What is venetoclax (Venclyxto®)?

Venetoclax (Venclyxto®) is a targeted therapy drug 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 targeted therapies and the type of leukaemia you have.

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

How venetoclax works

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.

How venetoclax is given

You have venetoclax as tablets. Venetoclax is usually given for as long as it is working well for you. Your nurse or doctor will discuss your treatment plan with 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 may not be able to cope with large amounts. This can cause a side effect called tumour lysis syndrome.

TLS can cause swelling and pain in the joints (gout) and may also cause more serious effects. This can include kidney problems, abnormal heartbeat, or in rare cases, seizures. But your doctor will talk to you about how to prevent or manage this. This involves having lots of fluids and taking medication.

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. 

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

The dose of venetoclax is increased each week for the first 5 weeks. During this time there is a risk of TLS. 

Your doctor may also give you tablets called allopurinol. You may start taking these before starting treatment with venetoclax. Some people may be given a drip with a drug called rasburicase (Fasturtec®) instead of allopurinol tablets to prevent TLS. 

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

Taking venetoclax 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.

You take venetoclax tablets once a day. You should take them with a meal. They must be swallowed whole with a glass of water. Follow the advice you have been given about drinking plenty of fluids. You must not chew, open or crush the tablets. Take them at the same time every day. 

For the first 5 weeks, take them in the morning. You may be told to wait for the results of your blood test before taking your next dose.

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.

If you forget to take the tablets, you should take the missed dose if it is less than 8 hours after you normally take them. If it is more than 8 hours, let your doctor or nurse know. Do not take a double dose.

Other things to remember about your tablets:

  • Do not eat grapefruit, Seville oranges (bitter oranges), or starfruit while taking venetoclax. It can increase the amount of venetoclax in your blood which can affect the way the drug works. You also need to avoid drinking the juice of these fruits or taking food or supplements that might contain them.
  • 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 capsules or tablets, contact the hospital. Do not take another dose.
  • If your treatment is stopped return any unused capsules or 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 called neutropenia.

If you have an infection, it is important to treat it as soon as possible. Contact the hospital straight away on the 24-hour contact number you have if:

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

Symptoms of an infection include:

  • feeling shivery
  • a sore throat
  • a cough
  • diarrhoea
  • needing to pass urine (pee) often.

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

You will have a blood tests during your treatment. If your white blood cell count is low, your doctor may reduce or delay your treatment for a short time.

Sometimes your doctor may give you a drug called G-CSF (granulocyte-colony stimulating factor). This is a type of drug called a growth factor. It encourages the body to make more white blood cells. 

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.

Anaemia (low number of red blood cells)

This treatment can reduce the number of red blood cells in your blood. These cells carry oxygen around the body. If the number of red blood cells is low, you may be tired and breathless. Tell your doctor or nurse if you feel like this. If you are very anaemic, you may need a drip to give you extra red blood cells. This is called a blood transfusion.


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

You may feel sick during treatment. Your doctor will give you anti-sickness drugs to help prevent or control sickness. 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 and eat small amounts often. If you continue to feel sick, or if you vomit more than once in 24 hours, contact the hospital as soon as possible. They will give you advice and may change the anti-sickness drug to one that works better for you.


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.


This treatment can cause constipation. 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

Other medicines

Some medicines can affect your treatment or be harmful when you are having it. This includes medicines you can buy in a shop or chemist. Tell your cancer doctor about any drugs you are taking, including vitamins, herbal drugs and complementary therapies.


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.


Women are advised not to breastfeed while having this treatment. This is because the drugs could be passed to the baby through breast milk.


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 that 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.