Venetoclax (Venclyxto®)

Venetoclax (Venclyxto®) is a targeted therapy drug. It is used to treat chronic lymphocytic leukaemia (CLL).

What is venetoclax (Venclyxto®)?

Venetoclax (Venclyxto®) is used to treat chronic lymphocytic leukaemia (CLL). It may sometimes be used to treat other cancers. It is best to read this information with our general information about the type of cancer you have.

Venetoclax is a type of targeted therapy drug called a BCL2 inhibitor (blocker). BCL2 is a protein some leukaemia or cancer cells make too much of. It prevents the leukaemia or cancer cells from dying, so they continue to grow.

Venetoclax blocks the BCL2 protein and helps destroy the leukaemia or cancer cells. Venetoclax is not helpful for everyone with CLL. Your doctor can tell you if it might be helpful for you.

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

How venetoclax is given

You will be given venetoclax at a day unit or you may be able to take it at home. Venetoclax can be given on its own or in combination with other cancer drugs.

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

You will have your blood checked regularly during treatment. A nurse or a person trained to take blood (phlebotomist) will take a blood sample from you. This is to check that your blood cells are at a safe level for you to have treatment. Your doctor will tell you when your blood will be checked.

You will see a doctor or nurse before you have treatment. They will ask you how you have been feeling. If your blood results are okay, the pharmacist will prepare your targeted therapy. Your nurse will tell you when your treatment is likely to be ready.

You take venetoclax as tablets (orally).

Your course of treatment

Venetoclax comes as tablets. You take it on its own, or with other targeted therapy drugs.

The nurse or pharmacist will give you the tablets to take at home. Always take them exactly as explained. This is important to make sure they work as well as possible for you. You may be given tablets of different strengths.

You take venetoclax tablets once a day. For the first week of treatment, you will have a low dose. The dose is gradually increased each week until you reach the standard dose in week 5. This helps to reduce the risk of tumour lysis syndrome.

During the first 5 weeks of treatment, you should take venetoclax in the morning. This is because you may need blood tests to check it is safe to have treatment. Your doctor will tell you when your blood will be checked.

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

You usually take venetoclax on its own for as long as it is working well for you. If you are taking venetoclax in combination with another targeted therapy drug, you will take it for a set amount of time. Your nurse or doctor will discuss your treatment plan with you.

Taking venetoclax tablets

You should take venetoclax tablets with or after food. Swallow them whole with a glass of water. Do not chew, open or crush them. Take them at the same time every day.

During treatment with venetoclax, you should avoid grapefruit, star fruit and Seville oranges. This is because they can affect how venetoclax works. You should avoid:

  • drinking the juice of these fruits
  • eating food that contains them
  • taking supplements that might contain them.

If you forget to take the tablets, you should take the missed dose as soon as possible within the same day. If a full day has gone by, let your doctor or nurse know. Do not take a double dose unless your doctor tells you to.

Other things to remember about your tablets:

  • Wash your hands after taking 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.
  • Read the label to check strength and how many tablets to take.
  • 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.

Risk of tumour lysis syndrome (TLS)

Venetoclax can cause the leukaemia or cancer cells to break down very quickly. This releases uric acid (a waste product) into the blood. The kidneys usually get rid of uric acid, but they may not be able to cope with large amounts. This can cause a side effect called tumour lysis syndrome (TLS).

TLS can cause swelling and pain in the joints (gout). It may also cause more serious effects such as:

Your doctor will talk to you about how to prevent or manage these effects.

You will be asked to drink plenty of water 2 days before you start venetoclax. You should try to drink 1½ to 2 litres (3 to 3½ pints) of water every day for the first 5 weeks of treatment while the dose is increased.

You may go into hospital to have fluids through a drip when you start venetoclax. You may also need to do this when the dose is increased. You may need to stay in hospital overnight so you can be monitored. You will have regular blood tests to check the uric acid levels and make sure your kidneys are working well.

To help prevent TLS, your doctor may give you tablets called allopurinol. You start taking allopurinol tablets before starting treatment with venetoclax. You may be given a drip with a drug called rasburicase (Fasturtec®) instead of allopurinol tablets. 

You can read more about the symptoms of TLS in our information on side effects of venetoclax. 

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.

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.

Tumour lysis syndrome (TLS)

If you have TLS it is very important it is treated early.

Contact the hospital straight away on the 24-hour contact number if you have symptoms that could be linked to TLS. These include: 

  • feeling very hot (feverish) or have chills and feeling sick
  • having pain in your joints or muscles 
  • feeling breathless or having an irregular heart beat
  • having dark or cloudy urine 
  • feeling confused
  • having pain or swelling in your tummy 
  • having fits or seizures. 

These symptoms can be caused by different things. They may not be caused by TLS, but it is important to get them checked. 

You will have blood tests to check for TLS. Your doctor will talk to you about how to prevent or manage TLS. You can read more about this in our information on having venetoclax.

Feeling tired

Feeling tired is a common side effect of this treatment. It is often worse towards the end of treatment and for some weeks after it has finished. 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 use machinery.


This treatment 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.

If you have diarrhoea:

  • try to 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 for advice.

Anaemia (low number of red blood cells)

This treatment can reduce the number of red blood cells in your blood. Red blood cells carry oxygen around the body. If the number of red blood cells is low, this is called anaemia. You may have symptoms such as:

  • pale skin
  • lack of energy
  • feeling breathless
  • feeling dizzy and light-headed.

Tell your doctor or nurse if you have these symptoms.

If you are very anaemic, you may need a drip to give you extra red blood cells. This is called a blood transfusion.

Feeling sick

Your doctor will give you anti-sickness drugs to help prevent or control sickness during your treatment. Take the drugs exactly as your nurse or pharmacist tells you. It is easier to prevent sickness than to treat it after it has started.

If you feel sick, take small sips of fluids often and eat small amounts regularly. It is important to drink enough fluids. If you continue to feel sick, or are sick (vomit) more than once in 24 hours, contact the hospital as soon as possible. They will give you advice. Your doctor or nurse may change the anti-sickness drug to one that works better for you.


This treatment can cause constipation. Constipation means that you are not able to pass stools (poo) as often as you normally do. It can become difficult or painful. Here are some tips that may help:

  • Drink at least 2 litres (3½ pints) of fluids each day.
  • Eat high-fibre foods, such as fruit, vegetables and wholemeal bread.
  • Do regular gentle exercise, like going for short walks.

If you have constipation, contact the hospital for advice. Your doctor can give you drugs called laxatives to help.

Other information

Blood clot risk

Cancer and some cancer treatments can increase the risk of a blood clot. Symptoms of a blood clot include:

  • throbbing pain, redness or swelling in a leg or arm
  • suddenly feeling breathless or coughing
  • sharp chest pain, which may be worse when you cough or take a deep breath.

If you have any of these symptoms, contact the hospital straight away on the 24-hour contact number you have been given. If you cannot get through to your doctor, call the NHS urgent advice number on 111.

A blood clot is serious, but it can be treated with drugs that thin the blood (anticoagulants). Your doctor or nurse can give you more information.

You can help reduce the risk of developing a blood clot by:

  • staying active during treatment
  • drinking plenty of fluids, especially water.

You may be given anticoagulants to help prevent a clot.

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.


Your doctor will advise you not to get pregnant or make someone pregnant while having this treatment. The drugs 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.


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.


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 doctors and nurses you are having cancer treatment. Give them the contact details for your cancer doctor so they can ask for advice.

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