What is fluorouracil (5FU)?

Fluorouracil is also called 5FU. It is used to treat many types of cancer. It is best to read this information with our general information about chemotherapy and the type of cancer you have.

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

How 5FU is given

You will be given 5FU in the chemotherapy day unit or during a stay in hospital. A chemotherapy nurse will give it to you. 5FU can be given in combination with other cancer drugs.

During treatment you usually see a cancer doctor, a chemotherapy nurse or 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) will take a blood sample from you. This is to check that your blood cells are at a safe level to have chemotherapy.

You will see a doctor or nurse before you have chemotherapy. 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 chemotherapy. Your nurse will tell you when your treatment is likely to be ready.

Your nurse usually gives you anti-sickness (anti-emetic) drugs before the chemotherapy. The chemotherapy drugs can be given through:

  • a short thin tube 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 into a vein with a drip (infusion) to flush it through.

Or they may give you the 5FU as a 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 portable 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 about 5FU that is given into a vein (intravenously).

Your course of chemotherapy

You usually have a course of several cycles of treatment over a few months. Your nurse or doctor will discuss your treatment plan with you.

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, but you are unlikely to get all of them. 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 can give you drugs to help control some side effects. It is important to take them exactly as your nurse or pharmacist explains. This means they will be more likely to work for you. Your nurse will give you advice about managing your side effects. After your treatment is over, most side effects start to improve.

Serious and life-threatening side effects

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

Rarely, fluorouracil (5FU) can cause severe or life-threatening side effects in people who have low levels of an enzyme called DPD. This is called DPD deficiency. You will not know before you start treatment whether you have DPD deficiency as there are no symptoms.

There are tests available which detect most cases of DPD deficiency, but not all cases. If you have DPD deficiency, this can affect the treatments that are available for you. Testing is available in some areas through the NHS. Some cancer centres across the UK will test all new patients.

Talk to your cancer doctor about the risk of you having DPD deficiency before you start treatment. You can ask what your options are if the test is not available through the NHS in your area.

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.

Common side effects of 5FU

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.

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.

Feeling sick

You may feel sick during chemotherapy or in the first few days of each treatment. Your doctor will give you anti-sickness drugs to help prevent or control sickness. Take the drugs exactly as your nurse or pharmacist tells you. It is easier to prevent sickness than to treat it after it has started.

If you feel sick, take small sips of fluids and eat small amounts often. If you continue to feel sick, or if you vomit more than once in 24 hours, contact the hospital as soon as possible. They will give you advice and may change the anti-sickness drug to one that works better for you.

Diarrhoea

This treatment may cause severe diarrhoea. Diarrhoea means passing more stools (poo) than is usual for you, or having watery or loose stools. If you have a stoma, it will be more active than usual.

Your hospital team may give you anti-diarrhoea drugs to take at home.

If you have diarrhoea or a mild increase in stoma activity:

  • follow any advice from your cancer team about taking anti-diarrhoea drugs
  • drink at least 2 litres (3½ pints) of fluids each day
  • avoid alcohol, caffeine, milk products, high-fat foods and high-fibre foods.

Contact the hospital straight away if:

  • you have diarrhoea at night
  • you have diarrhoea more than 4 times in a day
  • you have a moderate or severe increase in stoma activity
  • the anti-diarrhoea drugs do not work within 24 hours.

You may need to go to hospital to have fluids through a drip.

Sore mouth and throat

You may get a sore mouth and throat. You may also get mouth ulcers. This can make you more likely to get a mouth or throat infection. Use a soft toothbrush to clean your teeth or dentures in the morning, at night and after meals.

If your mouth or throat is sore:

  • tell your nurse or doctor – they can give you a mouthwash or medicines to help
  • try to drink plenty of fluids
  • avoid alcohol, tobacco and foods that irritate your mouth and throat.

Your treatment may need to be interrupted until you recover.

If you are having continuous treatment through a pump it is important to contact the hospital without delay if your mouth becomes sore.

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.

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.

Feeling tired

Feeling tired is a common side effect. It is often worse towards the end of treatment and for some weeks after it has finished. Try to pace yourself and plan your day so you have time to rest. Gentle exercise, like short walks, can give you more energy. 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. 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. It will start to get better when treatment ends.

Hair loss

Your hair may get thinner but you are unlikely to lose all the hair from your head. Hair loss usually starts after your first or second treatment. It is almost always temporary and your hair will usually grow back after treatment ends. Your nurse can talk to you about ways to cope with hair loss.

