Fluorouacil (5FU)

Fluorouracil (5FU) is a chemotherapy drug used to treat breast, bowel, skin, stomach, oesophageal (gullet) and pancreatic cancer. It may also be used to treat other cancers.

It is best to read this information with our general information about chemotherapy and the type of cancer you have.

Fluorouracil is usually given into a vein or as tablets. You usually have it as an outpatient or during a hospital stay. Your cancer doctor or nurse will tell you how often you will have it.

Like all chemotherapy drugs, fluorouracil can cause side effects. Some of the side effects can be serious, so it is important to read the detailed information below.

Your healthcare team can give you advice on how to manage any side effects. Tell your doctor or nurse straight away if you:

  • have a temperature
  • feel unwell
  • have severe side effects, including any we do not mention here.

Rarely, side effects may be life-threatening. Your cancer doctor or nurse can explain the risk of these side effects to you.

If you need medical attention for any reason other than cancer, always tell the healthcare staff that you are having this treatment.

What is 5FU?

How 5FU is given

You will have 5FU in the chemotherapy day unit, or during a short stay in hospital. 5FU can be given on its own or in combination with other chemotherapy drugs.

A chemotherapy nurse will give it to you. During treatment you will usually see a cancer doctor, a chemotherapy nurse or a specialist nurse. This is who we mean when we mention doctor or nurse in this information.

Before or on the day of treatment, a nurse or person trained to take blood (phlebotomist) will take a blood sample from you. This is to check that it is okay for you to have chemotherapy. 

You will also see a doctor or nurse before you have chemotherapy. They will ask you about how you have been. If your blood results are alright on the day of your treatment, the pharmacist will prepare your chemotherapy. Your nurse will tell you when your treatment is likely to be ready.

Your nurse will give you anti-sickness drugs before the chemotherapy, either as tablets or as an injection into a vein. They give you these drugs and the 5FU through one of the following:

  • a short thin tube that the nurse puts into a vein in your arm or hand (cannula)
  • a fine tube that goes under the skin of your chest and into a vein close by (central line)
  • a fine tube that is put into a vein in your arm and goes up into a vein in your chest (PICC line).

Your nurse may give you 5FU as an injection or drip (infusion). They usually run the drip through a pump that gives you the treatment over a set time. Sometimes you will have 5FU over a few days as a continuous infusion. This is usually given through a small pump connected to either a central line or PICC line. The pump can be carried on a belt or in a bag. You can go home with the pump. The nurses will tell you how to look after it.

5FU can also be given as a cream to treat some types of skin cancer. If you have 5FU as a cream, your doctor, nurse or pharmacist will tell you more about it. This information is mainly about 5FU that is given as an injection.

Your course of chemotherapy

You will have chemotherapy as a course of several sessions (or cycles) of treatment over a few months. This will depend on the type of cancer you have. Your doctor or nurse will tell you more about this and the number of cycles you are likely to have.

Going home

Before you go home, the nurse or pharmacist will give you anti-sickness drugs to take. Take all your tablets exactly as explained.

Possible side effects of 5FU

We explain the most common side effects of 5FU here. But we don’t include all the rare ones that are unlikely to affect you.

You may get some of the side effects we mention but you are very unlikely to get all of them. If you are having other chemotherapy drugs as well, you may have some side effects that we don’t list here. Always tell your doctor or nurse about the side effects you have.

Your doctor can prescribe drugs to help control some side effects. It is very important to take them exactly as your nurse or pharmacist has explained. This means they will be more likely to work better for you. Your nurse will give you advice about managing your side effects. After your treatment is over, the side effects will start to improve.

Serious and life threatening side effects

Sometimes cancer drugs can result in very serious side effects, which rarely may be life threatening. Your cancer doctor and nurse can explain the risk of these side effects to you.

Contact the hospital

Your nurse will give you telephone numbers for the hospital. You can call them if you feel unwell or need advice any time of day or night. Save these numbers in your phone or keep them somewhere safe.

More information about this drug

We are not able to list every side effect for this treatment here, particularly the rarer ones. For more detailed information you can visit the electronic Medicines Compendium (eMC).

Risk of infection

5FU can reduce the number of white blood cells in your blood. This will make you more likely to get an infection. When the number of white blood cells is low, it’s called neutropenia.

Contact the hospital straight away on the contact number you’ve been given if:

  • your temperature goes over 37.5°C (99.5°F) or over 38°C (100.4°F), depending on the advice given by your chemotherapy team
  • you suddenly feel unwell, even with a normal temperature you have symptoms of an infection.

Symptoms can include:

  • feeling shaky
  • a sore throat
  • a cough
  • needing to pass urine a lot.

The number of white blood cells will usually steadily increase and return to normal before your next treatment. You will have a blood test before having more chemotherapy. If your white blood cells are still low, your doctor may reduce the dose of your chemotherapy or delay your treatment for a short time.


Bruising and bleeding

5FU 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 you can’t explain. Bruising and bleeding includes:

  • nosebleeds
  • bleeding gums
  • blood spots
  • rashes on the skin.

Some people may need a drip to give them extra platelets.

Anaemia (low number of red blood cells)

5FU 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 (blood transfusion).

Feeling sick or being sick

This may happen in the first few days after chemotherapy. 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.


