Pembrolizumab (Keytruda®) is an immunotherapy drug used to treat many types of cancer.
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.
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.
We explain the main side effects of this treatment here. We also include some other possible side effects. You may get some of the side effects we mention, but you are unlikely to get all of them. You may have some rarer side effects that we have not listed here.
If you are also having treatment with other cancer drugs, you may have some side effects that we have not listed here. Always tell your doctor, nurse or pharmacist about any side effects you have.
Your doctor, nurse or pharmacist can give you drugs to help control some side effects. It is important to take them exactly as they tell you. This means they will be more likely to work for you. Your doctor, nurse or pharmacist will give you advice about managing your side effects.
Serious or delayed side effects
Some cancer treatments can cause severe side effects. Rarely, these may be life-threatening. Your doctor, nurse or pharmacist can explain the risk of these side effects to you.
This treatment may 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 finishes, contact your doctor or hospital straight away. Side effects can become serious if they are not treated.
Your patient alert card
Your doctor, nurse or pharmacist will give you a card with information about your treatment. It is important to keep the card with you during treatment and after treatment finishes. You should show it to any doctor or healthcare professional who looks after you.
Contact the hospital
Your nurse will give you telephone 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.
Sometimes side effects can become serious very quickly. If you have new symptoms or side effects, it is important to contact the hospital straight away.
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.
Some people have an allergic reaction while having this treatment. Signs of a reaction can include:
- feeling hot or flushed
- a skin rash
- feeling dizzy
- 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, it can be treated quickly.
Sometimes a reaction happens a few hours after treatment. If you develop any signs or feel unwell after you get home, contact the hospital straight away.
Some of these side effects are rare. But it is important to know about them. They can become serious if they are not treated or if you try to treat them yourself. Always contact your doctor or the hospital straight away if you have these side effects during treatment or after it finishes.
Feeling tired is a common side effect. But sometimes tiredness is a sign that treatment is affecting your hormone levels or nerves, and you may need treatment.
Contact your doctor if either of these things happen during treatment or after it finishes:
- 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.
Contact the hospital straight away if you have any of these symptoms during treatment or after it finishes:
- needing to empty your bowels (poo) more than 4 times a day
- waking up at night to empty your bowels
- pain in your tummy
- being sick
- blood or mucus in your stools (poo)
- a fever, with a temperature over 37.5°C (99.5°F).
You may need to go into hospital to have fluids through a drip (infusion) or other treatments, such as steroids.
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.
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 a day or 2. But if your appetite does not come back after a few days, or if you are losing weight, tell your nurse or dietitian. 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.
This treatment may cause headaches. If you have headaches, tell your doctor. They may give you painkillers to help.
This treatment may cause skin changes, including:
- a rash
- dry skin
- patches of white or paler skin.
If you are out in the sun, use a suncream with a high sun protection factor (at least SPF 30) to protect your skin. If you notice any skin changes during treatment or after it finishes, always tell your doctor or nurse straight away. You may need creams, steroids or other medicines. Your doctor may stop your treatment until the skin changes improve.
Rarely, a much more serious skin condition can develop. You may have:
- a skin rash which then blisters
- peeling skin
- flu-like symptoms, such as a high temperature and joint pain.
If you have any of these symptoms, contact your doctor or hospital straight away.
Muscle and joint pain
This treatment can cause sore or weak muscles. Your joints may also be stiff, sore or swollen. Contact your doctor if you have any of these symptoms during treatment or after it finishes:
- new aches or pains
- new stiffness in a joint
- swollen or red joints.
You may need steroids or other treatments such as painkillers.
You may gain weight, or your face, ankles and legs may swell. This improves slowly after your treatment has finished. Your doctor may give you drugs to help reduce the swelling.
Effects on the nervous system
This treatment can affect the brain and nerves. These effects are not common but if they are not treated quickly, they can become serious. These side effects may be mild to start with. Contact your doctor straight away if you have any of these symptoms during treatment or after it finishes:
- problems fastening buttons or doing other fiddly tasks
- any weakness, numbness or tingling in your arms, legs or face
- feeling drowsy or confused
- problems with moving or speaking
- any unexplained pain in other parts of your body.
You may need steroids and other treatments.
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 have symptoms such as:
- pale skin
- lack of energy
- feeling breathless
- feeling dizzy and light-headed.
Tell your doctor or nurse if you have these symptoms.
If you are very anaemic, you may need a drip to give you extra red blood cells. This is called a blood transfusion.
This treatment may affect how your body makes hormones. Hormones control many different processes in the body. You will have regular blood tests to check some of your hormone levels. Your doctor may give you drugs to control your hormone levels, and to reduce the risk of side effects such as:
- 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 tired (fatigue)
- mood changes.
