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At a glance
- Treatment
- Fluorouracil; often called 5-FU
- Used in selected settings
- Colorectal, stomach, oesophageal, pancreatic, head and neck, anal, breast
- How it is given
- Into a vein, as a short injection, an infusion or a continuous infusion through a portable pump. This page covers systemic chemotherapy, not fluorouracil cream for skin conditions.
- Before you start
- Save your 24-hour oncology number and have a thermometer and written medicine calendar available.
What this treatment is and how it works
An antimetabolite: it interferes with the materials and enzymes cells need to make DNA and RNA, the genetic material and messages cells use to function. This makes it harder for cancer cells to grow and divide.
When it may be used
Your team may use this treatment in the following settings: Colorectal, stomach, oesophageal, pancreatic, head and neck, anal, breast. The choice depends on the exact diagnosis, stage, previous treatment, health and treatment aim. Treatment may aim to cure cancer, lower the risk of recurrence, or control cancer and symptoms, depending on the setting.
A listed use is not a claim that every brand is licensed for that use in every country. Your oncologist will explain why this treatment is appropriate for you.
Before treatment
Before starting, tell the team about previous cancer treatment, allergies, infection symptoms, heart, lung, kidney or liver disease, hearing or nerve problems, diabetes and all medicines you take. Discuss pregnancy, contraception and fertility. Ask for a written treatment calendar, supportive-medicine instructions and a 24-hour oncology contact number. Baseline blood tests usually include a full blood count and kidney and liver function; additional tests depend on the medicine and your health. Before chemotherapy that reaches the whole body, the team should check for current or past hepatitis B infection. If a test is positive, extra monitoring or antiviral treatment may be needed to prevent the virus becoming active again. The team will coordinate testing and treatment; do not delay urgent cancer treatment yourself.
How treatment is given and the schedule
Into a vein, as a short injection, an infusion or a continuous infusion through a portable pump. This page covers systemic chemotherapy, not fluorouracil cream for skin conditions.
The calendar depends on the combination. Some colorectal regimens use a pump over about 46 hours every 2 weeks; other regimens use shorter injections or longer infusions. These schedules are not interchangeable.
The dose is calculated for your treatment and health. Rest periods, delays or dose changes may be needed. Do not change doses or treatment dates yourself, except to follow specific instructions to stop a medicine and seek help for warning symptoms. The team will give you a revised plan when needed.
During an infusion, tell the nurse immediately about pain, burning, swelling or leaking around the cannula or line. Do not massage the area or apply heat or ice unless the team instructs you.
Tests and monitoring during treatment
Before the first dose, the team should arrange assessment for reduced dihydropyrimidine dehydrogenase (DPD) activity, usually with a DPYD gene test and/or a blood test according to local practice. Known complete DPD deficiency means fluorouracil must not be used; partial deficiency may require a lower starting dose. A normal test does not rule out serious toxicity. Tell the team about heart disease or previous chest pain during treatment.
At each review, describe symptoms since the previous dose, including problems that have already settled. Blood tests can detect effects before you feel them. Your team may delay, reduce, omit or stop a medicine if results or symptoms make this necessary. The decision depends on the exact protocol; a normal result does not make new symptoms safe to ignore.
Side effects: what to expect
Frequency varies with the dose, schedule, cancer and other treatments. “Common or more frequent” is a practical grouping, not a single percentage for every patient. Some serious effects appear in more than one group because severity and frequency are different.
You are unlikely to have all of these effects. This is not an exhaustive list. Tell the team about any new or troublesome symptom, including one not mentioned here.
