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Learn about pembrolizumab (Keytruda) - a targeted therapy for breast cancer - including how it works, who might be offered it, and possible side effects.

1. What is pembrolizumab?

Pembrolizumab is a targeted (biological) therapy. It’s also referred to as immunotherapy.

Pembrolizumab is the drug’s non-branded name. Its brand name is Keytruda.

Pembrolizumab is given with chemotherapy.

2. Who might be offered pembrolizumab?

Pembrolizumab is used to treat .

Primary breast cancer

Pembrolizumab may be offered to people whose triple negative has a higher risk of .

Locally recurrent breast cancer that can’t be removed by surgery or secondary breast cancer

Pembrolizumab may be offered if you have triple negative, locally recurrent or :

  • That has a higher than normal level of a protein called PD-L1
  • Which has not previously been treated with chemotherapy

You may also be offered pembrolizumab as part of a clinical trial.

Tests before having pembrolizumab

If you have locally recurrent breast cancer that can’t be removed by surgery or secondary breast cancer, your treatment team will offer you a test to check if you would benefit from pembrolizumab.

You may be tested first to see if you would benefit from another similar drug called atezolizumab.

The tests measure levels of the protein PD-L1 using tissue removed during a biopsy or surgery.

3. How does pembrolizumab work?

Immunotherapy helps the immune system to recognise and attack cancer cells.

Cancer cells can use certain proteins such as PD-L1, sometimes called immune checkpoint proteins, to hide from the immune system enabling them to grow and spread.

Pembrolizumab works by blocking one of these proteins. It is then able to help the immune system to find the cancer cells and destroy them.

4. How pembrolizumab is given

It’s given as an intravenous (into a vein) infusion over 30 minutes.

You’ll be asked to stay in the hospital for a few hours after your first treatment to make sure you feel well.

The most common way of giving cancer drugs intravenously involves inserting a small needle and plastic tube called a cannula into a vein, either in the back of the hand or lower arm. The needle is removed, and the plastic tube left in place.

Other intravenous methods may be used depending on factors such as whether chemotherapy staff can find suitable veins, and your preferences.

To treat primary breast cancer

Pembrolizumab is given every three weeks before surgery for eight cycles in combination with chemotherapy (sometimes you may have the pembrolizumab every six weeks, even if this is the case, you'll still have chemotherapy every three weeks). Your treatment team will tell you which chemotherapy drugs you will be given.

It is then given after surgery every three or six weeks for up to nine cycles on its own.

Your treatment team can tell you whether you'll have pembrolizumab every three or six weeks.

To treat locally recurrent breast cancer that can’t be removed by surgery or secondary breast cancer

Pembrolizumab is given in combination with either paclitaxel or nab paclitaxel, also called Abraxane (which combines paclitaxel with a protein called albumin).

Pembrolizumab is given either every three weeks or every six weeks depending on the dose you are prescribed.

You’ll be given pembrolizumab with chemotherapy:

  • As long as your treatment team feels you’re benefitting from these drugs
  • Any side effects can be managed – this will vary from person to person
  • For up to two years (or after 35 treatments every three weeks or its equivalent if it’s given every six weeks)

5. Side effects of pembrolizumab and chemotherapy

Like any drugs, pembrolizumab and chemotherapy can cause side effects.

Side effects can be mild but are sometimes more severe. Some side effects are caused by the body’s own immune system affecting normal healthy organs.

Side effects from pembrolizumab can occur in any part of the body at any time during treatment. They can also sometimes occur months after treatment has finished.

Some side effects may need to be treated immediately, so it’s important to be familiar with them and report them as soon as possible.

You should be given an alert card to carry with you at all times while having pembrolizumab. This card lists the symptoms you must report to your treatment team and your team’s contact details, including who to contact out of hours. Getting medical treatment quickly is important to help prevent the symptoms worsening.

Everyone reacts differently to drugs and some people have more side effects than others. Many side effects can be controlled and those described here will not affect everyone.

This information does not list all the possible side effects of pembrolizumab and chemotherapy. If you’re concerned about any side effects, regardless of whether they’re listed here or on the alert card, contact your treatment team.

Effects of pembrolizumab on your immune system

Rarely, pembrolizumab may cause immune-related side effects. This is when healthy cells get destroyed by the body's immune system and cause inflammation of the tissues and organs.

