Idelalisib (Zydelig®)

Idelalisib (Zydelig®) belongs to a group of targeted therapy drugs called cancer growth inhibitors. 

It is best to read the information about this drug with our general information about targeted therapy drugs and the type of cancer you have. 

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

More information about this treatment

This information is correct at time of publishing. But sometimes the types of cancer this treatment is used for, or treatment side effects, may change between revision dates.

You can talk to your cancer team if you want more detailed information about this treatment. Or visit the electronic Medicines Compendium (eMC) website, which has patient information leaflets (PIL) for individual drugs.

How idelalisib is given

Idelalisib comes as tablets you take at home. Idelalisib may be given on its own or with other cancer drugs, such as rituximab.

During treatment, you usually see a:

  • cancer doctor 
  • chemotherapy nurse or a specialist nurse
  • specialist pharmacist.

This is who we mean when we mention doctor, nurse or pharmacist in this information

While you are taking idelalisib, you will have regular blood tests to check your blood cells are at a safe level. The tests will also check how well your liver is working.

You will speak to a doctor, nurse or pharmacist before you have treatment. They will ask how you have been feeling. If your blood results are okay, the pharmacy team will prepare your targeted therapy.

Your course of treatment

Your doctor, nurse or pharmacist will explain your treatment plan. Always take the tablets exactly as explained. This is important to make sure they work as well as possible for you. 

Your nurse or pharmacist may also give you anti-sickness drugs and other medicines to take home. Take all your tablets exactly as they have been explained to you.

You usually take idelalisib for as long as:

  • it is still working for you 
  • and the side effects can be managed.

Taking idelalisib tablets

The nurse or pharmacist will give you the tablets to take home. 

You usually take idelalisib twice a day. The tablets must be swallowed whole with a glass of water. They should not be chewed, broken or crushed. Take them at the same time every day. Idelalisib can be taken with or without food.

If you forget to take your tablets, take the missed dose as soon as possible if it is less than 6 hours late. If you miss a dose by more than 6 hours, tell your doctor or nurse. Do not take a double dose unless your doctor tells you to.

Other things to remember:

  • wash your hands after taking your tablets
  • keep the tablets in the original package and at room temperature 
  • keep the tablets away from moisture, heat and direct sunlight
  • keep the tablets safe and out of the sight and reach of children 
  • keep the tablets safe and out of direct contact with other people
  • if you are sick (vomit) just after taking your tablets, contact the hospital – do not take another dose
  • if you stop treatment, 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 that are less common. 

You may get some of the side effects we mention, but you are unlikely to get all of them. And you may have some side effects, including rarer ones, that we have not listed here. 

Other cancer treatments may cause different side effects. If you are also having other cancer treatment, you may have other side effects.

Always tell your doctor, nurse or pharmacist about any side effects you have. They can give you: 

  • drugs to help control some side effects 
  • advice about managing side effects. 

It is important to take any drugs exactly as explained. This means they will be more likely to work for you.

Serious and life-threatening side effects

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

Contact the hospital

Your doctor, nurse or pharmacist will give you 24-hour contact numbers for the hospital. If you feel unwell or need advice, you can call at any time of the day or night. Save these numbers in your phone or keep them somewhere safe.

Common side effects

Risk of infection

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

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. If you have any of the following symptoms, contact the hospital straight away on the 24-hour number:

  • a temperature above 37.5°C  
  • a temperature below 36°C 
  • you feel unwell, even with a normal temperature
  • you have symptoms of an infection.

Symptoms of an infection include:

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

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

Your white blood cell count 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

This treatment can increase the risk of getting a chest infection (pneumonia). You will be given antibiotics to take during your treatment to reduce this risk. You may also take antibiotics 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).


Idelalisib may cause diarrhoea. Diarrhoea means passing more stools (poo) than is usual for you or having watery or loose stools. If you have a stoma, it will be more active than usual. Idelalisib may also cause bowel inflammation (colitis). 

Your nurse or doctor may give you anti-diarrhoea drugs to take at home. Serious diarrhoea or colitis can also sometimes happen months after treatment has started. If this happens, your treatment may need to be delayed or stopped. Contact the hospital if you have diarrhoea. 

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
  • you have severe pain in your tummy (abdominal pain)
  • you have blood or mucus in your stools (poo).

