What is pembrolizumab (Keytruda®)?

Pembrolizumab is a type of immunotherapy drug called a monoclonal antibody. It may also be called a checkpoint inhibitor.

Pembrolizumab (Keytruda®) is used to treat many types of cancer. It is best to read this information with our general information about the type of cancer you have.

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 does pembrolizumab work?

T cells are part of the body’s immune system. Pembrolizumab targets and blocks a protein (receptor) called PD-1 on the surface of these T cells. It also blocks another protein called PD-L1. By blocking PD-1 or PD-L1, pembrolizumab helps the immune system to find and attack the cancer cells.

Before being offered treatment with pembrolizumab, your doctor may test the cancer cells to check if pembrolizumab is likely to be an effective treatment. This is not needed in every situation.

Your doctor will tell you more information about pembrolizumab and the possible side effects before you agree (consent) to have treatment.

How pembrolizumab is given

You will be given pembrolizumab at a day unit as an outpatient. Pembrolizumab can be given on its own or with other cancer drugs.

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

Before, or on the day of, treatment, a nurse or person trained to take blood (phlebotomist) may take a blood sample from you.

You will see a doctor or nurse before you have treatment. They will talk to you about your blood results and ask you how you have been feeling. If your blood results are okay, the pharmacist will prepare your treatment.

Pembrolizumab is given as a drip into a vein (intravenous infusion). The drip is usually run through a pump, which gives you the treatment over 30 minutes.

Your course of treatment

You may have pembrolizumab once every 3 weeks, or once every 6 weeks. For most types of cancer, pembrolizumab is usually given for 2 years. Your doctor, nurse or pharmacist will discuss your treatment plan with you.

About side effects

We explain the most common side effects of this treatment here. We also include some that are less common. This treatment can make the immune system too active. This can cause side effects that may be serious if not treated quickly.

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.

Do not try to treat side effects yourself. 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.

Delayed side effects

This treatment may also cause delayed side effects. These can start weeks, months, or sometimes more than a year after you finish treatment.

If you have symptoms or side effects that start during treatment or after treatment ends, contact the hospital straight away on the 24-hour number. Side effects can become serious if they are not treated.

Your patient alert card

They will also give you a card with information about your treatment. It is important to keep the card with you during treatment and after treatment ends. You should show it to any doctor or healthcare professional caring for 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.

Side effects while treatment is being given

Allergic reaction

Some people have an allergic reaction while having this treatment. Signs of a reaction can include: 

  • feeling hot or flushed 
  • shivering
  • itching
  • a skin rash
  • feeling dizzy or sick
  • a headache
  • feeling breathless or wheezy
  • swelling of your face or mouth
  • pain in your back, tummy or chest. 

Your nurse will check you for signs of a reaction during your treatment. If you feel unwell or have any of these signs, tell them straight away. If you do have a reaction, they can treat it quickly. 

Sometimes a reaction happens a few hours after treatment. If you develop any of these signs or feel unwell after you get home, contact the hospital straight away on the 24-hour number.

Immune system side effects

This treatment can make the immune system too active. This can cause the following side effects. Some of them are rare. But it is important to know about them.

Do not try to treat these side effects yourself. If you have these side effects during treatment or after it ends, always contact the hospital straight away on the 24-hour number.

Feeling tired (fatigue)

Feeling tired is a common side effect of this treatment. Try to pace yourself and plan your day so you have time to rest. Gentle exercise, like short walks, can help you feel less tired. 

Sometimes tiredness is caused by this treatment affecting glands that produce hormones. If your hormone levels are low, you may need hormone replacement drugs.

Contact your cancer team or hospital if either of these things happen during treatment or after it ends:

  • your tiredness gets worse
  • you feel weak or shaky.

If you feel sleepy, do not drive or use machinery.

Diarrhoea or tummy pain

You may have diarrhoea or tummy (abdominal) pain. This treatment can also cause inflammation of the bowel (colitis) which can be very serious. 

Diarrhoea means passing more stools (poo) than is normal for you, or having watery or loose stools. If you have a stoma, it may be more active than usual.

Contact the hospital straight away on the 24-hour number if you have any of the following symptoms during treatment, or after treatment ends:

  • You have diarrhoea 4 or more times in a day. 
  • You have a moderate increase in stoma activity. 
  • You have uncomfortable tummy cramps. 
  • You have diarrhoea at night. 
  • Your temperature is over 37.5°C.
  • There is blood or mucus in your stool.

