Drug Interaction Report

Allopurinol and Acenocoumarol: Interaction Details

A pharmacist-reviewed analysis of the Allopurinol and Acenocoumarol interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Jul 11, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Acenocoumarol

No brand names on record
+

Allopurinol

Aloprim Lopurin® Zyloprim Zyloprim®
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Jul 11, 2026
Editorial policy · Data sources
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Interaction severity
Major
Potentially serious — often needs a change or close monitoring.
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.

Of 32 documented Allopurinol interactions, 22 are rated major — including this one.
Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.
Onset
delayed
Evidence
probable
Severity
Major

What happens

Increased INR

Interaction Deep Dive

Exposure to warfarin (or another coumarin-derivative anticoagulant) might be increased, resulting in an increased INR, when coadministered with allopurinol, a CYP1A2 inhibitor. Consider the inhibition potential when initiating, adjusting, or discontinuing concomitant use and monitor INR closely4.

Why it happens (mechanism)

Inhibition of CYP1A2-mediated anticoagulant metabolism by allopurinol

Literature reports

3 reports — tap to read

a) Concomitant allopurinol and warfarin therapy has been reported to result in enhancement of warfarin activity as measured by prolongation of prothrombin time 567.

b) Allopurinol may increase the hypoprothrombinemic effect of oral anticoagulants in some patients 123.

c) An 83-year-old female received 300 mg daily of allopurinol for 12 days and developed a prolonged prothrombin time to a maximum value of 48.8 seconds three days after the patient had discontinued indomethacin but while she continued to receive warfarin and allopurinol 8.

Common questions

Can I take Allopurinol and Acenocoumarol together?

Increased INR Always confirm with your pharmacist or prescriber before making any change.

How serious is the Allopurinol and Acenocoumarol 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 Allopurinol or Acenocoumarol 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 (8)

  1. 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
  2. Pond SM, Graham GG, Wade DN, et al: The effects of allopurinol and clofibrate on the elimination of coumarin anticoagulants in man. Aust N Z J Med 1975; 5:324-328. DOI
  3. Jahnchen E, Meinertz I, & Gilfrich HJ: Interaction of allopurinol with phenprocoumon in man. Klin Wochensch 1977; 55:759-761. DOI
  4. Product Information: COUMADIN(R) oral tablets, warfarin sodium oral tablets. Bristol-Myers Squibb Company (per FDA), Princeton, NJ, 2017. DailyMed
  5. Avery GS: Check-list of potential clinically important interactions. Drugs 1973; 5:187-211.
  6. Koch-Weser J & Koch-Weser J: Drug interactions in cardiovascular therapy. Am Heart J 1975; 90:93-116. PubMed
  7. Koch-Weser J & Sellers EM: Drug interactions with coumarin anticoagulants (part 1). N Engl J Med 1971a; 285:487-498. DOI
  8. Self TH, Evans WE, & Ferguson T: Drug enhancement of warfarin activity (letter). Lancet 1975; 2:557-558. DOI
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Acenocoumarol

Keep reading about Acenocoumarol

All Acenocoumarol interactions (95)
Also serious with: Defibrotide Injection Bicalutamide Orlistat Fluorouracil Injection
Allopurinol

Keep reading about Allopurinol

All Allopurinol interactions (32) Allopurinol drug information
Also serious with: Fezolinetant Didanosine Azathioprine Pegloticase Injection
Beyond drug–drug

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:

Acenocoumarol + Activated Charcoal Danshen Piracetam Acetyl-l-carnitine Aconite Allspice Aloe Alpha-lipoic Acid + 213 more
Allopurinol + Activated Charcoal Asarabacca Beer Siberian Cocklebur Wine Aristolochia Ashwagandha Black Cohosh + 52 more

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This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.