Fluorouracil & Nutrient Depletion
Also searched as Efudex, Tolak. What the clinical evidence reports, what it may mean, and what to discuss with your pharmacist.
Fluorouracil has been associated with depletion of NIACIN and THIAMINE. This does not mean a deficiency will occur — it means these are worth knowing about and, if you take Fluorouracil long-term, worth raising with your pharmacist or healthcare provider.
Take more than one medication? Open the checker with Fluorouracil pre-loaded and add the rest to see overlapping nutrient signals.
Reported nutrient depletions with Fluorouracil
Each record below is shown as published in our licensed clinical database, with its evidence rating and citations. Expand a section for food sources and talking points.
NIACIN
Moderate Depletion5-FLUOROURACIL
Fluorouracil inhibits conversion of tryptophan to niacin and might cause pellagra, especially in people with poor nutritional intake or malabsorption. Niacin supplements can rapidly reverse early symptoms of deficiency in people taking fluorouracil.
Niacin is a B vitamin your body uses for energy production, healthy skin, and nerve function, and fluorouracil can interfere with it in an indirect way: your body normally makes some of its own niacin by converting the amino acid tryptophan, and fluorouracil appears to block that conversion. This is rated a moderate concern, meaning it does not affect every patient but is worth watching, particularly for people who already have poor nutritional intake or absorption issues. If you are on long-term fluorouracil therapy, it is worth asking your oncologist or pharmacist whether your niacin levels should be monitored.
What Vitamin B3 (niacin) does & food sources
Niacin supports energy metabolism, skin health, and nervous system function.
Common food sources:
- Poultry, beef, and fish
- Peanuts and legumes
- Fortified grains
HelloPharmacist pharmacist-reviewed education — separate from the licensed record above.
What to discuss with your clinician
Deficiency is rare in a typical diet. Discuss with your provider before using niacin as a supplement — at pharmacologic doses it behaves like a medication.
HelloPharmacist pharmacist-reviewed education — separate from the licensed record above.
Supplement considerations
High-dose niacin causes flushing and can affect the liver, blood sugar, and uric acid — it is not a casual supplement.
Do not start or stop medications or supplements without professional guidance — supplements can interact with prescription medications.
HelloPharmacist pharmacist-reviewed education — separate from the licensed record above.
Caution: Treat high-dose niacin as a medication decision, not a nutrition one.
Learn more about this nutrient: Niacin — uses, safety & interactions
THIAMINE
Insufficient Evidence5-FLUOROURACIL
People receiving 5-fluorouracil-containing chemotherapy regimens may be at risk for thiamine deficiency. 5-fluorouracil might interfere with activation of thiamine or increase its breakdown. However, there isn't enough research to recommend routine thiamine supplementation. Thiamine deficiency has been reported with other chemotherapy regimens, but this is likely due to reduced dietary intake associated with cancer and its treatment.
Thiamine is a B vitamin your body uses to convert food into energy and keep your nerves working properly. There is some early, theoretical concern that fluorouracil may interfere with how the body activates thiamine or may speed up its breakdown, but this has not been well studied and the clinical significance is simply not known yet. Keep in mind that people going through cancer treatment often eat less, which may play a bigger role in any thiamine changes than the drug itself. If you are concerned, bring it up with your oncologist or pharmacist before considering any supplement.
What Vitamin B1 (thiamine) does & food sources
Thiamine helps convert food into energy and is essential for nerve and heart function.
Common food sources:
- Whole grains and fortified cereals
- Pork
- Beans and lentils
- Nuts and seeds
HelloPharmacist pharmacist-reviewed education — separate from the licensed record above.
What to discuss with your clinician
Ask about thiamine if you take an associated medication (for example, a long-term loop diuretic in heart failure) and have symptoms like fatigue or worsening heart failure control — this is an area your care team can weigh.
HelloPharmacist pharmacist-reviewed education — separate from the licensed record above.
Supplement considerations
Thiamine is water-soluble and generally low-risk, but supplementing to treat a suspected problem should still be clinician-guided so the real cause is not missed.
Do not start or stop medications or supplements without professional guidance — supplements can interact with prescription medications.
HelloPharmacist pharmacist-reviewed education — separate from the licensed record above.
Learn more about this nutrient: Thiamine — uses, safety & interactions
References (3)
- Aksoy M, Basu TK, Brient J, Dickerson JW. Thiamin status of patients treated with drug combinations containing 5-fluorouracil. Eur J Cancer 1980;16:1041-5. PubMed
- Ulusakarya A, Vantelon JM, Munck JN, et al. Thiamine deficiency in a patient receiving chemotherapy for acute myeloblastic leukemia (letter). Am J Hematol 1999;61:155-6. DOI
- De Reuck JL, Sieben GJ, Sieben-Praet MR, et al. Wernicke's encephalopathy in patients with tumors of the lymphoid-hemopoietic systems. Arch Neurol 1980;37:338-41.. PubMed
This tool is educational and is not a substitute for professional medical advice. An association is not a diagnosis and does not mean a deficiency will occur. Always talk to your pharmacist or healthcare provider before starting, stopping, or changing any medication or supplement.
Disclaimer: This checker finds which nutrients might be depleted by prescription or over-the-counter medications. Each nutrient depletion issue is rated based on clinical significance. Use your own professional judgment prior to making clinical decisions.© 2026 Therapeutic Research Center Licensed records refresh weekly · last updated July 3, 2026.
More about Fluorouracil
What drug-induced nutrient depletion means
Some medications can, over time, lower the levels of certain vitamins, minerals, or other nutrients — through reduced absorption, increased loss, or changes in how the body uses them.
Association is not deficiency
An entry here means a depletion has been reported or studied — not that it happens to everyone, or that you are deficient. Dose, duration, diet, and your own health all matter.
Evidence varies by pair
Some drug–nutrient links (like metformin and vitamin B12) are well studied; others rest on limited or preliminary research. Each entry shows the evidence rating from our licensed clinical database.
When to ask about labs
If you take an associated medication long-term, ask your healthcare provider whether monitoring is appropriate for you. Not everyone needs testing — your provider can weigh your dose, duration, and symptoms.
Don't self-supplement blindly
Supplements are not risk-free: some can interact with medications, and some (like potassium or magnesium) can be risky with kidney disease. Talk to your pharmacist before adding anything.
Never stop a medication over this
These medications are prescribed for good reasons, and the benefit usually far outweighs a manageable nutrient consideration. Any change belongs in a conversation with your prescriber.
Food first, usually
For many nutrients, a varied diet covers the gap. Each entry lists common food sources; a supplement is a decision to make with your pharmacist or provider, not a default.
Fluorouracil & nutrients: FAQs
Does Fluorouracil deplete NIACIN?
Should I take a supplement because I take Fluorouracil?
Should I stop taking Fluorouracil?
Where does this information come from?
Questions about Fluorouracil and your nutrition?
Ask a licensed pharmacist — free, no appointment needed.
Sources & How We Checked
Nutrient depletion records for Fluorouracil are evidence-graded and sourced from the Natural Medicines database, and are displayed as published with their citations.
- Natural Medicines (Therapeutic Research Center) — Evidence-graded clinical reference for drug-induced nutrient depletions, supplements, and interactions — the basis for this page.
- MedlinePlus, U.S. National Library of Medicine — NIH consumer drug information, used for medication overviews across the site.
Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.