Drug Interaction Report

Gefitinib and Warfarin: Interaction Details

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

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Warfarin

Coumadin Jantoven
+

Gefitinib

Iressa Iressa®
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Moderate
Can be significant — usually manageable with 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 60 documented Gefitinib interactions, 2 are rated moderate — including this one.
Onset
unspecified
Evidence
established
Severity
Moderate

What happens

Increased prothrombin time and INR values and an increased risk of bleeding

Interaction Deep Dive

Concomitant use of gefitinib and warfarin has resulted in bleeding events and INR elevations in some patients1. In a retrospective study, 6 of 12 patients experienced prothrombin time (PT) and INR elevation following concomitant gefitinib and warfarin use 2. Therefore, If gefitinib and warfarin are coadministered, the patient should be monitored regularly for changes in PT or INR 1, especially during the first 2 weeks following warfarin initiation. If PT or INR values become elevated, a warfarin dose adjustment may be needed 2.

Why it happens (mechanism)

Unknown

Literature reports

1 report — tap to read

a) Prothrombin time (PT) and INR elevations were reported in 6 of 12 patients within 2 weeks of receiving gefitinib and warfarin concomitantly in a retrospective study of patients with non-small cell lung cancer in a hospital in Japan. Of the 12 patients, 2 (16.7%) had liver metastases. According to inpatient and outpatient database records, 296 patients received at least one gefitinib prescription between September 1, 2002 and January 31, 2007. Among the 296 patients, 12 (4.1%) had also received a warfarin prescription at the same time and were included in the study. None of the 6 patients with elevated PT and INR experienced a bleeding event. A warfarin dose adjustment was required in all 6 patients. Of the 7 factors evaluated for a possible association with the elevated PT and INR values (age, gender, smoking status, liver metastasis, performance status, liver dysfunction, and response to treatment), liver dysfunction was the only factor associated with increased PT and INR values (p=0.01). Authors postulate that competition between the R(+) enantiomer of warfarin and gefitinib for CYP3A4 inhibition or the inhibition of warfarin metabolism, also a CYP3A4 substrate, by gefitinib may be responsible for this interaction. Gefitinib also extensively binds (approximately 97%) to human plasma protein and warfarin is also highly (97% to 99%) but reversibly bound to albumin binding site I. As a result, the coadministration of gefitinib may cause displacement of the binding site, thereby increasing the unbound fraction of warfarin. The increased concentration of unbound warfarin may cause hypoprothrombinemia, which could result in a PT and INR elevation 2.

Common questions

Can I take Gefitinib and Warfarin together?

Increased prothrombin time and INR values and an increased risk of bleeding Always confirm with your pharmacist or prescriber before making any change.

How serious is the Gefitinib 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 "established". Well documented — supported by controlled studies or strong clinical data.

From our Q&A

Real reader questions about these medications, each personally answered by our pharmacist:

Questions for your pharmacist

  • Does my dose of Gefitinib 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 (2)

  1. Product Information: IRESSA(R) oral tablets, gefitinib oral tablets. AstraZeneca Pharmaceuticals, Wilmington, DE, 2005. DailyMed
  2. Arai S, Mitsufuji H, Nishii Y, et al: Effect of gefitinib on warfarin antithrombotic activity. Int J Clin Oncol 2009; 14(4):332-336. PubMed
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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:

Warfarin + Activated Charcoal Alfalfa American Ginseng Beer Cannabis Danshen Dong Quai Fasting + 353 more
Gefitinib + Activated Charcoal Beer Grapefruit Siberian Cocklebur St. John's Wort Wine Acacia Rigidula Adrafinil + 160 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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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.