What is pembrolizumab (Keytruda®)?

Pembrolizumab (Keytruda®) is used to treat melanoma that has spread or cannot be removed (advanced melanoma). It is also used for non-small cell lung cancer, Hodgkin lymphoma, and bladder cancer.

Pembrolizumab may also be used to treat other types of cancer as part of a clinical trial.

How pembrolizumab works

Pembrolizumab is a type of immunotherapy. It works by triggering your body’s immune system to fight the cancer cells. It belongs to a group of drugs called monoclonal antibodies. These drugs are sometimes called targeted therapies because they target specific proteins (receptors) on the surface of cells.

Pembrolizumab targets a protein (receptor) called PD-1. This is found on the surface of T-cells. T-cells are part of your body’s immune system and help your immune system to fight the cancer. Pembrolizumab targets a PD-1 receptor that switches off T-cells. It blocks (inhibits) the PD-1 receptor so the T-cells cannot be switched off. This keeps the T-cells active and may help shrink a tumour or stop it growing. Pembrolizumab is also known as an ‘immune checkpoint inhibitor’.

When is pembrolizumab used

Pembrolizumab is used to treat:

  • melanoma that has spread or cannot be removed (advanced melanoma)
  • non-small cell lung cancer (NSCLC)
  • Hodgkin lymphoma
  • bladder cancer.

It may also be used to treat other cancers as part of a clinical trial. In this case, you might have pembrolizumab on its own or with other cancer drugs.

Your cancer doctor can tell you if it might be appropriate for you. If a drug is not available on the NHS, there may be different ways you are still able to have it. Your cancer doctor can give you advice.

We have more information about what you can do if treatment is not available.

How pembrolizumab is given

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

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

A nurse will give you pembrolizumab into a vein as a drip (intravenous infusion). They will usually run the drip through a pump, which gives you the treatment over a set time. It is given over about 30 minutes, every 3 weeks. You will have treatment in the chemotherapy day unit.

Treatment usually continues for as long as it is controlling the cancer, but it may be stopped if the side effects are too troublesome. Your cancer doctor or specialist nurse will talk to you about how long the treatment will take.

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 here, but you will not get them all. Always tell your doctor or nurse about any side effects you have.

If you have other cancer drugs along with this treatment, some side effects may be worse. You may also have side effects not listed here. We have more information about chemotherapy and radiotherapy.

If a side effect is more severe, your doctors may need to reduce the dose, or stop the treatment for a short time. Some people may have this treatment stopped completely.

Your doctor can give you drugs to help control some side effects. It is very important to take them exactly as your nurse or pharmacist has explained. This will help the drugs work as well as possible for you. Your nurse will give you advice about managing side effects. Most side effects start to improve after treatment has finished.

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.

Possible side effects during your treatment

Some people may have side effects while they are being given the treatment or shortly after they have it:

Allergic reaction

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

  • feeling hot or flushed
  • a skin rash
  • itching
  • shivering
  • feeling dizzy
  • a headache
  • feeling breathless
  • 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, it can be treated quickly.

Sometimes a reaction can happen a few hours after treatment. If you get any signs or feel unwell after you get home, contact the hospital straight away.

Common side effects of pembrolizumab

Skin rash

Pembrolizumab can cause skin changes such as red patches or an itchy rash. This is a common side effect. Always tell your doctor or nurse about any skin changes. They can give you advice and may prescribe creams or medicines to help.

If the rash is on a lot of areas of skin or covers more than half of your body, tell your doctor straight away. You may need steroid creams or tablets to treat it. Your doctor may stop giving you pembrolizumab until the rash improves.

Less commonly, some people have a severe skin reaction. This can include large blisters, peeling skin, or sores in your mouth. You may also have a fever (high temperature). If you have a severe reaction you need to contact the hospital straight away and will probably need treatment in hospital.

During treatment, and for a few months after, you will be more sensitive to the sun. Your skin may burn more easily than usual. Wear a sun cream with a high sun protection factor (SPF) and cover up with clothing and a hat.

Diarrhoea or tummy pain

It is important to follow the advice your nurse or doctor gives you about diarrhoea. Contact the hospital if you have diarrhoea. They will give you advice on what to do and if you should see a doctor. They may give you anti-diarrhoea tablets to take at home.

If you have mild diarrhoea, it can usually be controlled with anti-diarrhoea tablets. Mild diarrhoea is when you have loose stool (poo) up to 4 times a day.

Try to drink at least 2 litres (3½ pints) of fluids every day.

It can help to avoid:

  • alcohol
  • caffeine
  • milk products
  • high-fat foods
  • high-fibre food.

Let the hospital know immediately if you have:

  • pain in your tummy (abdomen)
  • vomiting
  • blood or mucus in your poo
  • a fever.

You may need to go into hospital to have a drip (infusion) or other treatments. If your bowel is inflamed (colitis), you may need steroids.

Feeling sick

Your doctor will prescribe anti-sickness (anti-emetic) drugs to help prevent or control sickness. Take the drugs exactly as your nurse or pharmacist explains to you. It’s easier to prevent sickness than to treat it after it has started.