Symptoms to report include:

  • A new or worsening cough, shortness of breath or chest pain (inflammation of the lungs)
  • Persistent diarrhoea, blood or mucus in your poo, dark coloured poo or abdominal pain (inflammation of the bowel)
  • Yellowing of the skin or whites of the eyes, feeling very sick and being sick, bleeding and bruising, dark coloured urine or pain in the upper abdomen (inflammation of the liver)
  • Extreme tiredness, persistent headaches, weight change, change in mood, unusual bowel habits, dizziness or fainting, feeling more hungry or thirsty than usual, passing urine more often, increased sensitivity to heat or cold (inflammation of hormone-producing glands such as the thyroid or pituitary)
  • Chest pain, shortness of breath, increased sweating, irregular or rapid heartbeat, not being able to exercise for as long as normal, deeper voice (inflammation of the heart)
  • Neck stiffness, headache, fever, chills, being sick, not being able to sleep, feeling confused (inflammation of the brain tissue, lining or nerves)
  • Eye pain or blurred vision (inflammation of the eye)
  • Muscle aches (inflammation of the muscles)
  • Abdominal pain, feeling sick and being sick (inflammation of the pancreas)
  • Changes in how often you pass urine and changes to urine colour, swelling anywhere in the body, pain in the lower abdomen (inflammation of the kidneys)

Common side effects

Effects on the blood

Treatment with pembrolizumab and chemotherapy can temporarily affect the number of blood cells in the body. This is usually due to the chemotherapy.

You’ll have regular blood tests to check your blood count. Blood is made up of red cells, white cells and platelets. If the number of blood cells is too low, your next cycle of treatment may be delayed or the dose of chemotherapy reduced. 

If your blood tests show possible inflammation in your kidneys or liver, your treatment will be delayed and you may need steroids.

Risk of infection

Not having enough white blood cells can increase the risk of getting an infection.

Signs of an infection

Your treatment team may give you guidelines to follow for reporting signs of an infection. But generally you should contact your hospital immediately if you experience any of the following:  

  • A high temperature (over 37.5°C) or low temperature (under 36°C), or whatever your treatment team has advised  
  • Suddenly feeling unwell, even with a normal temperature  
  • Symptoms of an infection, for example a sore throat, a cough, a need to pass urine frequently or feeling cold or shivery 

Having too few red blood cells is called anaemia. If you feel particularly tired, out of breath or dizzy, let your treatment team know.

Bruising and bleeding

This treatment can reduce the number of platelets, which help the blood to clot. You may also bruise more easily, have nosebleeds or your gums may bleed when you brush your teeth. Tell your treatment team if you have any of these symptoms.

Nausea and vomiting

You may feel sick (nausea) and be sick (vomit). Anti-sickness drugs can be prescribed to help with this. If the anti-sickness drugs don’t help, contact your treatment team as they may be able to recommend a different drug.

Loss of appetite

You may not feel like eating, especially if you feel sick. It might help to eat small meals regularly and drink plenty of liquids. You can also ask your doctor to refer you to a dietitian for more advice.

Other effects on your digestive system

Pembrolizumab and chemotherapy may affect your digestive system in different ways. Some people get constipated, while others have diarrhoea.

Let your treatment team know if you have either of these. They can prescribe medication to help.

Fatigue (extreme tiredness)

It’s common to feel extremely tired during your treatment. There are different ways to cope with and manage fatigue.

You can speak to your treatment team or contact our helpline - details for which are at the top and bottom of this page - for more information and support.

If you feel extremely tired, unable to get up, are dizzy or faint, contact your treatment team straight away.

Painful muscles and joints   

Your back, muscles or joints may ache, become painful or be swollen after you have had your treatment. This usually wears off after a few days but sometimes may be longer.

Your treatment team may suggest taking pain relief or anti-inflammatory drugs, like ibuprofen. It’s a good idea to have some of these available before starting your treatment just in case you need them.

However, be aware that pain relief such as paracetamol or ibuprofen can mask the signs of infection such as a raised temperature.

Skin reactions

You may develop a rash on your body, or your skin might discolour. This could be red and itchy, or you may feel flushed.

If you have skin reactions, tell your treatment team so they can monitor your symptoms as they might be able to prescribe medicine to help.

