Fluorouracil | Cancer Research UK
Cancer Research UK on Google+ Cancer Research UK on Facebook Cancer Research UK on Twitter

What fluorouracil is

Fluorouracil is also known as FU or 5FU and is one of the most commonly used drugs to treat cancer. It is a treatment for many types of cancer including

  • Breast cancer 
  • Head and neck cancers
  • Anal cancer
  • Stomach cancer
  • Colon cancer
  • Some skin cancers

It may be combined with other cancer drugs or with radiotherapy.


How fluorouracil works

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 you have fluorouracil

You have fluorouracil into your bloodstream. You can have it through a thin, short tube (a cannula) put into a vein in your arm each time you have treatment. Or you may have it through a central line, a portacath, or a PICC line. These are long, plastic tubes that give the drugs directly into a large vein in your chest. You have the tube put in before or during your course of treatment and it stays in place as long as you need it.

You can read our information about having cancer drugs.

You usually have fluorouracil as part of a course of several cycles of treatment. The exact treatment plan depends on which type of cancer you have. You may have the fluorouracil over several days through a drip or using a small pump that you carry around.

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


Tests during treatment

You have blood tests before starting treatment and regularly during your treatment. The tests check your levels of blood cells. They also check how well your liver and kidneys are working.


About side effects

We've listed the side effects associated with fluorouracil. You can use the links to find out more about each side effect. Where there is no link, please go to our information about cancer drug side effects or use the search box at the top of the page.

You may have a few side effects. They may be mild or more severe. A side effect may get better or worse through your course of treatment. Or more side effects may develop as the course goes on. This depends on

  • How many times you've had the drug before
  • Your general health
  • The amount of the drug you have (the dose)

The side effects may be different if you are having fluorouracil with other medicines.

Tell your doctor, nurse or pharmacist straight away if any of the side effects get severe.


Common side effects

More than 10 in every 100 people have one or more of these side effects.

  • An increased risk of getting an infection from a drop in white blood cells – it is harder to fight infections and you can become very ill. You may have headaches, aching muscles, a cough, a sore throat, pain passing urine, or you may feel cold and shivery. If you have a severe infection this can be life threatening. Contact your treatment centre straight away if you have any of these effects or if your temperature goes above 38°C
  • Breathlessness and looking pale due to a drop in red blood cells (anaemia) – you may need a blood transfusion
  • Bruising more easily due to a drop in platelets – 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 (known as petechiae)
  • Tiredness and weakness (fatigue) during and after treatment – most people find their energy levels are back to normal from 6 months to a year after their treatment ends
  • Feeling or being sick, which is usually mild and well controlled with anti sickness medicines. Let your doctor or nurse know if you feel sick so that they can adjust your medicines if necessary
  • Mouth sores and ulcers
  • Diarrhoea – you need to drink plenty of fluids. If the diarrhoea becomes severe or continues for more than 3 days you could get dehydrated so tell your doctor or nurse
  • Changes in heart rhythm – let your doctor or nurse know if you have palpitations (a feeling of your heart beating)
  • Continuous infusion of fluorouracil can cause soreness, redness and peeling on the palms of the hands and soles of the feet (palmar – plantar syndrome). This may cause tingling, numbness, pain and dryness
  • Hair thinning
  • Loss of appetite
  • High uric acid levels in your blood due to cancer cells being broken down by the body (tumour lysis syndrome). You will have regular blood tests and will be asked to drink plenty of fluids to flush out the uric acid. Your doctor or nurse may give you medicines to control this

Occasional side effects

Between 1 and 10 in every 100 people have one or more of these.

  • Brittle, chipped and ridged nails – you may have a blue tinge, darkening or the nails or flaking of the nails. Some people have pain and thickening of the area where the nail starts growing (the nail bed)
  • Sensitivity of the skin to sunlight – you shouldn't use sunbeds or sit in the sun if having fluorouracil. Cover up or use a sun block
  • Skin changes, such as dryness, itching, rashes and reddening
  • Watery eyes from increased production of tears
  • Gritty, red, sore eyes and blurred vision – if you have this don't operate machinery or drive. Your nurse can give you eyedrops
  • A brown marking on the skin following the line of the vein where fluorouracil has been injected
  • Loss of fertility – you may not be able to become pregnant or father a child after this treatment. Talk to your doctor before starting treatment if you think you may want to have a baby in the future. Men may be able to store sperm before starting treatment

Rare side effects

Fewer than 1 in 100 people have these.

  • Total hair loss
  • Heart problems, such as angina, heart failure or a heart attack – tell your doctor or nurse straight away if you have any chest pain
  • Confusion or unsteadiness
  • Women may stop having periods (amenorrhoea) but this may only be temporary
  • The eyes moving quickly from side to side (nystagmus)
  • Low blood pressure, making you feel dizzy or faint
  • Headaches
  • Liver changes that are very mild and unlikely to cause symptoms – they will usually go back to normal when treatment is finished
  • A feeling of intense happiness and confidence (euphoria)
  • Numbness, tingling or shaking in the hands or feet
  • Skin may appear lighter or darker

Important points to remember

Talk to your doctor, pharmacist or nurse about all your side effects so they can help you manage them. They can give you advice or reassure you. Your nurse will give you a contact number to ring if you have any questions or problems. If in doubt, call them.

Other medicines and treatments

Tell your doctor about any other medicines you are taking, including vitamins, herbal supplements and over the counter remedies. Some drugs can react together.

Pregnancy and contraception

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


Don't breastfeed during this treatment because the drug may come through in the breast milk.

Low levels of DPD

If you know you have a condition that causes low levels of a protein called DPD (dihydropyrimidine dehydrogenase) tell your doctor. Low levels of DPD may increase the side effects of fluorouracil. Read our information about DPD and fluorouracil.  

Slow wound healing

Fluorouracil can cause slow wound healing. If you need to have an operation your doctor will normally advise you to stop having fluorouracil for a while beforehand. They will let you know when you can start taking it again.


Immunisations and chemotherapy

You shouldn't have immunisations with live vaccines while you are having chemotherapy or for at least 6 months afterwards. In the UK, these include rubella, mumps, measles (usually given together as MMR), BCG, yellow fever and Zostavax (shingles vaccine).

You can have other vaccines, but they may not give you as much protection as usual until your immune system has fully recovered from your chemotherapy. It is safe to have the flu vaccine.

It is safe for you to be in contact with other people who've had live vaccines as injections. There can be problems with vaccines you take by mouth (oral vaccines), but not many people in the UK have these now. So there is usually no problem in being with any baby or child who has recently had any vaccination in the UK. You might need to make sure that you aren't in contact with anyone who has had oral polio, cholera or typhoid vaccination recently, particularly if you live abroad.


Related information


More information about fluorouracil

This page doesn't list all the very rare side effects of this treatment that are very unlikely to affect you. For further information look at the Electronic Medicines Compendium website at www.medicines.org.uk.

If you have a side effect not mentioned here that you think may be due to this treatment you can report it to the Medicines Health and Regulatory Authority (MHRA) at www.mhra.gov.uk.

Rate this page:
Submit rating


Rated 5 out of 5 based on 95 votes
Rate this page
Rate this page for no comments box
Please enter feedback to continue submitting
Send feedback
Question about cancer? Contact our information nurse team

No Error

Updated: 27 January 2016