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Fluorouracil (5FU)

Find out what fluorouracil is, how you have it and other important information about taking fluorouracil.

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

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 may have fluorouracil through a drip or using a small pump that you carry around for several days.

Fluorouracil is also available as an ointment called Efudix 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.

Into your bloodstream

You have the treatment through a drip into your arm. A nurse puts a small tube (a cannula) into one of your veins and connects the drip to it.

You might need a central line. This is a long plastic tube that gives the drugs into a large vein, either in your chest or through a vein in your arm. It stays in while you’re having treatment, which may be for a few months.

When you have fluorouracil

You usually have fluorouracil as part of a course of several cycles of treatment. You generally have up to 6 cycles of treatment. Each cycle lasts 3 or 4 weeks.  

You 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.


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.

Side effects

Important information

Other medicines

Cancer drugs can interact with some other medicines and herbal products. Tell your doctor or pharmacist about any medicines you are taking. This includes vitamins, herbal supplements and over the counter remedies.

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 into your breast milk.

Treatment for other conditions

Always tell other doctors, nurses, pharmacists or dentists that you’re having this treatment if you need treatment for anything else, including teeth problems.


Don’t have immunisations with live vaccines while you’re having treatment and for at least 6 months afterwards.

In the UK, live vaccines include rubella, mumps, measles, BCG, yellow fever and shingles vaccine (Zostavax).

You can:

  • have other vaccines, but they might not give you as much protection as usual
  • have the flu vaccine (as an injection)
  • be in contact with other people who've had live vaccines as injections

Avoid close contact with people who’ve recently had live vaccines taken by mouth (oral vaccines) such as oral polio or the typhoid vaccine.

This also includes the rotavirus vaccine given to babies. The virus is in the baby’s poo for up to 2 weeks and could make you ill. So, avoid changing their nappies for 2 weeks after their vaccination if possible. Or wear disposable gloves and wash your hands well afterwards.

You should also avoid close contact with children who have had the flu vaccine nasal spray if your immune system is severely weakened. 

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.

Low levels of DPD

Around 5 out of 100 people (5%) have low levels of an enzyme called DPD in their bodies. 

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 can increase the side effects of fluorouracil.

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.

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.

Information and help

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About Cancer generously supported by Dangoor Education since 2010.