You may need to go into hospital to have fluids through a drip or treatments such as steroids.

Skin changes

This treatment may cause inflammation in the skin. This is usually mild, but can if it is not treated it can become serious. You may need creams, steroids or other medicines to treat skin problems. If you notice skin changes during treatment, or after treatment ends, contact the hospital on the 24-hour number as soon as possible. Skin changes can include: 

  • a rash or bumps on the skin
  • dry or itchy skin
  • patches of white or paler skin.

To protect your skin from the sun, use suncream with of at least SPF 30. SPF stands for sun protection factor. Cover up with clothing and a hat. 

Contact the hospital straight away on the 24-hour number if you have any of these symptoms:

  • a rash that is spreading
  • blistering or peeling skin
  • flu-like symptoms, such as a high temperature and joint pain. 

These symptoms can be a sign of a serious skin reaction that needs to be treated immediately in hospital.

Muscle and joint pain

This treatment can cause sore or weak muscles. Your joints may also be stiff, sore or swollen (arthritis). If you already have problems with joint pain, this may become worse. Contact the hospital on the 24-hour number if you have any of these symptoms during treatment or after it ends:

  • new or worsening aches or pains
  • new or worsening stiffness in a joint
  • swollen joints.

You may need steroids or other treatments, such as painkillers.

Effects on the nervous system

Rarely, this treatment can affect the brain and nerves. These effects may be mild at first, but can become serious if they are not treated. Contact the hospital straight away on the 24-hour number if you have any of these symptoms during treatment or after it ends:

  • any weakness, numbness or tingling in your arms, legs or face
  • a headache that does not get better
  • dizziness
  • drowsiness or confusion
  • problems with moving or speaking
  • any unexplained pain in other parts of your body.

You may need steroids and other treatments.

Hormone changes

This treatment may affect your hormones. Hormones control many different processes in the body. You will have regular blood tests to check some of your hormone levels. Contact the hospital on the 24-hour number if you have any of these side effects during treatment or after it ends:

  • increased sweating
  • weight gain or weight loss
  • dizziness or fainting
  • feeling more hungry or thirsty than usual
  • loss of sex drive
  • passing urine (peeing) more often than usual
  • headaches that do not go away
  • feeling more tired than usual (fatigue)
  • mood changes.

You may need drugs to control your hormone levels.

Effects on the lungs

This treatment can cause inflammation of the lungs. This is called pneumonitis. Contact the hospital straight away on the 24-hour number if you notice any of these changes during treatment or after it ends:

  • breathlessness
  • a cough that does not go away
  • wheezing
  • a fever, with a temperature over 37.5°C.

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

Effects on the liver

This treatment can cause liver inflammation and may affect how your liver works. This is called hepatitis. You will have regular blood tests to check this. If the blood tests show liver changes, you may have steroids.

Sometimes liver changes can be serious. Contact the hospital straight away on the 24-hour number if you have any of these symptoms during treatment or after it finishes:

  • yellow skin or eyes
  • feeling very sleepy
  • dark urine (pee)
  • unexplained bleeding or bruising
  • pain in the right side of your tummy (abdomen)
  • loss of appetite.

Effects on the kidneys

This treatment can cause kidney inflammation and may affect how the kidneys work. This is called nephritis. This is not common, but if it is not treated it can become serious. You will have regular blood tests to check how your kidneys are working. If tests show kidney changes, you may have steroids or other treatments.

Contact the hospital straight away on the 24-hour number during treatment, or after it ends, if you either:

  • have blood in your urine (pee) 
  • are passing less urine or peeing less often than usual.

Eyesight changes

This treatment can affect your eyes and eyesight. Contact the hospital straight away on the 24-hour number if you have any of these symptoms during treatment or after it ends:

  • sore, painful, or swollen eyes
  • eyes that are sensitive to light
  • changes to your eyesight.

You may need steroids or other treatments. A specialist eye doctor (ophthalmologist) may also need to check your eyes.

Effects on the heart

This treatment can affect how the heart works. This is not common, but it can be serious. You may have tests to check how well your heart is working. These may be done before, during and after treatment.

Contact the hospital straight away on the 24-hour number if you have any of these symptoms during or after treatment:

  • breathlessness
  • dizziness
  • changes to your heartbeat
  • swollen feet and ankles.

Other conditions can cause these symptoms, but it is important to get them checked by a doctor. 

Always call 999 if you have:

  • chest pain, pressure, heaviness, tightness or squeezing across the chest
  • difficulty breathing.

