PARP inhibitors side effects

Find out about the different side effects PARP inhibitors may cause, and how to manage them.

PARP inhibitors are a targeted treatment for cancer. Usually, you start taking them eight to 12 weeks after finishing chemotherapy. They work by stopping cancer cells from repairing DNA damage within themselves which causes the cancer cells to die.

PARP inhibitors are tablets taken orally (by mouth) at home. Your treatment team will tell you how often to take these.

There are three different types of approved PARP inhibitors in the UK:  

Access to PARP inhibitors depends on certain factors. To find out which PARP inhibitors you may be able to access use our PARP inhibitor tool.

PARP inhibitor tool

What are the common side effects of PARP inhibitors? 

All three PARP inhibitors share most of their common side effects. Often, side effects will appear early on in treatment and improve over time. It’s important to be honest with your team about any side effects you notice. There are many different ways they will be able to help you. They can give you management advice or medications to help with the side effect.

If the side effects from a PARP inhibitor are impacting your daily life, your treatment team may lower your daily dose or pause the treatment. Lowering the dose of PARP inhibitors has been shown to help reduce side effects without reducing the overall effectiveness of treatment. Sometimes PARP inhibitors need to be stopped if side effects are too severe.  

You will be given a 24-hour emergency contact number for your cancer centre to call for help or advice while on treatment. 

Impact on your blood

When you’re on PARP inhibitors your treatment team will monitor your blood with regular blood tests. This is because PARP inhibitors can sometimes affect your blood cells and your team need to make sure your blood stays healthy.  

They will take blood tests to measure:

  • the number of different blood cells in your blood. This is called a blood count. There are many different types of blood cells such as red blood cells, white blood cells and platelets  
  • how your kidneys are working
  • how your liver is working.

The most common PARP inhibitor side effects that affect your blood are: 

Low red blood cell count (anaemia)

A lack of red blood cells is also called anaemia. You may feel tired, weak, have headaches, a fast heartbeat, breathlessness, or pale skin. If you experience any of these symptoms, tell your treatment team.  

Depending on how severe your anaemia is, your doctor may consider:

  • replacing vitamin B12, folic acid or iron with supplements if your levels are low
  • pausing your PARP inhibitor treatment or lowering your dose
  • giving you a blood transfusion to ensure you have enough red blood cells. A blood transfusion is when you're given blood from a donor.
Infection risk from low white blood cell count

White blood cells are part of your immune system which help you fight infections. If your white blood cells are below a certain level, your doctor may need to pause your PARP inhibitor treatment or lower your dose. Having less white blood cells than usual is also called neutropenia.  

As white blood cells are part of your immune system, if you have less than normal you may be more likely to get infections. Macmillan has tips on how to avoid infection.  

Symptoms of an infection can include:

  • a cough
  • sore throat
  • achy muscles
  • fever  
  • feeling shivery or generally unwell.

It's important to check your temperature and to tell your treatment team urgently if you experience any of these symptoms.

Rarely, PARP inhibitors cause an abnormal increase in white blood cells which is called leukaemia. This will be monitored with regular blood tests. If you’re worried about this talk to your treatment team. 

Increased bruising or bleeding from a lack of platelets 

Platelets are a type of blood cell that help form clots when you injure or cut yourself to stop the bleeding. If you have a lack of platelets, you may bruise or bleed more than usual when injured. A lack of platelets is also called thrombocytopenia.  

You may experience nosebleeds, bleeding gums or blood in your wee or poo. If you notice these symptoms or any unexplained bruising or bleeding, you should tell your treatment team. Depending on how low your platelet count is, they may pause your PARP inhibitor treatment or put you on a lower dose. Sometimes PARP inhibitor treatment has to be stopped because of this.

High blood pressure

If you're taking niraparib, you will need to have your blood pressure checked regularly as niraparib can cause it to increase. High blood pressure is also called hypertension.  

You may need to start blood pressure medications if your blood pressure is too high, or have doses increased if you’re already on medication for blood pressure. If this doesn't help, your doctor may lower your dose of niraparib. 

Digestive system issues

Side effects which impact your digestive system are common with PARP inhibitors. In general, it can help to eat smaller, more frequent meals throughout the day rather than three big meals and to drink plenty of fluids.  

Macmillan and Cancer Research UK have more information on common digestive system issues caused by cancer treatments. 

Feeling sick (nausea) and being sick (vomiting)

Usually this impacts people early on in treatment and it often improves as time passes. If you taken a PARP inhibitor dose and then been sick, don't replace it and wait to take your next dose as normal.

If you're feeling or being sick, your treatment team can give you anti-sickness medication. There are a few types of medication they can offer so if you still feel sick after trying one, let them know and they can give you a different type. It can help to take the anti-sickness tablet 30 minutes before taking a PARP inhibitor.

For niraparib specifically, as it's taken only once a day, taking it before bed can help with nausea. 

Cancer Research UK and Macmillan have more tips on how to manage feeling sick or vomiting. 


You may poo more often than usual or have watery poo. This is called diarrhoea. It's important to make sure you stay hydrated if you have diarrhoea. Try to drink at least two litres (three and a half pints) of water each day.

It may help to:

  • avoid alcohol and caffeine  
  • eat less fibre by avoiding foods such as wholemeal bread and pasta. Peeling and cooking fruits and vegetables can also help to reduce your fibre whilst keeping a well-balanced diet.  

Your treatment team may be able to give you anti-diarrhoea medication.

Macmillan has more tips on how to manage diarrhoea.  


If you're not pooing as often as normal, you may be constipated. PARP inhibitors and anti-sickness medications can sometimes cause constipation.  

It can help to:

  • do gentle exercise
  • drink prune juice
  • eat foods with high fibre such as fruit and vegetables
  • drink lots of fluids.  

