Fluorouracil (5FU)

How does fluorouracil work?

Fluorouracil is part of a group of chemotherapy drugs known as anti metabolites. Anti metabolites are similar to normal body molecules but they have a slightly different structure.

These differences mean that anti metabolites stop cancer cells working properly. They stop the cells making and repairing ​DNA​. Cancer cells need to make and repair DNA so that they can grow and multiply.

How do you have fluorouracil?

You have fluorouracil into your bloodstream (intravenously).

You might have treatment through a thin short tube (a cannula) that goes into a vein in your arm. You have a new cannula each time you have treatment.

You might have treatment through a long plastic tube that goes into a large vein in your chest. The tube stays in place throughout the course of treatment. This can be a:

• central line

• PICC line

• portacath

You may have fluorouracil via a small pump that you carry around for several days.

As an ointment

Fluorouracil is also available as an ointment for skin cancer. When used as an ointment it does not cause the usual side effects but it can cause temporary irritation and inflammation in the treated areas of skin.

How often do you have fluorouracil?

You usually have fluorouracil as part of a course of several cycles of treatment. This means that you have the drug and then a rest to allow your body to recover.

How often you have fluorouracil can vary depending on your cancer type. It can be given weekly or a cycle can be every 2, 3 or 4 weeks. Your doctor will explain your treatment to you.

You might have continuous treatment through a small portable pump. The nurse attaches it to your central line. This means you can go home with it. You go back to hospital regularly for the nurse to change your pump, and to see how you’re doing.

Tests

You have blood tests before and during your treatment. They check your levels of blood cells and other substances in the blood. They also check how well your liver and kidneys are working.

DPD deficiency 

Before starting treatment with fluorouracil (5FU) you have a blood test to check levels of an enzyme called dihydropyrimidine dehydrogenase (DPD). A low DPD level means you are more likely to have severe side effects from this chemotherapy, so you may start treatment with a lower amount (dose) of the drug or have a different treatment. Your doctor, nurse or pharmacist will talk to you about this.

Find out more about having a DPD deficiency

What are the side effects of fluorouracil?

Side effects can vary from person to person. They also depend on what other treatments you're having. 

When to contact your team

Your doctor, nurse or pharmacist will go through the possible side effects. They will monitor you during treatment and check how you are at your appointments. Contact your advice line as soon as possible if:

• you have severe side effects 

• your side effects aren’t getting any better

• your side effects are getting worse

Early treatment can help manage side effects better. 

We haven't listed all the side effects here. Talk to your healthcare team if you have any new symptoms that you think might be a side effect of your treatment.

Remember it is very unlikely that you will have all of these side effects. But you might have some of them at the same time.

Common side effects

These side effects happen in more than 10 in 100 people (more than 10%). You might have one or more of them. They include:

Increased risk of getting an infection

Increased risk of getting an infection is due to a drop in white blood cells. Symptoms include a change in temperature, aching muscles, cough, headaches, feeling cold and shivery, pain or a burning feeling when peeing, or generally feeling unwell. You might have other symptoms depending on where the infection is.

Infections can sometimes be life threatening. You should contact your advice line urgently if you think you have an infection. 

Looking pale and breathlessness

You might be breathless and look pale due to a drop in red blood cells. This is called anaemia.

Increased risk of bleeding and bruising

This is due to a drop in the number of platelets in your blood. These blood cells help the blood to clot when we cut ourselves. You may have nosebleeds or bleeding gums after brushing your teeth. Or you may have lots of tiny red spots or bruises on your arms or legs. This is known as petechiae.

Diarrhoea

Contact your advice line if you have diarrhoea. For example, in one day you have 2 or more loose bowel movements than usual. If you have a ​stoma​, you might have more output than normal. Your doctor may give you anti diarrhoea medicine to take home with you after treatment.

Try to eat small meals and snacks regularly. It’s best to try to have a healthy balanced diet if you can. You don’t necessarily need to stop eating foods that contain fibre. But if your diet is normally very high in fibre, it might help to cut back on high fibre foods such as beans, nuts, seeds, dried fruit, bran and raw vegetables. 

Drink plenty to try and replace the fluid lost. Aim for 8 to 10 glasses per day.

Feeling or being sick

Feeling or being sick is usually well controlled with anti sickness medicines. It might help to avoid fatty or fried foods, eat small meals and snacks and take regular sips of water. Relaxation techniques might also help.

It is important to take anti sickness medicines as prescribed even if you don’t feel sick. It is easier to prevent sickness rather than treat it once it has started.

Difficulty breathing

You might have a sudden feeling of tightness in your chest, difficulty catching your breath, wheezing and a cough. This is caused by spasms in the muscles of your airways. Contact your healthcare advice line or tell your doctor. 

