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Fluorouracil (5-FU)


Trade/other name(s): 5-FU, Adrucil, Efudex (topical), 5-fluorouracil

Why would this drug be used?

Fluorouracil is used to treat several types of cancer including colon, rectum, and head and neck cancers. It is also used for other types of cancer, and the skin cream is used for other conditions as well.

How does this drug work?

Fluorouracil belongs to the class of chemotherapy drugs known as anti-metabolites. It interferes with cells making DNA and RNA, which stops the growth of cancer cells.

Before taking this medicine

Tell your doctor…

  • If you are allergic to anything, including medicines, dyes, additives, or foods.
  • If you have any medical conditions such as kidney disease, liver disease (including hepatitis), heart disease, congestive heart failure, diabetes, gout, or infections. These conditions may require that your medicine dose, regimen, or timing be changed.
  • If you have been treated for cancer with radiation or chemotherapy. Your doctor may need to watch you more closely during treatment with fluorouracil.
  • If you have ever been told you have a dihydropyrimidine dehydrogenase (DPD) deficiency. DPD is an enzyme the body uses to process this drug. This inborn genetic abnormality can cause extreme side effects if you use 5-FU (even on your skin) or capecitabine. DPD deficiency can be present without symptoms, so you may not know you have it until you get one of these drugs or are tested for it.
  • If you are pregnant, trying to get pregnant, or if there is any chance of pregnancy. This drug may cause birth defects if either the male or female is taking it at the time of conception or during pregnancy. Men and women who are taking this drug need to use some kind of birth control. It is important to check with your doctor about what kinds of birth control can be used with this medicine.
  • If you are breast-feeding. It is not known whether this drug passes into breast milk. If it does, it could harm the baby.
  • If you think you might want to have children in the future. Some drugs can cause sterility. Talk with your doctor about the possible risk with this drug and the options that may preserve your ability to have children.
  • About any other prescription or over-the-counter medicines you are taking, including vitamins and herbs. In fact, keeping a written list of each of these medicines (including the doses of each and when you take them) with you in case of emergency may help prevent complications if you get sick.

Interactions with other drugs

The drug leucovorin can increase the risk of side effects from fluorouracil.

Any drugs or supplements that interfere with blood clotting can raise the risk of bleeding during treatment with fluorouracil. These include:

  • vitamin E
  • non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen (Advil, Motrin), naproxen (Aleve, Naprosyn), and many others
  • warfarin (Coumadin)
  • ticlopidine (Ticlid)
  • clopidogrel (Plavix)

Note that many cold, flu, fever, and headache remedies contain aspirin or ibuprofen. Ask your pharmacist if you aren't sure what's in the medicines you take.

The drug levels of phenytoin (Dilantin) and fosphenytoin (Cerebyx) may be increased by fluorouracil. Your doctor will want to watch your drug levels closely.

Check with your doctor, nurse, or pharmacist about other medicines, vitamins, herbs, and supplements, and whether alcohol can cause problems with this medicine.

Interactions with foods

No serious interactions with food are known at this time. Check with your doctor, nurse, or pharmacist about whether foods may be a problem.

Tell all the doctors, dentists, nurses, and pharmacists you visit that you are taking this drug.

How is this drug taken or given?

Fluorouracil is given as a shot in the vein (intravenously) over 5 to 10 minutes, over 20 to 60 minutes, or as a continuous infusion over 22 to 24 hours for 1 to 4 days, or longer. The treatment can be repeated weekly, every other week, or every 3 weeks, depending on the treatment regimen. The dose depends on your weight. It may be lowered if you have problems taking the drug (severe diarrhea, or low white blood cell or platelet count).


Your doctor will likely test your blood throughout your treatment, looking for possible effects of the drug on blood counts (described below) or on other body organs. Based on the test results, you may be given medicines to help treat any effects. Your doctor may also need to reduce or delay your next dose of this drug, or even stop it altogether. Keep all your appointments for lab tests and doctor visits.

This drug can lower your white blood cell count, especially in the weeks after the drug is given. This can increase your chance of getting an infection. Be sure to let your doctor or nurse know right away if you have any signs of infection, such as fever (100.5° or higher), chills, pain when passing urine, a new cough, or coughing up sputum.