If you experience constipation for more than a few days, you should tell your treatment team as it's easier to treat constipation when it’s picked up early. They may be able to prescribe a laxative if needed. A laxative is a medication which can help you poo.

Cancer Research UK has more information about constipation. 

Taste changes

You may notice a bitter or metallic taste, or not be able to taste certain foods as well as normal. Macmillan has tips on how to deal with changes to taste.  

Stomach pain or indigestion

If you feel pain in or around your tummy area or have indigestion, you should tell your treatment team as they will want to rule out other causes. Indigestion is when you experience pain or discomfort after eating. This can include heartburn, acid reflux or stomach pain.

Macmillan has more information on how to help manage indigestion. 

Loss of appetite

If you're not feeling as hungry as normal, try eating smaller meals spread throughout the day rather than three big meals. If your appetite doesn’t return after a few days or you notice that you're losing weight, let your treatment team know. They may be able to recommend food or drink supplements to help make sure you are getting enough nutrients. 


Fatigue is when you feel extremely tired no matter how much sleep you get. This can really impact your day-to-day life. It’s important to let your treatment team know if you feel this way, as they will want to rule out other possible causes of fatigue such as anaemia.

Watch our session on how to cope with fatigue led by Mara Sheldon, an occupational therapist:

These are a few things that may help with fatigue:

  • Gentle exercise such as short walks or yoga, has been proven to help with fatigue. Try our free weekly yoga session which is run by a specialist cancer yoga instructor.
  • Practice sleep hygiene. This involves creating a regular sleep routine, going to bed and waking up at similar times every day and creating a calming space in your bedroom. The NHS has more information on how to do this.
  • A type of talking therapy called cognitive behavioural therapy (CBT) has been shown to help with fatigue and sleep. Depending on where you live in the UK you may be able to access CBT through the NHS, either by referring yourself or going through your GP. You can also find an accredited CBT therapist privately.
  • Macmillan and Cancer Research UK also have information on how to manage fatigue. 

 If these don't help, your team may lower your PARP inhibitor dose.  

Dizziness and headaches

If you experience dizziness or headaches tell your treatment team as they may want to look into the cause and rule out anaemia.  

For headaches, you can take over the counter painkillers. The NHS has tips on what to do when you're feeling dizzy.

Breathlessness (feeling short of breath)

If you feel breathless, or have difficulty breathing, contact your treatment team immediately. Breathlessness could be caused by many different things such as anaemia or an infection. Your doctor may want to do some extra tests to understand why you're breathless.

Difficulty sleeping (insomnia)

If you can't sleep you may be experiencing insomnia. Speak to your treatment team if it doesn't improve.

It can help to:  

Macmillan and Cancer Research UK have more information on how to help manage insomnia.   

Joint or muscle pain

If you experience joint or muscle pains, you can take over the counter pain killers to help with this.  

There are things you can do to help with muscle pain:

  • Try low impact, gentle exercise such as cycling, swimming or walking to help prevent additional stiffness in your joints and muscles.
  • Try hot and cold treatments. This includes taking warm baths or using heat pads to alleviate stiffness and then using ice packs to help relieve pain by reducing inflammation.  

Rashes or changes to your skin

Tell your treatment team if you notice a red, itchy patch or rash on your skin as they can prescribe medication to soothe the rash. Your skin may also feel dry or be more sensitive to the sun. You should use unscented creams to moisturise and wear a high SPF sun cream when outside. Try to wear clothes such as hats and long sleeves that can shield you from the sun.

Rucaparib may cause the soles of your feet or palms of your hands to become red or sore. This is called hand-foot, or palmar-plantar syndrome. Keeping your hands and feet moisturised and cool can help with this.  

Blood clot risk

A blood clot is when blood forms a blockage in your veins. Signs of a blood clot include:  

  • pain, swelling and redness in your arm or legs
  • sudden breathlessness
  • sharp chest pain.  

If you experience these symptoms, contact your 24-hour emergency number straightaway as blood clots need to be treated quickly. Your doctor can give you medication to thin the blood, which makes clots harder to form. This medication is called an anticoagulant.  

You can reduce the risk of blood clots by staying active through light exercise, moving regularly and drinking lots of fluids.

Impact on memory and concentration 

PARP inhibitors may affect your memory and ability to concentrate or think clearly. You may find yourself struggling to remember things, feeling more disorganised or distracted than usual, or having a low mood.  

Macmillan has more information on changes to memory or concentration due to cancer treatment. This is also called chemo brain as it was initially noticed in people undergoing chemotherapy, but it can also be a side effect of other types of cancer treatments such as PARP inhibitors. 

Signs of an allergic reaction

It's possible to have an allergic reaction to PARP inhibitors. If you notice any of the signs in the list below, contact your treatment team on the emergency telephone number and don't take any more doses until you have spoken with them.

Signs of an allergic reaction include:

  • an itchy rash (hives)
  • swelling of face, mouth or eyes
  • difficulty breathing
  • feeling hot or dizzy. 

Rare but serious side effects

There are some rarer but serious side effects that your treatment team should make you aware of before starting treatment. These include leukaemia, which will be monitored for through regular blood tests.

For more information:

More support and resources

Taking in side effect information and managing side effects can be overwhelming. There are many different ways we can support you.  

If you need someone to talk to, call our nurse-led support line. Our specialist nurse team have supported hundreds of people just like you. Whether you need practical information, or just a listening ear, our nurses are here for you.

If you want to speak to others who have gone through treatment, our online communities are safe and supportive spaces for those affected by ovarian cancer. 

Rachel and Val Target Ovarian Cancer nurse advisers

Our support line is open Monday-Friday, 9am-5pm

Last reviewed: May 2024

Next review: May 2027

To learn more about our review process, take a look at our information standards.


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