or GP can tell you more about live and inactivated vaccinations.

Cytomegalovirus (CMV)

CMV is a very common virus that many people have. It does not usually cause any problems or symptoms. Most people don’t know they have it. The body’s immune system usually controls it so that it is inactive (dormant). Once you have the virus in your body it stays there for life.

If you have CMV, treatment with idelalisib may make the virus active again. You will have regular blood tests to check for this. If CMV does becomes active you will be given treatment for this straightaway, usually with tablets.


Some cancer drugs can affect whether you can get pregnant or make someone pregnant. If you are worried about this, it is important to talk with your doctor before you start treatment.


Your doctor will advise you not to get pregnant or make someone pregnant while having this treatment. The drug may harm a developing baby. It is important to use contraception during your treatment and for a while after treatment finishes. Your doctor, nurse or pharmacist can tell you more about this.

Idelalisib may make hormonal contraceptive pills and implants less effective. You will need to use a barrier method of contraception like condoms as well. Your doctor, nurse or pharmacist can tell you more about this.


You are advised not to breastfeed while having this treatment, or for some time after treatment finishes. This is because the drugs could be passed to the baby through breast milk.

Your doctor or nurse can give you more information.

Medical treatment

If you need medical treatment for any other reason, always tell the doctors and nurses that you are having treatment with idelalisib. Give them the contact details for your haematologist or cancer doctor so they can ask for advice.

If you think you need dental treatment, talk to your blood doctor (haematologist), cancer doctor or nurse. Always tell your dentist you are having cancer treatment.