This drug may lower your platelet count in the weeks after it is given, which can increase your risk of bleeding. Speak with your doctor before taking any drugs or supplements that might affect your body's ability to stop bleeding, such as aspirin or medicines containing aspirin, warfarin (Coumadin), or vitamin E. Tell your doctor right away if you have unusual bruising, or bleeding such as nosebleeds, bleeding gums when you brush your teeth, or black, tarry stools.

This drug may lower your red blood cell count. If this is going to happen, it usually happens a few months after starting treatment. A low red blood cell count (known as anemia) can cause shortness of breath, or make you feel weak or tired all the time. Your doctor may give you medicines to help prevent or treat this condition, or you may need to get blood transfusions.

Do not get any immunizations (vaccines), either during or after treatment with this drug, without your doctor's OK. Fluorouracil may affect your immune system. This could make vaccinations ineffective, or could even lead to serious infections if you get a live virus vaccine during or soon after treatment. Try to avoid contact with people who have recently received a live virus vaccine, such as the oral polio vaccine or smallpox vaccine. Check with your doctor about this.

You may have nausea and vomiting on the day you receive this drug or in the first few days afterward. Your doctor may give you medicine before your treatment to help prevent nausea and vomiting. You will likely also get a prescription for an anti-nausea medicine that you can take at home. It is important to have these medicines on hand and to take them as prescribed by your doctor. Call your doctor if the medicines are not working to stop the vomiting.

This drug may cause sores in the mouth or on the lips, which often occur within the first few weeks after starting treatment. Call your doctor or nurse right away. You may need to stop taking this drug. Your doctor or nurse can suggest ways you can still eat or brush your teeth. If needed, your doctor can prescribe medicine to help with the pain.

Call your doctor if you have severe diarrhea or pain in your belly. You may need to get treatment for the problem. Your doctor may also want to stop this drug.

This drug can make you very sensitive to sunlight or bright ultraviolet light. When possible, avoid being outdoors between the hours of 10 and 4. Wear sunglasses, hat, and protective clothes when outside, even on hazy days. Always apply sunscreen half an hour before going out in the sun, and follow the instructions for repeat applications. Avoid tanning beds.

This drug can cause allergic reactions in some people. Symptoms include feeling lightheaded or dizzy (due to low blood pressure), itching, hives (welts on the skin), rapid heartbeat, coughing, wheezing, shortness of breath, or swelling of the face, tongue, or throat. Tell your doctor or nurse right away if you notice any of these symptoms as you are being given the drug

This drug can cause a condition known as hand-foot syndrome, in which a person may experience pain, numbness, tingling, reddening, or swelling in the hands or feet. Peeling, blistering, or sores on the skin in these areas are also possible. Let your doctor know right away if you notice any of these symptoms.

Men and women should avoid conceiving a child during and for a few months after treatment. Talk with your doctor about birth control.

Possible side effects

You will probably not have most of the following side effects, but if you have any talk to your doctor or nurse. They can help you understand the side effects and cope with them.


  • low white blood cell count with increased risk of infection*
  • low platelet count with increased risk of bleeding*
  • darkening of skin and nail beds
  • nausea*
  • vomiting*
  • poor appetite
  • sores in mouth, lips, or throat*
  • hair loss or thinning (may include face and body hair)
  • diarrhea
  • brittle nails
  • increased sensitivity to sun, with risk of severe sunburn*
  • dry, flaky, cracking skin

Less common

  • darkening and hardening of vein used for giving the drug
  • headache
  • weakness
  • muscle aches


  • trouble walking, trouble forming words, and poor coordination
  • irritated eyes
  • increased tears, watering eyes
  • blurred vision
  • heart problems (chest pain, heart attack, heart failure, changes in electrical conduction and more) that usually get better after the drug is stopped
  • confusion
  • tingling, numbness, or swelling in the hands and feet*
  • severe allergic reaction*
  • death due to infection, bleeding, not being able to process the drug (DPD deficiency), or other causes

*See "Precautions" section for more detailed information.

There are other side effects not listed above that can also occur in some patients. Tell your doctor or nurse if you develop these or any other problems.

FDA approval

Yes – first approved before 1984 (FDA cannot verify dates of drugs approved before 1984.)

Disclaimer: This information does not cover all possible uses, actions, precautions, side effects, or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for talking with your doctor, who is familiar with your medical needs.

Last Medical Review: 02/01/2010
Last Revised: 03/07/2014