This can sometimes be severe. Your doctor can prescribe anti-diarrhoea drugs to control it. You may be given these before you leave hospital. It’s important to take them exactly as your nurse or pharmacist explains. Make sure you drink at least two litres (three and a half pints) of fluids every day if you have diarrhoea. If you have diarrhoea for more than 4 to 6 times a day, or at night, contact the hospital straight away on the numbers your nurse gave you.

Sore mouth

Your mouth may become sore and you may get ulcers. This can make you more likely to get an infection in your mouth. Gently clean your teeth and/or dentures morning and night and after meals. Use a soft-bristled or children’s toothbrush. Your nurse might ask you to rinse your mouth regularly or use mouthwashes. It’s important to follow any advice you are given and to drink plenty of fluids.

Tell your nurse or doctor if you have any problems with your mouth. They can prescribe medicines to prevent or treat mouth infections and reduce any soreness.

Loss of appetite

You may lose your appetite during your treatment. Try to eat small meals regularly. Don’t worry if you don’t eat much for a day or two. If your appetite doesn’t improve after a few days, let your nurse or dietitian know. They can give you advice on getting more calories and protein in your diet. They may give you food supplements or meal replacement drinks to try. Your doctor can prescribe some of these and you can buy them from chemists.

Taste change

You may get a bitter or metallic taste in your mouth or find that food tastes different. This should go away when your treatment finishes. Try using herbs and spices (unless you have a sore mouth or ulcers) or strong-flavoured sauces to give your food more flavour. Sucking boiled sweets can sometimes help get rid of a bitter or metallic taste. Your nurse can give you more advice.


Feeling very tired is a common side effect. It’s often worse towards the end of treatment and for some weeks after. Try to pace yourself and get as much rest as you need. It helps to balance this with some gentle exercise, such as short walks. If you feel sleepy, do not drive or operate machinery.

Soreness and redness of palms of hands and soles of feet

This is called palmar-plantar or hand-foot syndrome. It gets better when treatment ends. Your doctor or nurse can give you advice and prescribe creams to improve the symptoms. It can help to keep your hands and feet cool and to avoid tight-fitting socks, shoes and gloves.

Hair loss

Your hair may thin but you’re unlikely to lose all the hair from your head. This usually starts after your first or second cycle of chemotherapy. It is almost always temporary and your hair will grow back after treatment ends. Your nurse can give you advice about coping with hair loss.

Changes in the way the heart works

5FU can affect the way the heart works. You may have tests to see how well your heart is working before, during and sometimes after treatment.

If you have pain or tightness in your chest, feel breathless or notice changes to your heartbeat at any time during or after treatment, tell a doctor straight away. These symptoms can be caused by other conditions but it’s important to get them checked by a doctor.

Less common side effects of 5FU

Sore eyes

Your eyes may become watery and feel sore and may become more sensitive to light. Your doctor can prescribe eye drops to help with this. If your eyes get red and inflamed (conjunctivitis), tell your doctor. This is because you may need antibiotic eye drops.

Skin changes

Chemotherapy may affect your skin. Your doctor or nurse can tell you what to expect. If your skin feels dry, try using an unperfumed moisturising cream every day.

During treatment and for several months afterwards, you'll be more sensitive to the sun and your skin may burn more easily than usual. You can still go out in the sun, but use a sun cream with a sun protection factor (SPF) of at least 30, and cover up with clothing and a hat.

Your skin may darken. It will return to its normal colour after you finish treatment. Always tell your doctor or nurse about any skin changes. They can give you advice and may prescribe creams or medicines to help. Any changes to your skin are usually temporary and improve when treatment finishes.

It’s important to let your doctor know straight away if you feel unwell or have any severe side effects, even if they’re not mentioned above.

Other information about 5FU

Blood clot risk

Cancer increases the chances of a blood clot (thrombosis) and chemotherapy can add to this. A clot can cause symptoms such as:

  • pain, redness and swelling in a leg
  • breathlessness
  • chest pain.

Contact your doctor straight away if you have any of these symptoms. A blood clot is serious but your doctor can treat it with drugs that thin the blood. Your doctor or nurse can give you more information.

Other medicines

Some medicines can interact with chemotherapy or be harmful when you are having chemotherapy. This includes medicines you can buy in a shop or chemist. Tell your doctor about any medicines you are taking, including over-the-counter drugs, complementary therapies and herbal drugs.


5FU can affect your fertility (being able to get pregnant or father a child). If you are worried about this, you can talk to your doctor before treatment starts.


Your doctor will advise you not to become pregnant or father a child during treatment. This is because the drugs may harm a developing baby. It’s important to use contraception during and for six months after chemotherapy. You can talk to your doctor or nurse about this.


If you have sex within the first couple of days of having chemotherapy you need to use a condom. This is to protect your partner in case there is any chemotherapy in semen or vaginal fluid.

Changes to your periods

Chemotherapy can sometimes stop the ovaries working. You may not get a period every month and they may eventually stop. In some women, this is temporary, but for others it is permanent and is the start of the menopause.


Women are advised not to breastfeed during treatment and for a few months after. This is in case there is chemotherapy in their breast milk.

Medical and dental treatment

If you need to go into hospital for any reason other than cancer, always tell the doctors and nurses that you are having chemotherapy. Give them the contact details for your cancer doctor.