Common or more frequent side effects
| Possible effect | What you can do | When to contact the team |
|---|---|---|
| Low blood counts | White cells fight infection, red cells carry oxygen and platelets help clotting. Treatment can lower one or more of these. Wash hands, avoid close contact with people who are unwell and keep blood-test appointments. | Call immediately for fever, shaking chills or feeling suddenly unwell. Report unusual bruising, bleeding, marked tiredness or increasing breathlessness. Do not wait for a blood-test result before seeking help. |
| Feeling or being sick | Take preventive anti-sickness medicines at the prescribed times. Try small meals and frequent small drinks; avoid strong food smells if they worsen nausea. | Contact the team the same day if medicines are not controlling symptoms. Seek urgent advice if you cannot keep fluids down, feel faint or pass much less urine. |
| Sore mouth or throat | Use a soft toothbrush and the mouth-care plan from your team. Choose soft, moist foods and avoid alcohol-containing mouthwash or foods that sting. | Contact the team if pain affects eating or drinking, or if white patches or ulcers develop. Inability to swallow fluids, fever or severe pain needs urgent assessment. |
| Loose or frequent stools | Keep a record of stools. Take frequent small drinks and use the diarrhoea medicine prescribed for your regimen. Oral rehydration solution may help if the team recommends it. | Contact the team promptly for diarrhoea that continues or increases. Fever, blood, severe pain, dizziness or difficulty drinking needs urgent assessment. Drug-specific stop instructions below take priority. |
| Sore palms or soles (hand–foot syndrome) | Use a plain moisturiser and avoid friction, tight shoes, prolonged heat and pressure on the skin. Check hands and feet each day. | Contact the team early for soreness. Pain affecting walking or hand use, blisters or broken skin may require treatment to be held. Your team will decide whether to pause fluorouracil or adjust the next dose. Do not disconnect an infusion pump yourself. |
| Tiredness and reduced stamina | Balance rest with short periods of gentle activity. Prioritise tasks and accept help with shopping, meals or transport. Tell the team about poor sleep or pain. | Report fatigue that prevents basic activities or is worsening. Sudden breathlessness, chest pain, fainting or confusion needs urgent assessment. |
| Reduced appetite or changes in eating | Try smaller, frequent meals with protein and energy-rich foods you tolerate. Ask for a dietitian if your weight or intake is falling. | Report ongoing weight loss or difficulty eating. Inability to drink or signs of dehydration needs same-day urgent advice. |
| Hair thinning or hair loss | Use gentle hair care and protect the scalp from sun and cold. Ask early about wigs, head coverings and whether scalp cooling is suitable for your particular treatment. | Tell the team about a painful or inflamed scalp. Hair often regrows after treatment, but the timing, texture and completeness of regrowth vary. |
Other reported side effects (frequency varies)
These effects may be common in some treatment settings, less common in others, or have no reliable frequency estimate. Their position here does not mean they are necessarily less frequent or less serious.
Skin pigmentation, sun sensitivity, nail changes, watery or sore eyes and problems with balance or concentration can occur. The pattern differs between a short injection and a prolonged infusion.
Report these at review, or sooner if worsening. Use the urgent guidance for symptoms that affect breathing, drinking, urination, walking, consciousness or bleeding.
Important serious side effects, including rare effects
Serious does not always mean rare. Where reliable incidence is unavailable, no numerical frequency is assigned. The action to take matters even when the likelihood is uncertain.
Unusually severe fluoropyrimidine toxicity
Reduced DPD activity can cause severe diarrhoea, mouth injury, low blood counts and neurological effects, sometimes early in the first course. Testing reduces but does not remove risk. Seek urgent oncology assessment for unexpectedly severe or rapidly worsening symptoms.
Chest pain or a heart-rhythm problem
New chest tightness, chest pain, severe breathlessness, fainting or a sustained racing heartbeat needs emergency assessment. Do not try to manage this with home remedies or wait for the next visit.
Serious neurological symptoms
New confusion, marked weakness, trouble walking or speaking, a seizure or sudden vision change needs immediate medical assessment. These symptoms may have several causes and must not be attributed to chemotherapy without evaluation.
Severe skin reaction
A spreading painful rash, blisters, peeling skin, or sores in the mouth, eyes or genital area needs urgent medical assessment. Severe reactions may require treatment to be stopped by the team.
Suspected overdose or unexpectedly rapid infusion
If a fluorouracil pump seems to have emptied too quickly, seek urgent medical assessment immediately, even if you feel well. Tell the oncology team what happened and when; bring the medicine packaging or pump details. Do not change pump settings yourself. A hospital may consider a time-sensitive antidote, so do not wait for symptoms or the next appointment. Local access to the antidote must be arranged by the treating service.
Practical advice for this treatment
Keep the pump and line protected as your nurse demonstrates. If it leaks, disconnects, alarms or appears to empty too quickly, contact the treatment unit immediately and follow its spill instructions. Do not repair or change the pump settings yourself.
Medicines, interactions and vaccinations
Give the oncology pharmacist a complete list of prescription medicines, over-the-counter products, vitamins, herbal and Ayurvedic remedies and supplements. Ask before starting anything new. Do not stop prescribed medicines, including anticoagulants or infection-prevention medicines, without a coordinated plan. Live vaccines are generally unsuitable while immunity is suppressed; discuss vaccine type and timing with the team. Non-live vaccines may be offered, although protection can be reduced. Follow any specific stop-and-call instructions for the cancer medicine on this page or in your written treatment plan.
Brivudine, an antiviral used in some countries, can cause a life-threatening interaction; at least 4 weeks must separate the end of brivudine treatment from fluorouracil. Warfarin, phenytoin and some other medicines need extra monitoring. Folinic acid is deliberately combined with 5-FU in some regimens and can also increase its side effects.
Pregnancy, fertility and breastfeeding
Chemotherapy can harm a developing baby and may affect fertility temporarily or permanently. Discuss sperm, egg or embryo storage before treatment when feasible. If you could become pregnant, or your partner could become pregnant, use effective contraception during treatment and for the drug-specific period afterwards. Ask for a written end date based on every medicine in your plan: the required period differs by product and can extend for months. Periods stopping does not guarantee infertility. Tell the team immediately about a suspected pregnancy. Do not breastfeed during treatment; restart only after the team confirms the interval for the exact medicine and formulation.