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

Skin changes

This treatment may cause skin changes, including:

  • a rash
  • itching
  • dry skin.

If you are out in the sun, use a suncream with a high sun protection factor (at least SPF 30) to protect your skin. 

If you notice any skin changes during treatment or after it, always tell your doctor or nurse straight away. They can give you advice and may prescribe creams or medicines to help.

Rarely, a much more serious skin condition can develop. You may have:

  • a skin rash which then blisters
  • peeling skin
  • flu-like symptoms, such as a high temperature and joint pain.

If you have any of these symptoms, contact your doctor or hospital straight away.

Effects on the liver

This treatment can affect how your liver works. You will have regular blood tests to check this. 

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)
  • pain in the right side of your tummy (abdomen).

If the blood tests show liver changes, your doctor may need to delay treatment until it improves. You may need steroids or other treatments. You may also be monitored until your liver recovers.

Increased levels of fat in the blood

The drug can increase the level of a type of fat called triglyceride in your blood. This will be monitored with blood tests.

Less common side effects

Effects on the lungs

This treatment can cause changes to the lungs. Tell your doctor, nurse or pharmacist if you develop: 

  • a cough that does not go away
  • wheezing
  • breathlessness.

You should also tell them if any existing breathing problems get worse. You may have tests to check your lungs.

Effects on the nervous system

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

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

Contact the hospital straight away if:

  • you have any of these symptoms
  • someone with you notices you have these symptoms.

If you notice these effects, it is important not to drive or operate machinery.

Other important information

Cytomegalovirus (CMV)

Cytomegalovirus (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, taking idelalisib may make CMV 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. Treatment is usually with tablets.

Blood clot risk

Cancer and some cancer treatments can increase the risk of a blood clot. Contact the hospital straight away on the 24-hour number if you have any of these symptoms during or after treatment:

  • throbbing pain or swelling in a leg or arm
  • reddening of the skin in the area – if you have black or brown skin, this can be harder to notice, but the skin might become darker
  • suddenly feeling breathless or coughing.

Always call 999 if you have:

  • chest pain
  • difficulty breathing.

A blood clot is serious, but it can be treated with drugs called anticoagulants. These thin the blood. Your doctor, nurse or pharmacist can give you more information about preventing and treating blood clots.

Other medicines

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


Doctors usually recommend that people with cancer have vaccinations for flu and for coronavirus (covid). These help reduce your risk of serious illness from these infections. Most people can have these vaccines, including people with weak immune systems.

If your immune system is weak, you should not have live vaccinations. Live vaccines can make you unwell because they contain a very weak version of the illness they will protect you against. Live vaccines include Zostavax®, which is a shingles vaccine, and the yellow fever vaccine.

It is important to ask your doctor, nurse or pharmacist for advice about having vaccinations. They can explain what vaccines are right for you and when it is best to have them.


Your doctor will advise you not to get pregnant or make someone pregnant while having this treatment. Idelalisib may harm a developing baby. It is important to use contraception during your treatment and for a while after treatment finishes. 

Idelalisib may affect how well hormonal contraception works during treatment and up to a month afterwards. If you use hormonal contraception, it is important to also use a barrier method such as a condom to prevent pregnancy during this time.

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 ends. This is because the drugs could be passed to the baby through breast milk. 

Your doctor, nurse or pharmacist can give you more information.


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.

Medical and dental treatment

If you need medical treatment for any reason other than cancer, always tell the healthcare professional that you are having cancer treatment. Give them the contact details for your cancer doctor or cancer team so they can ask for advice.

If you have appointments with a dentist, always tell them you are having cancer treatment. Talk to your cancer team before you have any dental treatment.

About our information

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert health professionals and people living with cancer.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

The language we use

We want everyone affected by cancer to feel our information is written for them.

We want our information to be as clear as possible. To do this, we try to:

  • use plain English
  • explain medical words
  • use short sentences
  • use illustrations to explain text
  • structure the information clearly
  • make sure important points are clear.

We use gender-inclusive language and talk to our readers as ‘you’ so that everyone feels included. Where clinically necessary we use the terms ‘men’ and ‘women’ or ‘male’ and ‘female’. For example, we do so when talking about parts of the body or mentioning statistics or research about who is affected.

You can read more about how we produce our information here.

Date reviewed

Reviewed: 01 July 2023
Next review: 01 July 2025
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.