If your symptoms are caused by this treatment, you may need steroids. You may be monitored in hospital for a time.

Raised blood sugar levels

This treatment may raise your blood sugar levels and increase the risk of diabetes. 

You will have regular blood tests to check this. 

Signs of raised blood sugar include:

  • feeling thirsty
  • needing to pass urine (pee) more often than usual
  • feeling more tired than usual.

Contact the hospital on the 24-hour number if you have these symptoms during treatment or after it ends.

If you already have diabetes, your blood sugar levels may be higher than usual. You may need advice and support from your GP or diabetes team about managing this.

Other possible side effects

Feeling sick

If you feel sick, take small sips of fluids and try to eat small amounts often. Your doctor can prescribe anti-sickness drugs. Take the drugs exactly as your nurse or pharmacist explains.  

If you continue to feel sick, contact the hospital. They will give you advice and may change the anti-sickness drug to one that works better for you.

Constipation

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 on the 24-hour number for advice. They can give you drugs called laxatives to help. 

If you have not been able to pass stools for over 2 days and are being sick, contact the 24-hour number straight away. 

Loss of appetite

This treatment can affect your appetite. Don’t worry if you do not eat much for 1 or 2 days. But if your appetite does not come back after a few days, or if you are losing weight, tell your doctor, nurse or pharmacist. They can give you advice. They may give you food or drink supplements. Or they may suggest changes to your diet or eating habits to help.

Headaches

This treatment may cause headaches. If you have headaches, tell your doctor, nurse or pharmacist. They can give you advice about painkillers that may help. Tell them if the headache does not get better, or gets worse.

Fluid build-up

This treatment can cause a build-up of fluid in the body. This will slowly get better after treatment ends. Contact the hospital on the 24-hour number if you:

  • are gaining weight 
  • have swelling in your face, legs or ankles.

They can give you advice and treatment to help.

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 feel:

  • very low in energy
  • breathless 
  • dizzy and light-headed. 

If you have these symptoms, contact the hospital straight away on the 24-hour number. You may need treatment for anaemia. If you are very anaemic, you may need a drip to give you extra red blood cells. This is called a blood transfusion.

Less common side effects

Bruising and bleeding

This treatment can reduce the number of platelets in your blood. Platelets are cells that help the blood to clot.

If the number of platelets is low, you may bruise or bleed easily. You may have:

  • nosebleeds
  • bleeding gums
  • heavy periods
  • blood in your urine (pee) or stools (poo)
  • tiny red, brown or purple spots that may look like a rash – these spots can be harder to see if you have black or brown skin. 

If you have any unexplained bruising or bleeding, contact the hospital straight away on the 24-hour number. You may need a drip to give you extra platelets. This is called a platelet transfusion.

Dry mouth

You may get a dry mouth. Drink plenty of fluids, and add sauces and gravies to your food to keep it moist. If you have dry lips, using a lip balm or Vaseline® can help.

Changes to your taste

Some foods may taste different or have no taste. Try different foods to find out what tastes best to you. You may also get a bitter or metallic taste in your mouth. Your doctor, nurse or pharmacist can give you advice. It might help to try:

  • sucking sugar-free sour or boiled sweets
  • eating cold foods
  • eating sharp-tasting fresh fruit.

Taste changes usually get better after treatment ends. We have more information about coping with changes to taste.

Low number of white blood cells

This treatment can reduce the number of white cells in your blood. These cells fight infection. If you have any signs of an infection, tell your doctor, nurse or pharmacist. These signs include:

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

Contact the hospital straight away on the 24-hour number if you have any of these symptoms:

  • a temperature above 37.5°C 
  • a temperature below 36°C
  • you feel unwell, even with a normal temperature.

High blood pressure

This treatment can cause high blood pressure. Tell your doctor or nurse if you have ever had any problems with your blood pressure. Your nurse will check it regularly during your treatment.

Other information

Driving

Do not drive if you:

  • feel dizzy
  • have blurred vision
  • feel very tired.

If you are worried about this, talk to your doctor.

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: 

Vaccinations

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.

Contraception

Your doctor, nurse or pharmacist will advise you not to get pregnant or make someone pregnant while having this treatment and for some time afterwards. The drugs may harm a developing baby. It is important to use contraception to prevent pregnancy. Follow their advice about:

  • what types of contraception to use 
  • how long after treatment you should continue to use contraception. 

Breastfeeding

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.

Fertility

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. Show them your patient alert card. 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 or optician, 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 2022
|
Next review: 01 January 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.