Fluorouracil is also called 5FU. It is a chemotherapy drug used to treat many types of cancer.
Your doctor will talk to you about this treatment and its possible side effects before you agree (consent) to have treatment.
Before starting this treatment, you should have a blood test to check whether you have low levels of an enzyme called DPD. This is called DPD deficiency. People who have low DPD levels can develop serious or life-threatening side effects if they have 5FU. If you have DPD deficiency, this can affect the treatments that are available for you.
You will not know without a test if you have DPD deficiency, as there are no symptoms. Testing can detect most cases of DPD deficiency, but not all cases. You can talk to your cancer doctor about your risk of having DPD deficiency before you start treatment.
You will be given fluorouracil (5FU) in the chemotherapy day unit or during a stay in hospital. A chemotherapy nurse will give it to you. Fluorouracil (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. You may have the chemotherapy drugs through:
- a cannula – a short, thin tube the nurse puts into a vein in your arm or hand
- a central line – a fine tube that goes under the skin of your chest and into a vein close by
- a PICC line – a fine tube that is put into a vein in your arm and goes up into a vein in your chest
- an implantable port (portacath) – a disc that is put under the skin on your chest or arm and goes into a vein in your chest.
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.
5FU may also be given over a few days as a continuous infusion. This is usually given through a small portable pump connected to either a central line or a PICC line. The pump can be carried on a belt or in a bag. You can go home with the pump. The nurse will tell you how to look after 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.
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.
If you have low levels of an enzyme called DPD (DPD deficiency), you may have a higher risk of severe or life-threatening side effects.
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.
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.
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 sometimes called neutropenia.
An infection can be very serious when the number of white blood cells is low. It is important to get any infection treated as soon as possible. Contact the hospital straight away on the 24-hour contact number you have been given 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
- your temperature goes below 36°C (96.8°F).
Symptoms of an infection include:
- feeling shivery and shaking
- a sore throat
- a cough
- needing to pass urine (pee) a lot, or discomfort when you pass urine.
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, until your cell count increases.
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.
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.
Your doctor will give you anti-sickness drugs to help prevent or control sickness during your treatment. 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 often and eat small amounts regularly. It is important to drink enough fluids. If you continue to feel sick, or are sick (vomit) more than once in 24 hours, contact the hospital as soon as possible. They will give you advice. Your doctor or nurse may change the anti-sickness drug to one that works better for you.
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.
Sucking ice chips may sometimes help relieve mouth or throat pain. But if you are having radiotherapy to the head or neck, do not suck on ice. It can cause damage.
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. 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.
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.
Sore and red palms of hands and soles of feet
You may get sore and red palms of hands and soles of feet. The skin may also begin to peel. This is called palmar-plantar or hand-foot syndrome. It usually gets better after treatment ends.
Tell your doctor or nurse about any changes to your hands or feet. They can give you advice and prescribe creams to improve any symptoms you have. It can help to:
- keep your hands and feet cool
- moisturise your hands and feet regularly
- avoid tight-fitting socks, shoes and gloves.
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 finishes. Your nurse can talk to you about ways to cope with hair loss.
Changes in the way the heart works
5FU can affect how the heart works. You may have tests to see how well your heart is working. These may be done before, during and after treatment.
It is still possible for the heart to be affected even if these test results are normal. Very rarely, this can cause heart failure or a heart attack. The risk of this happening is very low (less than 1 in 100 or 1%). But it is important that you know about it.
Call your doctor straight away on the 24-hour number the hospital has given you if you have any of these symptoms at any time during treatment:
- pain or tightness in your chest
- changes to your heartbeat.
If you cannot get through to your doctor, call the NHS urgent advice number which is 111.
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.
Tumour lysis syndrome (TLS)
This treatment may cause cancer cells to die and break down quickly. When cancer cells break down very quickly, it can cause a sudden release of large amounts of chemicals into the blood. This is called tumour lysis syndrome (TLS).
