Fluorouracil (5-FU)
Say it: flure-oh-YOOR-uh-sill
Fluorouracil is a chemotherapy medicine that blocks cancer cells from making the DNA they need to grow.
What it is
Fluorouracil looks like one of the building blocks of DNA. Cancer cells take it in by mistake. It blocks an enzyme they need to make DNA, so the cells cannot grow and die.
Often used for
- colon and rectal cancer
- breast cancer
- stomach cancer
- pancreatic cancer
- and some other cancers, such as head and neck cancer
How you get it
Fluorouracil is given through a vein (IV). It is often given as a quick dose, then as a slow drip through a small pump you wear at home for about 2 days. This is often repeated every 2 weeks. Your schedule may be different.
Common side effects and what helps
- Diarrhea. Drink plenty of fluids and use the diarrhea medicine your team suggests.
- Mouth sores. Use a soft toothbrush and rinse often with a mild salt and baking soda rinse.
- Red, sore, or peeling hands and feet. Keep them moisturized and avoid heat and rubbing.
- Nausea. Take your anti-nausea medicine as your team tells you.
- Low blood counts, which raise your risk of infection and bleeding. Wash your hands often.
- Feeling tired. Rest when you need to.
- Skin that burns easily in the sun. Use sunscreen and cover up.
Call your care team right away if you have
- A fever of 100.4 degrees Fahrenheit (38 degrees Celsius) or higher, or chills
- Diarrhea more than 4 times a day, or diarrhea at night
- Mouth sores that keep you from eating or drinking
- Chest pain or pressure
- Painful blisters or peeling on hands or feet
- Confusion or extreme sleepiness
Before you start, tell your team
- If you are pregnant, may become pregnant, or are breastfeeding
- If you know you have a DPD enzyme deficiency, or if you have had a bad reaction to fluorouracil or capecitabine
- If you take warfarin or other blood thinners
- If you have heart problems
- All the medicines, vitamins, and herbal products you take
Good to know
- Some people lack an enzyme called DPD that breaks down this medicine. They can have very serious or even deadly side effects. Your team should test you for this before you start, unless you need treatment right away.
- If you take warfarin, you will need more frequent blood tests.
- You will have blood tests to check your blood counts.
- Rarely, it can cause chest pain or heart problems.
At home
- If you go home with a pump, keep it dry and do not disconnect it yourself.
- Keep the tubing from getting pulled or kinked.
- If the pump alarms or leaks, call your team. Use the spill kit and gloves you were given.
- Ask your team how to safely handle body fluids during and for a short time after the pump.
Questions to ask
- How long will I wear the pump?
- Should I be tested for DPD deficiency?
- What should I take for diarrhea?
- Who do I call if the pump has a problem?
Part of these treatment plans
Side effect guides
- Fatigue (tiredness)
- Nausea and vomiting
- Diarrhea
- Mouth sores
- Low white blood cells and infection risk
- Low platelets and bleeding
- Hand-foot syndrome
Sources
Quick answers
- What is Fluorouracil (5-FU)?
- Fluorouracil is a chemotherapy medicine that blocks cancer cells from making the DNA they need to grow.
- What is Fluorouracil (5-FU) used for?
- colon and rectal cancer; breast cancer; stomach cancer; pancreatic cancer; and some other cancers, such as head and neck cancer.
- How is Fluorouracil (5-FU) given?
- Fluorouracil is given through a vein (IV). It is often given as a quick dose, then as a slow drip through a small pump you wear at home for about 2 days. This is often repeated every 2 weeks. Your schedule may be different.
- What are the common side effects of Fluorouracil (5-FU)?
- Diarrhea. Mouth sores. Red, sore, or peeling hands and feet. Nausea. Low blood counts, which raise your risk of infection and bleeding. Feeling tired. Skin that burns easily in the sun.
- When should I call my care team while taking Fluorouracil (5-FU)?
- A fever of 100.4 degrees Fahrenheit (38 degrees Celsius) or higher, or chills; Diarrhea more than 4 times a day, or diarrhea at night; Mouth sores that keep you from eating or drinking; Chest pain or pressure; Painful blisters or peeling on hands or feet; Confusion or extreme sleepiness.
- Which chemotherapy plans include Fluorouracil (5-FU)?
- FLOT, FOLFIRI, FOLFIRINOX, FOLFOX, FOLFOXIRI.
Last updated: 2026-10-27. Medically reviewed by Veli Bakalov, MD, medical oncologist (October 2026). General education from Step by Step, the patient portal of HemeOncBuddy.