Fluorouracil; Uses, Dosage, Side Effects, Drug Interactions

Last updated: February 7, 2026Reviewed date: February 7, 2026Reading time: 9 min read

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Fluorouracil is an antimetabolite fluoropyrimidine analog of the nucleoside pyrimidine with antineoplastic activity. Fluorouracil and its metabolites possess a number of different mechanisms of action. In vivo, fluoruracil is converted to the active metabolite 5-fluorodeoxyuridine monophosphate (F-UMP); replacing uracil, F-UMP incorporates into RNA and inhibits RNA processing, thereby inhibiting cell growth. Another active metabolite, 5-5-fluoro-2'-deoxyuridine-5'-O-monophosphate...

For severe symptoms, danger signs, pregnancy, child illness, or sudden worsening, seek urgent medical care.

বাংলা রোগী নোট এখনো যোগ করা হয়নি। পোস্ট এডিটরে “RX Bangla Patient Mode” বক্স থেকে সহজ বাংলা সারাংশ যোগ করুন।

এই তথ্য শিক্ষা ও সচেতনতার জন্য। এটি ডাক্তারি পরীক্ষা, রোগ নির্ণয় বা প্রেসক্রিপশনের বিকল্প নয়।

Article Summary

Key Takeaways

This article explains Mechanism of Action of Fluorouracil in simple medical language.
This article explains Indications of Fluorouracil in simple medical language.
This article explains Contraindications of Fluoruoracil in simple medical language.
This article explains Dosage of Fluorouracil in simple medical language.

Educational health guideWritten for patient understanding and clinical awareness.

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Emergency safety firstUrgent warning signs are highlighted below.

Seek urgent medical care if you notice

These warning signs are general safety guidance. Local emergency numbers and clinical judgment should always come first.

Severe symptoms, breathing difficulty, fainting, confusion, or rapidly worsening illness.
New weakness, severe pain, high fever, or symptoms after a serious injury.
Any symptom that feels urgent, unusual, or unsafe for the patient.

Emergency now

Use emergency care for severe, sudden, rapidly worsening, or life-threatening symptoms.

See a doctor

Book a professional medical evaluation if symptoms persist, worsen, recur often, affect daily activities, or occur in a high-risk patient.

Learn safely

Use this article to understand possible causes, tests, treatment options, prevention, and questions to ask your clinician.

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Definition

Fluorouracil is an antimetabolite fluoropyrimidine analog of the nucleoside pyrimidine with antineoplastic activity. Fluorouracil and its metabolites possess a number of different mechanisms of action. In vivo, fluoruracil is converted to the active metabolite 5-fluorodeoxyuridine monophosphate (F-UMP); replacing uracil, F-UMP incorporates into RNA and inhibits RNA processing, thereby inhibiting cell growth. Another active metabolite, 5-5-fluoro-2′-deoxyuridine-5′-O-monophosphate (F-dUMP), inhibits thymidylate synthase, resulting in the depletion of thymidine triphosphate(TTP), one of the four nucleotide triphosphates used in the in vivo synthesis of DNA. Other fluorouracil metabolites incorporate into both RNA and DNA; incorporation into RNA results in major effects on both RNA processing and functions.

Mechanism of Action of Fluorouracil

The precise mechanism of action has not been fully determined, but the main mechanism of fluorouracil is thought to be the binding of the deoxyribonucleotide of the drug (FdUMP) and the folate cofactor, N5–10-methylenetetrahydrofolate, to thymidylate synthase (TS) to form a covalently bound ternary complex. This results in the inhibition of the formation of thymidylate from uracil, which leads to the inhibition of DNA and RNA synthesis and cell death. Fluorouracil can also be incorporated into RNA in place of uridine triphosphate (UTP), producing a fraudulent RNA and interfering with RNA processing and protein synthesis.

