Pembrolizumab (Keytruda) | Cancer Research UK
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What pembrolizumab is

Pembrolizumab (pronounced pem-bro-lee-zoo-mab) is also called by its brand name, Keytruda.

It is a treatment for melanoma that has spread or cannot be removed with surgery. You may also have it as part of clinical trials for other cancers.


How pembrolizumab works

Pembrolizumab is a type of immunotherapy. This type of treatment stimulates the body’s immune system to fight cancer cells.

Pembrolizumab targets and blocks a protein called PD-1 on the surface of certain immune cells called T-cells. Blocking PD-1 triggers the T-cells to find and kill cancer cells.


How you have pembrolizumab

You have pembrolizumab into a vein as a drip.  You can have the drug through a thin, short tube (a cannula) put into a vein in your arm or hand each time you have treatment.

Or you may have it through a central line, a portacath, or a PICC line. These are long, plastic tubes that give the drugs directly into a large vein in your chest. The tube stays in place as long as you need it.

You can read our information about having cancer drugs into a vein.

You have the treatment over 30 minutes once, every 3 weeks.

You usually carry on having pembrolizumab for as long as it works, unless it causes bad side effects.


Tests during treatment

You have blood tests before starting treatment and regularly during your treatment. The tests check your levels of blood cells and other substances in the blood. They also check how well your liver and kidneys are working.


About side effects

We've listed the side effects associated with pembrolizumab below. You can use the links to find out more about each side effect. Where there is no link, please go to our information about cancer drug side effects or use the search box at the top of the page.

You may have a few side effects. They may be mild or more severe. A side effect may get better or worse through your course of treatment. Or more side effects may develop as the course goes on. This depends on

  • How many times you've had the drug before
  • Your general health
  • The amount of the drug you have (the dose)

The side effects may be different if you are having them with other drugs.

Tell your doctor or nurse straight away if any of the side effects get severe.


Common side effects

More than 10 in 100 people have one or more of these side effects.

  • Tiredness and weakness affects 3 in 10 people (30%) during and after treatment. Most people find their energy levels are back to normal within 6 months to a year
  • Skin reactions occur in around 2 out of 10 people (20%) – you may have a rash similar to acne on your face, neck and trunk. Or your skin may be dry and itchy
  • Diarrhoea affects 2 out of 10 people (20%). Drink plenty of fluids. Contact your treatment centre straight away if you have any signs of diarrhoea. Your doctor or nurse will give you anti diarrhoea tablets to take
  • Feeling or being sick affects around 2 out of every 10 people (20%). It is usually well controlled with anti sickness medicines
  • Pain and swelling in the joints occurs in just under 2 out of 10 people (20%)

Occasional side effects

Between 1 and 10 in every 100 people have one or more of these side effects

  • Bruising more easily from a drop in platelets – you may have nosebleeds, or bleeding gums after brushing your teeth. Or you may have lots of tiny red spots or bruises on your arms or legs (known as petechiae)
  • Tiredness and breathlessness from a drop in red blood cells (anaemia) – you may need a blood transfusion
  • Loss of appetite and taste changes
  • A dry, sore mouth
  • Headaches
  • Numbness or tingling in fingers and toes. It can cause difficulty with fiddly things such as doing up buttons
  • Weakness of hands and feet
  • Dry eyes
  • Feeling dizzy
  • Hot flushes
  • Breathlessness, chest pain and a cough. Contact your treatment centre straight away if you develop these symptoms or they are getting worse
  • Tummy pain and bloating
  • Constipation – your doctor or nurse may give you medicines to help prevent this, tell them if you are constipated for more than 3 days
  • Hair loss
  • Patchy changes in your skin colour
  • Flu like symptoms – you may have fever, chills and muscle aches but taking paracetamol can help
  • Swelling of your legs and arms due to fluid buildup (known as peripheral oedema)
  • Change to your thyroid gland – your thyroid may become overactive making you feel hot, sweaty, agitated, lose weight, and have problems concentrating and sleeping. Or it may be underactive and cause tiredness and weight gain. Let your doctor or nurse know if you have any of these symptoms
  • Changes in the amounts of hormones your adrenal glands and pituitary glands make
  • Liver changes that are very mild and unlikely to cause symptoms
  • A reaction to pembrolizumab during the drip, causing flu like symptoms such as fever, chills and shivering (rigors), dizziness and breathing problems. You will have anti allergy medicines beforehand to try to prevent a reaction. If you have a reaction, your nurse will slow your drip down, or stop it for a while

Rare side effects

Fewer than 1 in 100 people have these side effects.

  • Inflammation of your liver or pancreas causing abdominal pain and feeling or being sick
  • Diabetes type 1
  • Infections – contact your nurse or doctor if you have symptoms such as a high temperature, feeling shivery, pain passing urine, red sore eyes, or sore mouth or skin
  • Feeling anxious, confused, or depressed
  • Problems with remembering or concentrating
  • Decreased interest in having sex (low libido) 
  • Difficulty sleeping
  • Muscle weakness and muscles tiring easily (myasthenic syndrome)
  • Heart problems, for example an irregular heart beat
  • Yellowing of skin and whites of the eyes (jaundice) due to a blocked bile duct
  • Soreness, redness and peeling on the palms of the hands and soles of the feet (hand-foot syndrome), which may cause tingling, numbness, pain and dryness
  • Sensitivity to sunlight
  • Numbness, tingling and colour change in fingers and toes when exposed to cold temperatures
  • Difficulty walking
  • Difficulty swallowing
  • Voice changes
  • Pain – this may be pain in the area of the cancer, bone pain or neck and jaw pain
  • Kidney failure – let you nurse know straight away if you are passing less urine than normal, or no urine at all
  • Heavy periods (in women)

Important points to remember

Talk to your doctor, pharmacist or nurse about all your side effects so they can help you manage them. They can give you advice or reassure you. Your nurse will give you a contact number to ring if you have any questions or problems. If in doubt, call them.

Other medicines

Tell your doctor about any other medicines you are taking, including vitamins, herbal supplements and over the counter remedies. Some drugs can react together.

Pregnancy and contraception

This drug may harm a baby developing in the womb. It is important not to become pregnant or father a child while you are having treatment and for 4 months afterwards. Talk to your doctor or nurse about effective contraception before starting treatment.


Don’t breastfeed during this treatment because the drug may come through in the breast milk.



You shouldn't have immunisations with live vaccines while you are having this treatment or for at least 6 months afterwards. In the UK, these include rubella, mumps, measles (usually given together as MMR), BCG, yellow fever and Zostavax (shingles vaccine).

You can have other vaccines, but they may not give you as much protection as usual until your immune system has fully recovered from your treatment. It is safe to have the flu vaccine.

It is safe for you to be in contact with other people who've had live vaccines as injections. There can be problems with vaccines you take by mouth (oral vaccines), but not many people in the UK have these now. So there is usually no problem in being with any baby or child who has recently had any vaccination in the UK. You might need to make sure that you aren't in contact with anyone who has had oral polio, cholera or typhoid vaccination recently, particularly if you live abroad.


More information about pembrolizumab

We don’t list all the very rare side effects of this treatment that are very unlikely to affect you. For further information look at the Electronic Medicines Compendium website at

If you have a side effect we don’t mention here and you think may be due to this treatment, you can report it to the Medicines Health and Regulatory Authority (MHRA) at

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Updated: 23 December 2015