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Pembrolizumab (836 + A1004)

Pembrolizumab (836 + A1004)

This leaflet provides information on a treatment known as pembrolizumab.  It will explain what this is, when and how it will be given.  It will also tell you about common side effects that you may experience.  Contact telephone numbers and details of how to obtain further information on this treatment are given at the end of the leaflet.

What is pembrolizumab?

Pembrolizumab is an artificially made antibody which attaches itself to cancer cells and stops them from growing.  It is an immunotherapy, it works by helping the immune system to attack the cancer cells. It is not chemotherapy. 

Why am I having this treatment?

Your doctor has prescribed this treatment because it has been found to be effective in treating your type of cancer. 

How often will I receive this treatment?  

For the treatment to be most effective it is usually given once every three weeks. Your doctor will discuss with you how long you will have your treatment for.

How will my treatment be given?

Your treatment is given through a drip into a vein in the back of your hand or arm. 

Alternatively, if finding a vein is difficult, it may be suggested that a fine tube called a PICC is inserted into a large vein in your upper arm.  This line can remain in place for the whole of your treatment.  Your doctor or nurse will explain this in more detail, if required. 

We have a leaflet that tells you more about PICC lines, if needed. 

How long will I be in hospital?

You should be prepared to spend 1 – 2 hours at the hospital for your first treatment, and approximately 1 hour for the following treatments.

Can I bring relatives and friends with me? 

You are welcome to bring someone to stay with you during your treatment. Space is limited so there is not usually room for more than one person.  Treatment areas are not suitable for young children.

How often will I see the specialist team?

You will usually see the specialist team every 3 weeks.  Your team will let you know how often they will see you.  You will have regular blood tests and we will check how you are feeling and discuss any problems you may have. This is so we can check how the treatment is affecting you.  

What are the possible side effects?

There are many possible side effects with this treatment we have described the main side effects below. However as this is a new drug there may be some side effects which are rare. If you feel unwell and are unsure if it is related to this treatment please contact Velindre. The doctor, nurses and pharmacists can give you advice or answer any questions you may have.


Hair loss

This treatment should not cause hair loss.

Tiredness and fatigue

This treatment may make you feel more tired than usual.  It is important to listen to your body and rest if you need to, but carry out your normal activities if you feel able. Some people find it beneficial to take gentle exercise as well as rest. However, if you are becoming increasingly tired, telephone the treatment helpline; the number is at the end of the leaflet. 

Skin reactions

Use a non-perfumed body wash and moisturising cream to help to prevent skin reactions. 

A skin reaction with pembrolizumab may cause a rash which is dry and itchy and cause mild discomfort. 

For a small number of people this reaction may be more severe.  A severe rash would affect a large part of your body, may be painful and may become infected. 

If you develop a severe rash you should contact the treatment helpline; the number is at the end of the leaflet. 

Effects on your bowels

This treatment has been known to cause either constipation or diarrhoea.  

  • If you experience constipation it is important that you increase the amount of fluids you drink.  You may need laxatives.  You can speak to us or your GP for advice. 
  • If you experience diarrhoea it is important that you drink plenty of fluids.  Medication is available to control diarrhoea.  If you have 4 or more bowel movements in 24 hours above what is normal for you please contact Velindre Cancer Centre immediately.  


Nausea and vomiting are uncommon these days as we will give you anti-sickness medicines which are usually extremely effective.  If you are sick more than once in 24 hours despite taking regular anti-sickness medicine when you are at home after your treatment, contact Velindre Cancer Centre for advice.  The telephone number is at the end of the leaflet.

Fluid retention

You may experience fluid retention which can result in swelling of your ankles and legs.  Rarely this can result in breathlessness because of fluid on the lungs.  Please tell your doctor or nurse if this is a problem.

Myalgia (pain in the muscles)

Some patients may experience myalgia which is muscle or joint pain.   If you already have painkillers at home you may find they relieve the pain.  If this does not work please contact Velindre Cancer Centre.  The telephone number is at the end of the leaflet.

Effects on your thyroid hormone 

Pembrolizumab may either increase or decrease the level of thyroid hormone.  We will be monitoring this regularly with blood tests.  If your thyroid hormone is increased, you may feel more anxious and have difficulty sleeping, have hot flushes or have a fast heart beat and feel very tired.  If your thyroid hormone is decreased you will feel tired, feel the cold easily, you may put on weight and become constipated.

Other side effects

A small number of people may experience lung problems following pembrolizumab treatment.  If you develop a dry cough or notice that you are short of breath please tell us at your next hospital visit.

Some patients may find that this treatment can affect your vision.

You may get headaches whilst on this treatment.  Try taking whatever painkillers you would normally take.  If these do not work please contact Velindre Cancer Centre or your GP for advice.

It is important you do not become pregnant or father a child whilst having this treatment as it could damage the unborn baby.  

Rarely people on this treatment may experience shortness of breath.  This is more likely if you have an existing lung problem.  If you wish to discuss this further please speak to your doctor.

A small number of patients may find that they have a reduced ability to fight infection.

Sometimes cancer drugs can have very serious side effects which can be life threatening. It is important to inform Velindre cancer centre if you are concerned about any side-effects.

A diagnosis of cancer can increase your risk of developing a blood clot (thrombosis), and having cancer treatment may increase this risk further.  It is important to tell your doctor immediately if you have symptoms such as pain, redness and swelling in your leg, or breathlessness and chest pain.  

Blood clots can be very serious.  However, most clots can usually be successfully treated with drugs to thin the blood.  Your doctor or nurse can give you more information

Side effects may continue for up to 18 months after treatment.  If you have any side effects contact the treatment helpline. If you are admitted to hospital or see your GP during this period, inform the doctor you have had Pembrolizumab, an immune therapy and give them your alert card. 

Manufacturer’s patient information leaflets

Velindre leaflets provide information about very common and commonly reported side-effects (we are unable to list all of the common side effects), for more information regarding these and the less common side-effects please refer to the manufacturers patient information leaflets, obtained from Velindre pharmacy and/or on the internet at Sometimes patients may find these leaflets difficult to read however. Please ask if you would like a copy from your doctor or from Velindre pharmacy

Contact telephone numbers

Velindre Cancer Centre 029 2061 5888

Ask for the treatment helpline if you are unwell at home and need immediate advice at any time of the day or night. 

Pharmacy department 029 2061 5888 ext 6223

Monday – Friday 9am – 5pm for queries about your medicines

Tenovus freephone 0808 808 1010

cancer helpline 

Macmillan freephone Helpline 0808 808 0000


This leaflet was written by health professionals.  The information contained in this leaflet is evidence based.  It has been approved by doctors, nurses and patients.  It is reviewed and updated every 2 years.

Prepared August 2014                  Reviewed January 2019