5-FU requires enzymatic conversion to the nucleotide (ribosylation and phosphorylation) in order to exert its cytotoxic activity. Several routes are available for the formation of the 5′-monophosphate nucleotide (F-UMP) in animal cells. 5-FU may be converted to fluorouridine by uridine phosphorylase and then to F-UMP by uridine kinase, or it may react directly with 5-phosphoribosyl-1-pyrophosphate (PRPP), in a reaction catalyzed by orotate phosphoribosyltransferase, to form F-UMP. Many metabolic pathways are available to F-UMP, including incorporation in to RNA. A reaction sequence crucial for antineoplastic activity involves reduction of the diphosphate nucleotide by the enzyme ribonucleoside diphosphate reductase to the deoxynucleotide level and the eventual formation of 5-fluoro-2′-deoxyuridine-5′-phosphate (F-dUMP). 5-FU also may be converted directly to the deoxyriboside 5-FUdR by the enzyme thymidine phosphorylase and further to F-dUMP, a potent inhibitor of thymidylate synthesis, by thymidine kinase. The interaction between F-dUMP and the enzyme thymidylate synthase leads to depletion of TTP, a necessary constituent of DNA … The folate cofactor, 5,10-methylenetetrahydrofolate, and F-dUMP form a covalently bound ternary complex with the enzyme. The inhibitory complex resembles the transition state formed during the normal enzymatic reaction when dUMP is converted to thymidylate. Although the physiological complex progresses to the synthesis of thymidylate by transfer of the methylene group and 2 hydrogen atoms from folate to dUMP, this reaction is blocked in the inhibitory complex by the stability of the fluorine-carbon bond on F-dUMP; sustained inhibition of the enzyme results

Indications of Fluorouracil

For the topical treatment of multiple actinic or solar keratoses. In the 5% strength, it is also useful in the treatment of superficial basal cell carcinomas when conventional methods are impractical, such as with multiple lesions or difficult treatment sites. Fluorouracil injection is indicated in the palliative management of some types of cancer, including colon, esophageal, gastric, rectum, breast, biliary tract, stomach, head and neck, cervical, pancreas, renal cell, and carcinoid.
Colorectal Cancer
Breast Cancer
Stomach Cancer
Pancreatic Cancer
Malignant Neoplasm of Colon
Malignant Neoplasm of Pancreas
Malignant Neoplasm of Stomach
Rectal Carcinoma
Superficial Basal Cell Carcinoma
Hyperkeratotic actinic keratosis

Therapeutic Uses

Antimetabolites; Antimetabolites, Antineoplastic; Immunosuppressive Agents
Fluorouracil is indicated for palliative treatment of carcinoma of the colon, rectum, breast, stomach, and pancreas in patients considered to be incurable by surgery or other means.
Fluorouracil is also indicated for the treatment of bladder carcinoma, prostatic carcinoma, epithelial ovarian carcinoma, cervical carcinoma, endometrial carcinoma, anal carcinoma, esophageal carcinoma, metastatic tumors of skin carcinoma, and hepatoblastoma, and is used by intra-arterial injection for treatment of hepatic tumors and head and neck tumors.
Fluorouracil, in combination therapy, is reasonable medical therapy at some point in the management of adrenocortical carcinoma, vulvar carcinoma, penile carcinoma and carcinoid tumors (gastrointestinal and neuroendocrine tumors).
Although fluorouracil has been used for the treatment of malignant pleural effusions, the USP Division of Information Development Hematology-Oncology Advisory Panel believes there is insufficient evidence to support the effectiveness of fluorouracil in the treatment of malignant pleural effusions.
Fluorouracil is used for the treatment of glaucoma during or following trabeculectomy surgery. Recommended for topical treatment of multiple actinic or solar keratoses.

