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5-FU (fluorouracil) (fluorouracil)
Chemotherapy (antimetabolite) · Used for: Colorectal cancer
💉 Given by IV or injection · Chemotherapy (antimetabolite)
Cliff notes
What it is: Chemotherapy (antimetabolite).
How it's given: An IV push and then a 46-hour infusion through a portable pump you take home (in FOLFOX and FOLFIRI).
How it works: 5-FU looks like a building block of DNA. Cancer cells take it in and can't make new DNA properly.
Common side effects and what helps at home
| Side effect | What can help |
|---|---|
| Diarrhea | Fluids; call if severe. |
| Mouth sores | Salt and baking soda rinses. |
| Hand-foot syndrome (red, sore palms and soles) | Moisturize; avoid heat and friction. |
| Low blood counts | Hand washing. |
| Nausea | Small meals. |
Not everyone gets these. Your team may suggest other ways to manage them.
Call your care team now if you have:
- Fever of 100.4°F (38°C) or higher, or chills (possible infection when blood counts are low)
- Vomiting or diarrhea that won't stop, or you can't keep fluids down
- Unusual bleeding or bruising
- Shortness of breath or chest pain
- Signs of an allergic reaction during or after an infusion: trouble breathing, swelling of the face or throat, hives
- Chest pain (5-FU can rarely affect the heart, call 911 for chest pain)
- Painful mouth sores that stop you eating or drinking
- A problem with your pump or line
- Confusion, extreme sleepiness or trouble walking (rare; can happen within 3 days of starting)
- You take warfarin (Coumadin). 5-FU can raise your INR, so your team needs to check it closely
Call 911 for trouble breathing, chest pain or signs of stroke.
🗓 What to expect
1️⃣ Before you start
- Ask your team for a written schedule and an after-hours phone number.
- A DPYD gene test, which the FDA label now recommends before starting 5-FU.
- Bring a list of all your medicines and supplements.
2️⃣ During each infusion
- You'll get the medicine through an IV in a clinic; nurses check you during the infusion.
- Tell the nurse right away about chills, itching, flushing or trouble breathing.
- Bring a snack, water and something to pass the time.
3️⃣ Between doses
- Watch for the side effects below and use the checklist.
- Keep up with blood tests and visits.
- Call your team for anything in the red box.
✅ Side-effect checklist
Print this and check off what you notice so you can tell your team.
| Side effect | Noticed? | Notes for my team |
|---|---|---|
| Diarrhea | ☐ | |
| Mouth sores | ☐ | |
| Hand-foot syndrome (red, sore palms and soles) | ☐ | |
| Low blood counts | ☐ | |
| Nausea | ☐ |
Help paying for 5-FU (fluorouracil)
Generic chemo has no single manufacturer program, so the copay foundations below are your best bet.
- PAN Foundation: Copays and coinsurance, including for people with Medicare, when a disease fund is open.
- HealthWell Foundation: Grants for copays, deductibles and premiums when a fund is open.
- CancerCare Co-Payment Assistance Foundation: Help with treatment copays for insured people under income limits.
- Good Days: Copay help and some travel costs.
- NeedyMeds: Free database of assistance programs, open to anyone.
📋 Medicare: Medicare Part B usually covers drugs given by infusion in a doctor's office or hospital outpatient clinic. You typically pay 20% after the Part B deductible. See Medicare and your cancer drugs.
More: Our guide to paying for cancer treatment.
Regimens that include 5-FU (fluorouracil)
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❓ Questions to ask your doctor
- What is the goal of this treatment for me?
- How will we know if it's working?
- Which side effects should I call about right away, and what number do I call after hours?
- Which medicines, vitamins or supplements should I avoid?
- Will this affect my ability to work, drive or care for my family?
- What will this cost me, and who can help me with coverage?
Support groups
- Fight Colorectal Cancer: Patient guides, advocacy and community
- Colorectal Cancer Alliance: Helpline (877-422-2030) and patient navigation
- CancerCare: Free counseling, support groups and financial help
- Cancer Support Community: Support groups, a helpline and the MyLifeLine online community
Common questions
What is DPD deficiency, and should I be tested?
DPD is an enzyme that breaks down 5-FU. People who lack it can have severe or deadly side effects. The current FDA label has a boxed warning and says patients should be tested for DPYD gene variants before starting 5-FU unless treatment is urgent. Ask your team if you've been tested.
Can I shower with the 5-FU pump?
Usually not while it's connected. Follow your clinic's instructions.
📄 The official FDA label for 5-FU (fluorouracil), including its patient information section, is on DailyMed.
Written by the Cancer Side Effects Guide team from FDA labels and leading cancer centers. Not medical advice. · Last updated: October 15, 2026 · How we write