Your kidneys can usually keep these chemicals balanced. But they might not be able to cope with very large amounts. The chemical imbalance can affect how well your kidneys work and cause problems with your heart rhythm.
You will have regular blood tests to check the levels of these chemicals.
If you are at risk of TLS, your doctor can give you treatment to help prevent it. You may have:
- extra fluids through a drip
- medicines such as rasburicase or allopurinol.
Drinking at least 2 litres (3½ pints) of fluid a day will also help.
This treatment may affect your skin. 5FU may cause a rash, which might be itchy. 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 will be more sensitive to the sun and your skin may burn more easily than usual. Here are some tips to protect your skin if you are out in the sun:
- Apply a suncream with a sun protection factor (SPF) of at least 30.
- Wear loose clothes made of cotton or natural fibres to cover up.
- Stay out of the sun during the hottest part of the day. This is usually between 11am and 3pm.
- Wear a wide brimmed hat and sunglasses.
Always tell your doctor or nurse about any changes to your skin. They can give you advice and may prescribe creams or medicines to help. Skin changes usually improve when treatment finishes.
This treatment can affect your nails. They may grow more slowly or break more easily. You might notice ridges or white or dark lines across your nails. Sometimes nails can become loose or fall out. When treatment finishes, any changes usually disappear as the nails grow out.There are things you can do to look after your nails:
- Moisturise your nails and cuticles regularly.
- Keep your nails clipped short.
- Wear gloves to protect your nails when you are doing things in the house or garden.
- Keep your hands and nails clean to help avoid infection, but avoid bathing in very hot water.
- Do not use false nails, gels or other acrylics during this treatment, as they may increase the risk of infection.
- It is fine to wear nail varnish, but try to use a water-based polish. Avoid using harsh chemicals, such as acetone, when taking off the polish.
- If your toenails are affected, wear well-fitted shoes to cushion them.
Tell your doctor or nurse if you notice changes to your nails. They can give you advice or arrange for you to see a podiatrist for foot care advice if needed.
Effects on nervous system
Rarely, 5FU can affect the nervous system. If you or anyone close to you notices any of the following, contact the hospital straight away:
- difficulty with speech, balance, movement or coordinationmood changes (such as feeling excitable or agitated)
- mood changes (such as feeling excitable or agitated).
Your doctor or nurse will assess you and give you treatment if needed.
Rarely, this treatment can affect the brain. It can cause a condition called posterior reversible encephalopathy syndrome (PRES). This is sometimes called reversible posterior leukoencephalopathy syndrome (RPLS).
This condition can cause:
- a headache that does not get better
- drowsiness or confusion
- changes in eyesight
- fits or seizures.
You should either:
- call the 24-hour emergency number the hospital has given you
- go to the hospital straight away.
You should not drive yourself to hospital.
You can make a full recovery from PRES. But it needs to be diagnosed and treated quickly.
Your eyes may become watery and feel sore. Your doctor can give you eye drops to help with this. If your eyes get red and inflamed (conjunctivitis), tell your doctor as you may need treatment.
This treatment may 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.
Blood clot risk
Cancer and some cancer treatments can increase the risk of a blood clot. Symptoms of a blood clot include:
- throbbing pain, redness or swelling in a leg or arm
- suddenly feeling breathless or coughing
- sharp chest pain, which may be worse when you cough or take a deep breath.
If you have any of these symptoms, contact the hospital straight away on the 24-hour contact number you have been given. If you cannot get through to your doctor, call the NHS urgent advice number on 111.
A blood clot is serious, but it can be treated with drugs that thin the blood (anticoagulants). Your doctor or nurse can give you more information.
You can help reduce the risk of developing a blood clot by:
- staying active during treatment
- drinking plenty of fluids, especially water.
You may be given anticoagulants to help prevent a clot.
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 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.
If you have sex in the first few days after this treatment, you should use barrier protection such as a condom or dental dam. This will protect your partner if any of the drug is in your semen or vaginal fluids.
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 doctors and nurses 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.
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.
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