Contraindications of Fluoruoracil

Pregnancy
A mother who is producing milk and breastfeeding
Dihydropyrimidine dehydrogenase deficiency
Allergies to Pyrimidine Analogues & Adhesive

Dosage of Fluorouracil

Strengths: 50 mg/mL

Colorectal Cancer

In combination with leucovorin, or in combination with leucovorin and oxaliplatin or irinotecan: 400 mg/m2 by IV bolus on Day 1, followed by 2400 to 3000 mg/m2 as a continuous IV infusion over 46 hours every 2 weeks.
When administered in a bolus dosing regimen in combination with leucovorin: 500 mg/m2 by IV bolus on Days 1, 8, 15, 22, 29, and 36 in 8-week cycles; if no toxicity is observed, 3 mg/kg may be administered on days 5, 7, and 9; no therapy is to be administered on days 4, 6, or 8; discontinue at the end of day 9, even with no apparent toxicity

Breast Cancer

Administered as a component of a cyclophosphamide-based multidrug regimen: 500 or 600 mg/m2 IV on Days 1 and 8 every 28 days for 6 cycles

Stomach Cancer

Administered as a component of a platinum-containing multidrug chemotherapy regimen: 200 to 1000 mg/m2 IV as a continuous infusion over 24 hours; the frequency of dosing in each cycle and the length of each cycle will depend on the dose of fluorouracil and the specific regimen administered

Pancreatic Cancer

Administered in combination with leucovorin or as a component of a multidrug chemotherapy regimen that includes leucovorin: 400 mg/m2 by IV bolus on Day 1, followed by 2400 mg/m2 IV as a continuous infusion over 46 hours every 2 weeks

Side Effects of Fluorouracil

The Most Common

confusion or disorientation
diarrhea
heartburn
severe nausea or vomiting
signs of anemia (low red blood cells; e.g., pale skin, unusual tiredness or weakness)
signs of bleeding (e.g., unusual bruising or bleeding, pinpoint red spots on skin, black tarry stools, bloody nose, blood in urine, coughing blood, cuts that don’t stop bleeding)
signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)

More Common

Diarrhea
heartburn
sores in mouth and on lips
Black, tarry stools
cough or hoarseness, accompanied by fever or chills
fever or chills
lower back or side pain, accompanied by fever or chills
nausea and vomiting (severe)
painful or difficult urination, accompanied by fever or chills
stomach cramps

Rare

Blood in urine or stools
pinpoint red spots on skin
unusual bleeding or bruising
Chest pain
cough
shortness of breath
tingling of hands and feet, followed by pain, redness, and swelling
trouble with balance

Drug Interactions of Fluorouracil

Fluorouracil may interact with following drugs, supplements & may change the efficacy of the drug

celecoxib
dapsone
echinacea
fluoxetine
ganciclovir
glipizide
glyburide
interferon
ketamine
leflunomide
leucovorin
levamisole
live vaccines (e.g., BCG, yellow fever)
losartan
montelukast
nateglinide
other cancer medications (including but not limited to cytarabine, flucytosine, methotrexate)
pimecrolimus
phenytoin
sulfamethoxazole
tacrolimus
tamoxifen
tolbutamide
trimethoprim
voriconazole
warfarin
zafirlukast

Pregnancy Category of Fluorouracil

AU TGA Pregnancy Category: D
US FDA Pregnancy Category: D

Pregnancy:

There is a possibility of birth defects if fluorouracil is being used at the time of conception, or if it is taken during pregnancy. Use effective birth control while you are being treated with this medication. If you become pregnant while taking this medication, contact your doctor immediately.

Lactation

It is not known if fluorouracil passes into breast milk. Due to the potential for serious harm to the baby, women receiving fluorouracil should not breast-feed.

References

Fluorouracil; Uses, Dosage, Side Effects, Drug Interactions

Safety note: This is not a prescription or diagnosis. For severe symptoms, pregnancy danger signs, children with serious illness, chest pain, breathing difficulty, stroke-like weakness, or major injury, seek urgent care.

Which doctor may help?

Start with a registered doctor or the nearest qualified health center.

What to tell the doctor

Write when the problem started and how it changed.
Bring old prescriptions, investigation reports, and current medicines.
Write allergies, pregnancy status, diabetes, kidney/liver disease, and major past illnesses.
Bring one family member if the patient is weak, elderly, confused, or a child.

Questions to ask

What is the most likely cause of my symptoms?
Which danger signs mean I should go to hospital quickly?
Which tests are necessary now, and which can wait?
How should I take medicines safely and what side effects should I watch for?
When should I come for follow-up?