If you have any of these symptoms during treatment or after it finishes, tell your doctor.
Effects on the lungs
This treatment can cause changes to the lungs. Contact the hospital straight away if you notice any of these changes during treatment or after it finishes:
- a cough
- a fever, with a temperature over 37.5°C (99.5°F).
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.
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:
- bleeding gums
- heavy periods
- blood in your urine (pee) or stools (poo)
- tiny red or purple spots on the skin that may look like a rash.
Tell your doctor if you have any unexplained bruising or bleeding. You may need a drip to give you extra platelets. This is called a platelet transfusion.
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 can help.
Changes to your taste
You may get a bitter or metal taste in your mouth. Sucking sugar-free sweets may help with this. Some foods may taste different or have no taste. Try different foods to find out what tastes best to you. Taste changes usually get better after treatment finishes. Your nurse can give you more advice.
Helpful hints to improve taste
- Season food or add spices and herbs to add flavour when cooking.
- Use strong, flavoured sauces or gravies to make food tastier.
- Eat sharp-tasting fresh fruit and juices or try sugar-free sweets to leave a pleasant taste in your mouth.
- Try cold foods as they may have a stronger taste than hot foods.
Effects on the liver
This treatment can affect how your liver works. You will have regular blood tests to check this. If the blood tests show liver changes, your doctor may give you steroids.
Sometimes liver changes can be serious. Contact the hospital straight away 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.
You may need steroids or other treatments, and may be monitored until your liver recovers.
Effects on the kidneys
This treatment can affect how your kidneys work. 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, your doctor may give you steroids or other treatments.
Contact the hospital straight away if either of these things happen during treatment, or after it finishes:
- you are not passing urine (peeing) at all
- you are only passing very small amounts of urine.
This treatment can affect your eyes and eyesight. Contact your doctor straight away if you have any of these symptoms during treatment or after it finishes:
- 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 see how well your heart is working. These may be done before, during and after treatment.
Contact your doctor straight away on the 24-hour number the hospital has given you if you have any of these symptoms during or after treatment:
- pain or tightness in your chest
- changes to your heartbeat.
Other conditions can cause these symptoms, but it is important to get them checked by a doctor. If you cannot contact your doctor, call the NHS urgent advice number on 111.
If your symptoms are caused by this treatment, you may need steroids. You may be monitored in hospital for a time.
Low number of white blood cells
This treatment can reduce the number of white cells in your blood. These cells fight infection. Tell your doctor or nurse if you have any signs of an infection. These include:
- a cough
- a sore throat
- needing to pass urine (pee) often.
Contact your doctor or the hospital straight away if:
- your temperature goes over 37.5°C (99.5°F)
- you suddenly feel unwell, even with a normal temperature
Raised blood sugar levels
This treatment may raise your blood sugar levels. You will have regular blood tests to check this. Symptoms of raised blood sugar include:
- feeling thirsty
- needing to pass urine (pee) more often
- feeling tired.
Tell your doctor or nurse if you have these symptoms during treatment or after it finishes.
If you have diabetes, your blood sugar levels may be higher than usual. Your doctor will talk to you about how to manage this.
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.
Do not drive if you:
- feel dizzy
- have blurred vision
- feel very tired.
If you are worried about this, talk to your doctor.
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:
- medicines you have been prescribed
- medicines you buy in a shop, pharmacy or online
- vitamins or supplements
- herbal drugs and complementary or homeopathic therapies
- recreational drugs – for example, cannabis.
Vaccinations can reduce your risk of getting certain infections. Your doctor or nurse may talk to you about having vaccinations.
Doctors usually recommend that people with cancer have a flu vaccination and a coronavirus (covid) vaccination. These are both inactivated vaccinations that can help reduce the risk of infection. People with weak immune systems can have these, as they are not live vaccinations.
If your immune system is weak, you need to avoid live vaccinations. This is because they can make you unwell. Live vaccines contain a very weak version of the illness they are vaccinating you against. Your cancer doctor or GP can tell you more about live and inactivated vaccinations.
Your doctor will advise you not to get pregnant or make someone pregnant while having this treatment. The drugs may harm a developing baby. It is important to use contraception during your treatment and for a while after treatment finishes. Your doctor, nurse or pharmacist can tell you more about this.
You are advised not to breastfeed while having this treatment, or for some time after treatment finishes. This is because the drugs could be passed to the baby through breast milk.
Your doctor or nurse can give you more information.
Medical and dental treatment
If you need medical treatment for any reason other than cancer, always tell the doctor and nurse that you are having cancer treatment. Show them your patient alert card. 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.
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.
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