Cimetidine and Fluorouracil Injection: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Cimetidine
Fluorouracil Injection
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
An increased risk of fluorouracil toxicity
Interaction Deep Dive
Concomitant fluorouracil and cimetidine therapy has resulted in increased peak plasma levels and area under the plasma concentration-time curve (AUC) for fluorouracil2, and coadministration of cimetidine with fluorouracil or one of its prodrugs should be approached with caution 1.
Why it happens (mechanism)
Inhibition of fluorouracil metabolism
Literature reports
1 report — tap to read
a) Concomitant oral 5-fluorouracil and cimetidine therapy resulted in increased peak plasma levels and area under the plasma concentration-time curve (AUC) for 5-fluorouracil in 15 ambulant patients with carcinoma. These effects were observed after doses of cimetidine 1 gram daily for 4 weeks, but a single pretreatment dose of cimetidine 400 mg or daily doses of 1 g for 1 week did not alter the pharmacokinetics of fluorouracil. Similarly, the AUC for intravenous 5-fluorouracil was significantly increased following 4 weeks of cimetidine pretreatment 2.
Common questions
Can I take Cimetidine and Fluorouracil Injection together?
An increased risk of fluorouracil toxicity Always confirm with your pharmacist or prescriber before making any change.
How serious is the Cimetidine and Fluorouracil Injection interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
Questions for your pharmacist
- Does my dose of Cimetidine or Fluorouracil Injection need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (2)
- Product Information: Teysuno oral capsules, tegafur gimeracil oteracil oral capsules. Nordic Group BV (per EMA), Hoofddorp, The Netherlands, 2012.
- Harvey VJ, Slevin ML, Dilloway MR, et al: The influence of cimetidine on the pharmacokinetics of 5-fluorouracil. Br J Clin Pharmacol 1984; 18:421-430. DOI
Keep reading about Cimetidine
Keep reading about Fluorouracil Injection
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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