Tests to discuss

Vital signs: temperature, pulse, blood pressure, oxygen saturation
Basic physical examination by a clinician
CBC, urine test, blood sugar, or imaging only when clinically needed

Avoid these mistakes

Do not use antibiotics, steroid tablets/injections, or strong painkillers without proper medical advice.
Do not hide pregnancy, kidney disease, ulcer, allergy, or blood thinner use.
Do not delay emergency care when danger signs are present.

Medicine safety and first-aid guide

This section is for patient education only. It does not replace a doctor, pharmacist, or emergency care.

Safe first steps

Avoid heavy lifting, sudden bending, and prolonged bed rest.
Use comfortable posture and gentle movement as tolerated.
Discuss physiotherapy, X-ray, or MRI only when clinically needed.

OTC medicine safety

For mild back pain, pain-relief medicine may be discussed with a doctor or pharmacist.
Avoid repeated painkiller use if you have kidney disease, stomach ulcer, uncontrolled blood pressure, or are taking blood thinners.

Avoid these mistakes

Do not start antibiotics without a proper medical decision.
Do not use steroid tablets or injections casually for quick relief.
Do not delay emergency care because of home remedies.

Get urgent help if

Back pain with leg weakness, numbness around private area, loss of urine/stool control, fever, cancer history, or major injury needs urgent care.

Medicine names, dose, and timing must be decided by a qualified clinician or pharmacist after checking age, pregnancy, allergy, other diseases, and current medicines.

For rural patients and family caregivers

Patient health record and symptom diary

Write your symptoms, medicines already taken, test results, and questions before visiting a doctor. This note stays on your device unless you print or copy it.

Doctor to discuss: Medicine doctor / pediatrician for children / qualified clinician

Tests to discuss with doctor

Temperature chart and hydration assessment
CBC with platelet count if fever persists or dengue/other infection is possible
Urine test, malaria/dengue tests, chest evaluation, or blood culture only when clinically indicated

Questions to ask

What is the most likely cause of my symptoms?
Which warning signs mean I should go to emergency care?
Which tests are really needed now?
Which medicines are safe for my age, pregnancy status, allergy, kidney/liver/stomach condition, and current medicines?
Do I need antibiotics, or is this more likely viral?

Emergency warning signs such as chest pain, severe breathing difficulty, sudden weakness, confusion, severe dehydration, major injury, or loss of bladder/bowel control need urgent medical care. Do not wait for online information.

Go to emergency care if you notice:

Severe or rapidly worsening symptoms
Breathing difficulty, chest pain, fainting, confusion, severe weakness, major injury, or severe dehydration

Doctor / service to discuss: Qualified healthcare provider; specialist depends on symptoms and examination.

Step 1
Check danger signs first

If danger signs are present, seek emergency care and do not wait for online information.
Step 2
Record the symptom story

Write when symptoms started, severity, medicines already taken, allergies, pregnancy status, and test results.
Step 3
Visit a qualified clinician

A doctor, nurse, or qualified healthcare provider can examine you and decide which tests or treatment are needed.
Step 4
Do only useful tests

Do tests after clinical assessment. Avoid unnecessary tests, random antibiotics, or repeated medicines without diagnosis.
Step 5
Follow up and return early if worse

If symptoms worsen, new warning signs appear, or treatment is not helping, return for review quickly.

Rural patient practical tips

Take a written symptom diary and all previous prescriptions/test reports.
Do not hide medicines already taken, even herbal or over-the-counter medicines.
Ask which warning signs mean urgent referral to hospital.

This roadmap is for education. A real diagnosis and treatment plan requires history, examination, and clinical judgment.

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Frequently Asked Questions

Is this article a replacement for a doctor?

No. It is educational content only. Patients should consult a qualified clinician for diagnosis and treatment.

When should I seek urgent care?

Seek urgent care for severe symptoms, rapidly worsening condition, breathing difficulty, severe pain, neurological changes, or any emergency warning sign.

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