Warfarin and Amitriptyline: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 2, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Amitriptyline
Warfarin
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.
You're taking amitriptyline (Elavil), an antidepressant, along with warfarin (Coumadin), a blood thinner. When these are used together, the amitriptyline can slow down how your body breaks down warfarin and let more of it get absorbed. That means warfarin can build up a bit, which can make your blood thinner than intended and raise your risk of bleeding.
Warfarin is a medicine where even small changes matter, so this is worth watching. The good news is your care team handles this all the time. They can check your INR (a blood test that shows how thin your blood is) and fine-tune your warfarin dose. Keep taking both as prescribed and tell your doctor about any unusual bruising or bleeding.
Effect: TCAs including amitriptyline may increase warfarin half-life and bioavailability, increasing anticoagulant effect and bleeding risk.
- Mechanism: decreased hepatic metabolism of warfarin plus increased GI absorption; net increase in warfarin exposure.
- Direction: increased warfarin effect (neither drug is a prodrug requiring activation here).
- Onset: delayed. Evidence: probable. Severity: moderate.
- Variability: considerable interindividual differences.
- Management: monitor PT ratio/INR closely for stability; anticipate individualized warfarin dose adjustment to maintain target anticoagulation. Warfarin is narrow therapeutic index, so small shifts are clinically meaningful.
What happens
An increased risk of bleeding
Interaction Deep Dive
Oral anticoagulants may show a prolonged half-life and greater bioavailability when combined with tricyclic antidepressants (TCAs)23. There may be substantial variability from one individual to another 1.
Why it happens (mechanism)
Decreased warfarin metabolism; increased warfarin absorption
How to manage this interaction
This interaction is very manageable with routine monitoring. Here's what typically happens and what you can do:
- Keep taking both exactly as prescribed unless your prescriber tells you otherwise.
- Your care team may check your INR more closely to watch your anticoagulant level, especially after starting, stopping, or changing either drug.
- Your warfarin dose may need to be adjusted and individualized by your team to keep you in your target range.
- Watch for and report signs of bleeding: unusual bruising, nosebleeds, pink or red urine, black stools, or bleeding gums.
Bring this up with your pharmacist or prescriber so they can plan your monitoring.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
3 reports — tap to read
a) In an investigation involving healthy volunteers, taking nortriptyline or amitriptyline at the same time led to a rise in dicumarol levels along with a lengthening of the plasma half-life of dicumarol, though this effect was not uniform across all participants 1. This effect was not seen when warfarin was used.
b) A single oral dose of bishydroxycoumarin given after eight days of nortriptyline produced a significantly prolonged half-life and a reduced volume of distribution of the coumarin anticoagulant in six healthy volunteers 2. The proposed mechanism was decreased metabolism of bishydroxycoumarin and/or a change in coumarin absorption.
c) Drug dosing was examined in 16 patients receiving long-term anticoagulant therapy who were concurrently taking TCAs 3. The TCAs influenced the stability of anticoagulant control, resulting in frequent adjustments to anticoagulant doses. The suggested mechanism was inhibition of coumarin metabolism.
Common questions
Can I take Warfarin and Amitriptyline together?
Amitriptyline can raise warfarin levels and increase bleeding risk, so your team should monitor your INR closely and adjust your warfarin dose as needed. Keep taking both as prescribed and report any unusual bleeding. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Warfarin and Amitriptyline 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 is the Warfarin and Amitriptyline interaction managed?
This interaction is very manageable with routine monitoring. Here's what typically happens and what you can do: Keep taking both exactly as prescribed unless your prescriber tells you otherwise. Your care team may check your INR more closely to watch your anticoagulant level, especially after starting, stopping, or changing either drug. Your warfarin dose may need to be adjusted and individualized… Management is individual — always follow your own care team's guidance.
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 Warfarin or Amitriptyline 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 (3)
- Pond SM, Graham GG, Birkett DJ, et al: Effects of tricyclic antidepressants on drug metabolism. Clin Pharmacol Ther 1975; 18:191-199. PubMed
- Vesell ES, Passananti GT, & Greene FE: Impairment of drug metabolism in man by allopurinol and nortriptyline. N Engl J Med 1970; 283:1484-1488. PubMed
- Williams JR, Griffin JP, & Parkins A: Effect of concomitantly administered drugs on the control of long term anticoagulant therapy. Q J Med 1976; 45:63-73. DOI
Keep reading about Amitriptyline
Keep reading about Warfarin
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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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