If you still feel sick or are vomiting, contact the hospital as soon as possible. They can give you advice and change the anti-sickness drug to one that works better for you.


Feeling very tired (fatigue) is a common side effect. Try to pace yourself and get as much rest as you need. If you can, balance this with taking some gentle exercise, such as short walks. This can help lessen tiredness. If you feel sleepy, do not drive or operate machinery.

If your tiredness is a lot worse after starting pembrolizumab, let your doctor know. This can be a sign of changes in your hormone levels (see below).

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.

Joint and muscle pain

You may have pain and stiffness in your joints, and sometimes in your muscles. Tell your doctor or nurse if this happens. They can prescribe painkillers and give you advice.

Changes in hormone levels

Pembrolizumab can make some glands that make hormones inflamed. Glands that may be affected are the thyroid, pituitary and adrenal glands. If a gland is affected it may change the levels of hormone that gland makes. Usually, a blood test finds changes to hormone levels before you start to have symptoms.

Tell your doctor if you have any of the following:

  • headaches that do not go away
  • extreme tiredness or sleepiness
  • feeling cold
  • weight gain
  • feeling dizzy or faint
  • changes in behaviour or mood, such as irritability or feeling confused.

These side effects may continue after you have finished your treatment. You may have blood tests to see if you need treatment to regulate your hormone levels.

Loss of appetite

This treatment can affect your appetite. Do not worry if you do not eat much for a day or two. But if your appetite does not come back after a few days, tell your nurse or dietitian. They will give you advice. They may give you food or drink supplements.


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.

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.


This treatment may cause headaches. If this happens, tell your doctor or nurse. They can give you painkillers.

Build-up of fluid

Your ankles and legs may swell because of fluid building up. If fluid collects in the lining of the lungs (pleural effusion), this may make you breathless. Tell your doctor or nurse if you notice this. They can prescribe medicines to help.

Breathlessness or a cough

Some people may become breathless or have a cough. Always tell your doctor if you develop wheezing, a cough, a fever, or feel breathless. You should also let them know if any existing breathing problems get worse. They can give you medicines to help. They may arrange for you to have tests to check your lungs.

Less common side effects of pembrolizumab

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 (99.5°F)
  • 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 often.

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.

Bruising and bleeding

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

Tell your doctor if you have any bruising or bleeding that you cannot explain. This includes:

  • nosebleeds
  • bleeding gums
  • tiny red or purple spots on the skin that may look like a rash.

Some people may need a drip to give them extra platelets. This is called a platelet transfusion.

Raised blood sugar levels

Pembrolizumab may raise your blood sugar levels. Your nurse will check your blood regularly for this. Symptoms of raised blood sugar include feeling thirsty, needing to pass urine more often, and feeling tired. Tell your doctor or nurse if you have these symptoms.

If you have diabetes, your blood sugar levels may be higher than usual. Your doctor will talk to you about how to manage this.

Numb or tingling hands or feet

These symptoms are caused by the effect of pembrolizumab on nerves. It is called peripheral neuropathy. You may also find it hard to fasten buttons or do other fiddly tasks.

Tell your doctor if you have these symptoms. They may stop treatment to help this improve.

Rarely, pembrolizumab causes serious problems with the nerves. Contact the hospital straight away if you have unusual weakness, numbness or tingling in your arms, legs or face, or pain that is getting worse.

Changes in the way your kidneys work

You will have blood tests before and during treatment to check how well your kidneys are working. Your nurse will ask you to drink plenty of fluid. It is important to do this as it helps protect your kidneys. Tell your nurse straight away if you stop passing urine or are only passing small amounts. This may mean the pembrolizumab has affected your kidneys.

Changes in the way the liver works

You will have regular blood tests to check your liver is working properly.

Rarely, pembrolizumab may have a more severe effect on the liver. If this happens, the whites of your eyes and your skin may look a yellow colour (jaundice). You may also have itchy skin, pee (urine) that is darker than usual, and pain on your right side of your tummy.

If you have any of these symptoms it is very important to contact the doctor straight away. They can do tests and may give you treatment for liver inflammation.

Changes to your heartbeat

Pembrolizumab may cause changes to your heartbeat. Tell your doctor if you notice your heartbeat is irregular, slow, or fast.

Eye problems

Pembrolizumab may make your eyes sore, red, or dry. It can also cause blurry vision or your eyes may become more sensitive to light. Always tell your doctor or nurse if you have eye pain or notice any change in your vision.

Other information about pembrolizumab


Pembrolizumab may affect your ability to drive. Do not drive if you have dizziness, blurred vision, or feel very tired. Talk to your doctor if you are worried about this.

Other medicines

Some medicines, including those you can buy in a shop or chemist, may be harmful to take while you are having pembrolizumab. Tell your cancer doctor know about any medicines you are taking, vitamins, complementary therapies, and herbal drugs.


Your doctor will advise you not to become pregnant or father a child while having pembrolizumab. This because pembrolizumab may harm the developing baby. It is important to use effective contraception while having this drug and for at least a few months after your last dose of treatment. You can discuss this with your doctor or specialist nurse.


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.

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