Cephalexin and Warfarin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Warfarin
Cephalexin
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
Increased INR and an increased risk of bleeding
Interaction Deep Dive
Although data are conflicting as to whether cephalexin is associated with an increased risk of bleeding for patients taking warfarin241, the concomitant use of cephalexin with warfarin may result in increased INR. The mechanism of the interaction appears to involve alterations in intestinal flora that synthesize vitamin K. In a nested case-control study, there was an association between cephalosporin use in patients on warfarin therapy and an increased risk of bleeding. If concomitant use is required, more frequent monitoring of INR is recommended 1 (eg, within 3 to 14 days of antibiotic initiation 2), and especially during initiation and discontinuation of the antibiotic 3.
Why it happens (mechanism)
Additive anticoagulant effects; alteration in intestinal flora that synthesize vitamin K
Literature reports
3 reports — tap to read
a) Initiation of antibiotics in patients on warfarin therapy resulted in a significantly increased risk of bleeding, according to a nested case-control study of United States Medicare part D beneficiaries 65 years or older (n=38,762). Patients on warfarin who received any antibiotic were approximately twice as likely to be hospitalized for bleeding within 15 days compared with a control group on warfarin who were not exposed to antibiotics (adjusted odds ratio [aOR], 2.01; 95% CI, 1.62 to 2.5). Specific antibiotics with the highest bleeding risk were azole antifungals (aOR, 4.57; 95% CI, 1.9 to 11.03), followed by cephalosporins (aOR, 2.45; 95% CI, 1.52 to 3.95), cotrimoxazole (aOR, 2.7; 95% CI, 1.46 to 5.05), penicillins (aOR, 1.92; 95% CI, 1.21 to 2.07), macrolides (aOR, 1.86; 95% CI, 1.08 to 3.21), and quinolones (aOR, 1.69; 95% CI, 1.09 to 2.62) 1.
b) Data from a retrospective cohort study over a 6-year period of veterans who were prescribed warfarin for 30 days without interruption through the US Department of Veterans Affairs show warfarin users who were prescribed high-risk antibiotics were at a higher risk for serious bleeding risks. Antibiotics considered to be high risk for interaction with warfarin included trimethoprim/sulfamethoxazole (TMP/SMX), ciprofloxacin, levofloxacin, metronidazole, fluconazole, azithromycin, and clarithromycin and low-risk antibiotics included clindamycin and cephalexin. A total of 22,272 patients were enrolled, including 14,078 and 8194 receiving high- and low-risk antibiotics, respectively. Bleeding events occurred in 93 patients in the high-risk antibiotic group and 36 patients in the low-risk group. Concomitant use of a high-risk antibiotic (hazard ratio [HR], 1.48; 95% CI, 1 to 2.19) was associated with increased risk of bleeding as a primary diagnosis, and TMP/SMX (HR, 2.09; 95% CI, 1.45 to 3.02), ciprofloxacin (HR, 1.87; 95% CI, 1.42 to 2.5), levofloxacin (HR, 1.77; 95% CI, 1.22 to 2.5), azithromycin (HR, 1.64; 95% CI, 1.16 to 2.33), and clarithromycin (HR 2.4; 95% CI, 1.16 to 4.94) were associated with serious bleeding as a primary or secondary diagnosis. Alterations in INR occurred frequently, including 9.7% of patients receiving fluconazole experiencing INR greater than 6. Decreased risk of serious bleeding occurred in patients with INR drawn within 3 to 14 days of coadministration (HR, 0.61; 95% CI, 0.42 to 0.88). Monitoring INR within 3 to 14 days of antibiotic prescription may decrease risk of hospitalization for serious bleeding. If possible, substitute an antibiotic with low potential to interact with warfarin 2.
c) Patients 65 years or older on stable warfarin therapy and with at least 1 prescription of an oral antibiotic of interest during an 8-year period were included in a retrospective cohort study involving a Veterans Affairs medical center outpatient anticoagulation clinic. The primary outcome was postantibiotic INR values. One of the secondary outcomes included overanticoagulation in which cephalexin was chosen as a comparator drug because it is mostly excreted unchanged by the kidneys and does not undergo metabolism via CYP2C9. A total of 205 patients (mean age, 75.7 years) had 364 prescriptions for warfarin and antibiotics concomitantly. The mean preantibiotic INR ranged from 2.3 to 2.5. On average, preantibiotic INR was obtained 9.8 days before the start of the antibiotic, and postantibiotic INR was obtained 10.2 days after the start of the antibiotic. Antibiotics associated with a significant increase in INR were amoxicillin, azithromycin, ciprofloxacin, levofloxacin, and moxifloxacin. Cephalexin use comprised approximately 13% of total prescriptions and did not result in significant INR changes. The percentage of patients who had a warfarin dose adjustment (either reduced or withheld) was highest for levofloxacin (25%), followed by moxifloxacin (24%), ciprofloxacin (17%), and azithromycin (12%). The percentage of patients with INR values above the therapeutic range was significantly higher for azithromycin (41%), levofloxacin (46%), and moxifloxacin (40%), compared with cephalexin (16%). INR should be closely monitored in older adults receiving warfarin and antibiotics, especially with fluoroquinolones, both during and after antibiotic therapy 4.
Common questions
Can I take Cephalexin and Warfarin together?
Increased INR and an increased risk of bleeding Always confirm with your pharmacist or prescriber before making any change.
How serious is the Cephalexin and Warfarin interaction?
It is rated moderate. Can be significant — usually manageable with 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.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Cephalexin or Warfarin 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 anything you'd monitor while I'm on both?
References (4)
- Baillargeon J, Holmes HM, Lin YL, et al: Concurrent use of warfarin and antibiotics and the risk of bleeding in older adults. Am J Med 2012; 125(2):183-189. PubMed
- Lane MA, Zeringue A, & McDonald JR: Serious bleeding events due to warfarin and antibiotic co-prescription in a cohort of veterans. Am J Med 2014; 127(7):657-663.e2. DOI
- Product Information: WARFARIN SODIUM oral tablets, warfarin sodium oral tablets. Teva Pharmaceuticals (per Dailymed), Parsippany, NJ, 2023. DailyMed
- Ghaswalla PK, Harpe SE, Tassone D, et al: Warfarin-antibiotic interactions in older adults of an outpatient anticoagulation clinic. Am J Geriatr Pharmacother 2012; 10(6):352-360. PubMed
Keep reading about Warfarin
Keep reading about Cephalexin
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.
Check another combination
Our instant two-drug interaction checker is almost here.
Still have questions about this combination?
Every question gets a real answer from a licensed pharmacist — free, and usually within a day.
Ask the pharmacist