Changes in the way the heart works

5FU can affect the way your heart works. You may have tests to see how your heart is working before, during and sometimes after treatment. But it is still possible for your heart to be affected even if these tests are normal. Very rarely, this can cause heart failure or a heart attack. The risk of this happening is less than 1 in 100 (1%). This risk is very low but it is important that you know about it.

Tell a doctor immediately if you have any of these symptoms at any time during treatment:

  • pain or tightness in your chest
  • breathlessness
  • changes to your heartbeat.

If you have any of these symptoms, call your doctor or the 24-hour number the hospital has given you. If you cannot get through, call the NHS urgent advice number which is 111.

Less common side effects of 5FU

Difficulty breathing

5FU can cause a sudden tightness in your chest, making it difficult to breath. This can happen if the muscles in your airway contract for a short time (spasm). This can also cause a cough or wheeze. Contact your doctor straight away if you have difficulty breathing.

Effects on nervous system

Rarely 5FU can affect the nervous system. Contact the hospital straight away for advice if you develop any of the following:

  • confusion
  • mood changes (such as feeling excitable or agitated)
  • drowsiness
  • difficulty with speech, balance, movement or coordination.

Your doctor or nurse will assess you and give you treatment if needed.

Eye problems

This treatment may make your eyes feel sore, red and itchy (conjunctivitis). Your doctor can prescribe eye drops to help treat this. Your pharmacist will tell you how to use your eye drops. It is important to follow their advice.

It may also make your eyes more sensitive to light and cause blurry vision. If you have pain or notice any change in your vision, always tell your doctor or nurse.

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. 5FU can cause a rash, which may be itchy.

During treatment and for several months afterwards, you will 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 suncream with a sun protection factor (SPF) of at least 30
  • cover up with clothing
  • wear 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 give you creams or medicines to help. Any changes to your skin are usually temporary and improve when treatment finishes.

Nail changes

Your nails may grow more slowly or break more easily. You may notice white lines across your nails, or other changes to their shape or colour. Once the treatment has ended, any changes usually take a few months to grow out.

There are things you can do to look after your nails:

  • Keep your nails and hands moisturised.
  • Wear gloves to protect your nails when you are doing things in the house or garden.
  • Do not use false nails during this treatment. It is fine to wear nail varnish.
  • If your toenails are affected, wear well-fitted shoes to cushion them.

Other information

Blood clot risk

Cancer and some cancer treatments can increase the risk of a blood clot. Symptoms of a blood clot include:

  • pain, redness or swelling in a leg or arm
  • breathlessness
  • chest pain.

If you have any of these symptoms, contact a doctor straight away.

A blood clot is serious, but can be treated with drugs that thin the blood. Your doctor or nurse can give you more information.

Driving and using machines

Do not drive or use machines if you:

  • have blurred vision
  • feel drowsy
  • feel tired
  • have difficulty concentrating.

Hearing changes or effects on the nervous system may also affect your ability to drive or use machines. Your doctor can tell you more about this.

Other medicines

Some medicines can affect how this treatment works or be harmful when you are having it. Always tell your cancer doctor about any drugs you are taking or planning to take, such as:

  • medicines you have been prescribed
  • medicines you buy in a shop or chemist
  • vitamins, herbal drugs and complementary therapies.

Tell other doctors, pharmacists or dentists who prescribe or give you medicines that you are having this cancer treatment.

You can visit the electronic Medicines Compendium (eMC) for more detailed information about your treatment.

Vaccinations

Your doctor or nurse may talk to you about vaccinations. These help reduce your risk of getting infections.

Doctors usually recommend that you have a flu jab, which is an inactivated vaccination. People with weak immune systems can have this type of vaccination.

If your immune system is weak, you need to avoid live vaccinations such as shingles. Your cancer doctor or GP can tell you more about live vaccinations.

Contraception

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.

Breastfeeding

Women are advised not to breastfeed while having this treatment. This is because the drugs could be passed to the baby through breast milk.

Fertility

Some chemotherapy drugs can affect whether you can get pregnant or father a child.

If you are a woman, your periods may become irregular or stop. This may be temporary, but for some women it is permanent. Your menopause may start sooner than it would have done.

There may be ways to preserve fertility for men and women. If you are worried about fertility, it is important to talk with your doctor before you start chemotherapy treatment.

Sex

If you have sex in the first few days after chemotherapy, you need to use a condom. This is to protect your partner in case there is any chemotherapy in semen or vaginal fluids.

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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