Niacin and Warfarin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Warfarin
Niacin
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 risk of bleeding
Interaction Deep Dive
Concomitant use of niacin and warfarin may increase the risk of bleeding. A 69-year-old woman treated with warfarin 2.5 mg/day experienced a critically elevated INR level of 8 after her niacin ER dose was increased to 1000 mg/day. After warfarin holiday, discontinuation of niacin ER, and vitamin K, her INR re-stabilized with warfarin 2.5 mg/day. The mechanism of interaction maybe multifactorial, including niacin-mediated reduction in coagulation factors, decreased CYP450-mediated metabolism of warfarin by niacin metabolites, and saturable first-pass metabolism of niacin. If concomitant therapy is required, more frequent monitoring of INR and/or prothrombin time is recommended, particularly when niacin is initiated or with dosage increase1.
Why it happens (mechanism)
Additive anticoagulation effect and decreased CYP450-mediated metabolism of warfarin by niacin metabolites
Literature reports
1 report — tap to read
a) A 69-year-old woman treated with warfarin experienced a critically elevated INR after her niacin extended-release (ER) dose was increased. She had a stable INR on warfarin 2.5 mg/day for mechanical aortic valve replacement for over a year. She started niacin ER 500 mg/day about 17 months after warfarin was initiated without changes in INR over 3 months. The dose of niacin ER was increased to 1000 mg/day. One week after niacin ER was increased, her INR increased from 2.4 to 8 (venous INR greater than 12.3). However, she was asymptomatic, she reported no changes in diet or regular medications, and she denied alcohol use, except red wine in the evening. No other abnormal laboratory values were observed. Subsequently, warfarin was held, she received a dose of vitamin K 2.5 mg orally, and she discontinued niacin ER without physician consent. The next day, her INR remained elevated at 10.43 (venous), the warfarin hold was continued, and she received another dose of vitamin K. The following day, her INR was 4.8 and warfarin was resumed at 1 mg alternating with 2 mg for 3 days, with an INR recheck on day 4. Her INR remained in therapeutic range with warfarin 2.5 mg. Causation assessment using the Horn Drug Interaction Probability Scale suggested the interaction was probable 1.
Common questions
Can I take Niacin and Warfarin together?
Increased risk of bleeding Always confirm with your pharmacist or prescriber before making any change.
How serious is the Niacin and Warfarin interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
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 Niacin 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 (1)
- Christopher A: Critically elevated INR in a patient on warfarin after increase in extended-release niacin dose. Ann Pharmacother 2011; 45(11):e58. PubMed
Keep reading about Warfarin
Keep reading about Niacin
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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