Heart problems

You may get changes to your heart rhythm. Your doctor might ask you to have tests to check your heart, such as a heart trace (ECG).

Rarely your heart might stop beating (heart attack). Contact 999 straight away if you have chest pain. 

Read about having an ECG

Sore inflamed mouth and throat

It may be painful to swallow drinks or food. Painkillers and mouth washes can help to reduce the soreness and keep your mouth healthy. 

Loss of appetite

You might lose your appetite for various reasons while having cancer treatment. Sickness, taste changes or tiredness can put you off food and drinks.

Hair loss

You could lose your hair. This includes your eyelashes, eyebrows, underarms, legs and sometimes pubic hair. Your hair will usually grow back once treatment has finished but it is likely to be softer. It may grow back a different colour or be curlier than before. 

Tiredness and feeling weak (fatigue)

You might feel very tired and as though you lack energy.

Various things can help you to reduce tiredness and cope with it, for example exercise. Some research has shown that taking gentle exercise can give you more energy. It is important to balance exercise with resting.

Soreness, redness and peeling of palms and soles (hand foot syndrome)

The skin on your hands and feet may become sore, red, or may peel. You may also have tingling, numbness, pain and dryness. This is called hand-foot syndrome or palmar plantar syndrome.

Moisturise your skin regularly. Your healthcare team will tell you what moisturiser to use.

Rare side effects

These side effects happen in fewer than 1 in 100 people (fewer than 1%). You might have one or more of them. They include:

• feeling happy and excited

• eye problems including rapid eye movement, inflammation, blocked tear ducts, blurred vision, eyelid turning outwards, watery eyes and double vision

• headaches and dizziness

• shaking and trembling, stiffness and slow movement (symptoms of Parkinson's disease)

• an increase in the muscle tone of your legs and feet causing them to be stiff

• low blood pressure

• stomach ulcers or bleeding in your stomach

• skin problems including rash, dry skin, increased sensitivity to sunlight, hives, darkening of the skin

• nail problems including thickening, pain, changes in colour of the nail and nail bed

• changes to the way your kidneys work - you have regular blood tests to check for this

Coping with side effects

We have more information about side effects and tips on how to cope with them.

Read more about how to cope with side effects

What else do you need to know

Other medicines, food and drink

Cancer drugs can interact with medicines, herbal products, and some food and drinks. We are unable to list all the possible interactions that may happen. An example is grapefruit or grapefruit juice which can increase the side effects of certain drugs.

Tell your healthcare team about any medicines you are taking. This includes vitamins, herbal supplements and over the counter remedies. Also let them know about any other medical conditions or allergies you may have.

Loss of fertility

You may not be able to become pregnant or get someone pregnant after treatment with this drug. Talk to your doctor before starting treatment if you think you may want to have a baby in the future.

Men might be able to store sperm before starting treatment. And women might be able to store eggs or ovarian tissue. But these services are not available in every hospital, so you would need to ask your doctor about this.    

Contraception and pregnancy

This drug may harm a baby developing in the womb. It is important not to become pregnant while you are having treatment with this drug and for at least 6 months afterwards. Men receiving treatment with this drug are advised not to father a child during treatment and for 3 months after stopping treatment. Talk to your doctor or nurse about effective contraception before starting treatment

Breastfeeding

Don’t breastfeed during this treatment because the drug may come through into your breast milk.

Treatment for other conditions

If you are having tests or treatment for anything else, always mention your cancer treatment. For example, if you are visiting your dentist.

Immunisations

Don’t have immunisations with live vaccines while you’re having treatment and for up to 12 months afterwards. The length of time depends on the treatment you are having.

Ask your doctor or pharmacist how long you should avoid live vaccinations.

In the UK, live vaccines include rubella, mumps, measles, BCG, and yellow fever.

You can usually have:

• other vaccines, but they might not give you as much protection as usual

• the flu vaccine (as an injection)

• the coronavirus (COVID-19) vaccine

Talk to your doctor or pharmacist about the best time to have a vaccine in relation to your cancer treatment.

Contact with others who have had immunisations 

You can be in contact with other people who have had live vaccines as injections. If someone has had a live vaccine by mouth or nasal spray there may be a small risk the vaccine virus can be passed onto you if your ​immune system​ is weakened.

Your healthcare team will let you know if you need to take any precautions if you are in close contact with someone who has had a live vaccine.

Read more about immunisations and cancer treatment

More information about this treatment

We haven't listed all the very rare side effects of this treatment. For further information see the electronic Medicines Compendium (eMC) website.

eMC website

You can report any side effect you have that isn’t listed here to the Medicines Health and Regulatory Authority (MHRA) as part of their Yellow Card Scheme.

Report a side effect to the MHRA