Everyday life
Gentle activity, regular meals and rest can help you cope. Prepare food hygienically and drink according to your individual fluid plan. Avoid smoking; ask whether alcohol is safe with your exact treatment and other medicines. Plan work, travel and driving around symptoms and monitoring visits. Do not drive if dizzy, sleepy, weak, visually affected or impaired by neuropathy. You can usually be close to family and friends: chemotherapy does not make you radioactive. Follow the unit’s instructions for handling urine, vomit, laundry and sexual precautions for the period it specifies.
When to contact the oncology team
Contact the oncology team the same day for persistent vomiting or diarrhoea, difficulty drinking, reduced urine, mouth pain affecting eating, new cough, jaundice, a painful or spreading rash, unusual bruising, worsening constipation or new nerve symptoms. Use the more urgent instructions for the particular symptom when given above. If the team cannot be reached and you are unwell, seek urgent medical care.
Write down your hospital’s daytime and out-of-hours numbers before treatment. CTI’s general contact details do not replace the emergency number provided by your treating hospital.
After treatment
Keep the planned blood tests and follow-up appointments after the last dose. Blood counts and immunity may take time to recover, and some effects can start or persist later. Contact the team about symptoms rather than waiting for a routine visit. Keep a treatment summary, including drug names, dates and any serious reactions, for future doctors. Response may be assessed with examination, scans or blood or marrow tests, depending on the cancer.
Words you may hear
- DPD / DPYD
- DPD is an enzyme that helps break down fluorouracil. DPYD is the gene that carries its instructions. Reduced activity can increase serious toxicity.
Frequently asked questions
Why do I need a DPD test?
DPD helps the body break down fluorouracil. Low activity can cause unusually severe side effects. Testing helps the team choose safer treatment, but you still need to report symptoms promptly.
Is a 5-FU pump the same as an injection?
It contains the same medicine, but the dose and time over which it enters the body differ. Follow the calendar for your prescribed regimen.
Will I get every side effect listed?
No. Many people have only some of them. Your dose, other medicines, health and previous treatment all affect risk. Reporting symptoms early often makes them easier to manage.
Does having few side effects mean treatment is not working?
No. Side effects do not reliably show how well cancer treatment is working. The team assesses response using the appropriate tests.
Can a dose be reduced or delayed?
Yes. Blood counts, organ function or side effects may require a delay, reduction, omission or change. The team balances safety with the treatment aim; do not change doses yourself. Follow the specific instructions to stop a medicine and seek help if a warning symptom occurs.
Can I take vitamins, herbal or Ayurvedic products?
Check every product with the oncology pharmacist first. “Natural” does not mean interaction-free, and some products can affect the liver, bleeding or drug levels.
What should I have ready at home?
A thermometer, the 24-hour oncology number, a treatment calendar, your medicine list and prescribed symptom medicines. Arrange help with transport or meals if needed.
When should I call instead of waiting for the next appointment?
Use the urgent-help instructions on this page. Fever, sudden illness, uncontrolled symptoms or difficulty drinking should not wait until the next treatment visit.
Related combination guides
Sources and further reading
Summary of Product Characteristics
Product-specific details vary by formulation and treatment protocol.
Professional reference resources. These describe the specific product, warnings, interactions, monitoring and adverse effects. For US substitutes, the document is prescribing information rather than an SmPC. Check the product actually dispensed in India.
- Fluorouracil 50 mg/ml injection — Hospira UK. Accessed 16 October 2026. Current company-supplied/regulatory product information; not asserted to be the originator brand.
Other authoritative references
- Cancer Research UK — Fluorouracil (5-FU). Accessed 16 October 2026.
- FDA — current fluoropyrimidine DPD safety labelling update. Accessed 16 October 2026.
- NCI — infection and neutropenia. Accessed 16 October 2026.
- NCI — mouth and throat problems. Accessed 16 October 2026.
- NCI — diarrhoea. Accessed 16 October 2026.
- NCI — fatigue. Accessed 16 October 2026.
- BC Cancer — Fluorouracil Monograph. Accessed 16 October 2026. Independent cross-check: USES; SPECIAL PRECAUTIONS; SIDE EFFECTS; INTERACTIONS; DOSAGE GUIDELINES.
- ASCO hepatitis B screening and management opinion. Accessed 16 October 2026. Independent cross-check: Provisional Clinical Opinion: universal three-test screening before or at systemic anticancer therapy; chronic/past infection management.
- eviQ fluoropyrimidine overdose guidance. Accessed 16 October 2026. Independent cross-check: Background; assessment and management; uridine triacetate timing and asymptomatic overexposure.
Editorially approved by Dr (Prof) Vijay Agarwal
Professor and Lead Consultant – Medical Oncology