Nail changes

After a few cycles of chemotherapy, the colour of your nails may change. The nails could also become brittle, crack or change in texture, for example ridges may form. Some people might lose nails on their fingers or toes during treatment, but they usually grow back.

Hair loss

You may experience hair loss when having pembrolizumab and chemotherapy.

Numbness and tingling in hands or feet

Some people may experience numbness or tingling in their hands and feet. This is due to the effect chemoterhapy drugs can have on the nerves and is known as peripheral neuropathy.

In most cases it’s mild and goes away soon after treatment stops. If it’s severe, the dose of the chemotherapy may be reduced or stopped completely. It normally improves a few months after the treatment has finished, but it may not disappear completely.

Sometimes this can be caused by the pembrolizumab rather than the chemotherapy.

If you have pain, tingling or numbness (such as difficulty doing up buttons, or difficulty feeling the difference between hot and cold water with your fingertips), mention this to your treatment team when you see them next so that the symptoms can be monitored.

If your symptoms rapidly get worse, let your treatment team know straight away.

Less common side effects

Sore mouth and taste changes

Your mouth may become sore and small ulcers can develop. You’ll usually be given mouthwash to try to reduce soreness of the mouth and gums.

Looking after your mouth, including your teeth and gums, is very important during treatment.

It’s advisable to see your dentist for a check-up before treatment begins.

If you need dental work during treatment you can talk to your oncologist about the best time to have this.

Your taste can change and some food may taste different, for example more salty, bitter or metallic.

Our information on diet during breast cancer treatment includes tips on dealing with sore mouth and taste changes.

Allergic and infusion reactions

Allergic reactions to pembrolizumab or chemotherapy can occur. Reactions can vary from mild to severe, although severe reactions are uncommon.

If you have any swelling, wheezing, chest pain or difficulty breathing during or after having treatment with pembrolizumab and chemotherapy, contact your treatment team immediately.

6. Other important information

Blood clots

People with breast cancer have a higher risk of blood clots, such as deep vein thrombosis (DVT). Their risk is higher because of the cancer itself and some treatments for breast cancer. Nab paclitaxel and paclitaxel are treatments that can cause blood clots. If the cancer has spread to other parts of the body this also increases the risk.

People with a DVT are at risk of developing a pulmonary embolism (PE). This is when part of the blood clot breaks away and travels to the lung.

Blood clots can be life-threatening but are treatable, so it’s important to report symptoms as soon as possible.

Blood clot symptoms

If you have any of the following symptoms, contact your treatment team or go to your local A&E department immediately:

  • Pain, redness/discolouration, heat and swelling of the arm or leg
  • Swelling, redness or tenderness where a central line is inserted to give chemotherapy - for example in the arm, chest area or up into the neck
  • Shortness of breath
  • Pain or tightness in the chest
  • Unexplained cough or coughing up blood

Find out more about blood clots.

Taking other medications

It’s important to tell your specialist about any prescribed or over-the-counter medicines you’re taking including any herbal medicines or supplements. This includes steroids or drugs that affect the body’s immune system.

Sex, contraception and pregnancy

Having pembrolizumab while pregnant may be harmful to a developing baby. Some women can still become pregnant even if their periods are irregular or have stopped.

Women who haven’t been through the menopause should continue using an effective barrier contraception during treatment, such as condoms or Femidoms, and for at least four months after treatment has finished.

Chemotherapy may affect male and female fertility. It’s not known whether pembrolizumab affects fertility.

Find out more about how breast cancer and its treatment can affect sex and intimacy and read our tips on how to manage these changes.


You’ll be advised not to breastfeed during treatment. This is because there is a chance your baby may absorb the drug through your breast milk, which could cause harm.


Always check with your treatment team before having any kind of immunisation or vaccination while receiving treatment with pembrolizumab and chemotherapy.

Coronavirus (Covid-19) vaccination 

People having chemotherapy or targeted therapies are advised to speak to their treatment team about the best time to have a coronavirus (Covid-19) vaccination. 

Flu vaccination

Anyone at risk of a weakened immune system, and therefore more prone to infection, should have the flu vaccine. This includes people due to have, or already having, chemotherapy.

The flu vaccine is not a live vaccine so doesn’t contain any active viruses. Talk to your chemotherapy team or breast care nurse about the best time to have your flu jab.

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Quality assurance

This information was published in November 2022. We will revise it in November 2024.

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