Oseltamivir with Warfarin Interaction Details

Brand Names Associated with Oseltamivir

  • Oseltamivir
  • Tamiflu®

Brand Names Associated with Warfarin

  • Coumadin®
  • Jantoven®
  • Warfarin

Medical Content Editor
Last updated Nov 25, 2023

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Interaction Effect

Increased risk of bleeding

Interaction Summary

Concomitant administration of oseltamivir and warfarin resulted in elevated international normalized ratio (INR) levels, and hospitalization of 3 patients for bleeding events, after initiation of oseltamivir therapy in 7 of 15 patients who were previously stable on warfarin therapy. Although no probable mechanism was identified, a temporal association was established.. Caution is advised if these agents are coadministered. Consider monitoring INR more frequently and adjust the warfarin dose accordingly to achieve desired level of anticoagulation.







How To Manage Interaction

Concomitant administration of oseltamivir and warfarin has resulted in elevated international normalized ratio (INR) levels. Monitor patient for signs and symptoms of increased bleeding. Consider monitoring INR more frequently and adjust the warfarin dose as necessary to maintain the desired level of anticoagulation.

Mechanism Of Interaction


Literature Reports

A) Addition of oseltamivir therapy resulted in a 56% to 289% elevation of international normalized ratio (INR) levels in patients who had previously stable INR levels on warfarin therapy, according to a retrospective case series (n=15). Patients included in the study had INR test results available within 10 days of initiating oseltamivir, had been taking warfarin for at least 8 months, and had no dose changes or INR level more than 1 standard deviation outside of their 1-year mean level for at least 3 months prior to initiating oseltamivir therapy. All patients received oseltamivir 150 mg orally daily for 5 days. Increased INR levels were observed between 2 and 9 days after initiation of therapy. At a mean 5.7 +/- 2.5 days after the first dose of oseltamivir, the INR level was increased in 46.7% of patients (7 of 15) from a 1-year mean of 2.08 +/- 0.46 to a mean of 5.15 +/- 2.00. Hospitalization was required in 3 patients due to bleeding events (blood-tinged sputum, in 2 patients, INR 5.28 and 5.64; underlying hepatoma with bloody ascites in 1 patient, INR 5.22). Warfarin was discontinued in all 3 patients, and the INR level recovered within 2 to 5 days. After discontinuation of oseltamivir, the INR level was maintained at the previous stable value in all 7 patients who experienced elevations. Positivity for influenza A H1N1 was similar among both groups; however, in the elevated-INR group, concomitant medications may have been contributory in 3 patients (azithromycin and levofloxacin, 1 patient each; prednisolone and paracetamol, 1 patient). This study demonstrated a temporal association between initiation of oseltamivir therapy and increased INR level in patients receiving warfarin; however, no probable mechanism was elucidated .

Oseltamivir Overview

  • Oseltamivir is used to treat some types of influenza infection ('flu') in adults, children, and infants (older than 2 weeks of age) who have had symptoms of the flu for no longer than 2 days. This medication is also used to prevent some types of flu in adults and children (older than 1 year of age) when they have spent time with someone who has the flu or when there is a flu outbreak. Oseltamivir is in a class of medications called neuraminidase inhibitors. It works by stopping the spread of the flu virus in the body. Oseltamivir helps shorten the time that flu symptoms such as a stuffy or runny nose, sore throat, cough, muscle or joint aches, tiredness, headache, fever, and chills last. Oseltamivir will not prevent bacterial infections, which may occur as a complication of the flu.

See More information Regarding Oseltamivir

Warfarin Overview

  • Warfarin is used to prevent blood clots from forming or growing larger in your blood and blood vessels. It is prescribed for people with certain types of irregular heartbeat, people with prosthetic (replacement or mechanical) heart valves, and people who have suffered a heart attack. Warfarin is also used to treat or prevent venous thrombosis (swelling and blood clot in a vein) and pulmonary embolism (a blood clot in the lung). Warfarin is in a class of medications called anticoagulants ('blood thinners'). It works by decreasing the clotting ability of the blood.

See More information Regarding Warfarin

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Severity Categories


These drugs, generally, should not be used together simultaneously due to the high risk of severe adverse effects. Combining these medications can lead to dangerous health outcomes and should be strictly avoided unless otherwise instructed by your provider.


This interaction could result in very serious and potentially life-threatening consequences. If you are taking this drug combination, it is very important to be under close medical supervision to minimize severe side effects and ensure your safety. It may be necessary to change a medication or dosage to prevent harm.


This interaction has the potential to worsen your medical condition or alter the effectiveness of your treatment. It's important that you are monitored closely and you potentially may need to make adjustments in your treatment plan or drug dosage to maintain optimal health.


While this interaction is unlikely to cause significant problems, it could intensify side effects or reduce the effectiveness of one or both medications. Monitoring for changes in symptoms and your condition is recommended, and adjustments may be made if needed to manage any increased or more pronounced side effects.


Rapid: Onset of drug interaction typically occurs within 24 hours of co-administration.

Delayed: Onset of drug interaction typically occurs more than 24 hours after co-administration.


Level of documentation of the interaction.

Established: The interaction is documented and substantiated in peer-reviewed medical literature.

Theoretical: This interaction is not fully supported by current medical evidence or well-documented sources, but it is based on known drug mechanisms, drug effects, and other relevant information.

How To Manage The Interaction

Provides a detailed discussion on how patients and clinicians can approach the identified drug interaction as well as offers guidance on what to expect and strategies to potentially mitigate the effects of the interaction. This may include recommendations on adjusting medication dosages, altering the timing of drug administration, or closely monitoring for specific symptoms.

It's important to note that all medical situations are unique, and management approaches should be tailored to individual circumstances. Patients should always consult their healthcare provider for personalized advice and guidance on managing drug interactions effectively.

Mechanism Of Interaction

The theorized or clinically determined reason (i.e., mechanism) why the drug-drug interaction occurs.

Disclaimer: The information provided on this page is for informational purposes only and should not be considered medical advice. Always consult with a qualified healthcare professional regarding your specific circumstances and medical conditions.

Where Does Our Information Come From?

Information for our drug interactions is compiled from several drug compendia, including:

The prescribing information for each drug, as published on DailyMED, is also used. 

Individual drug-drug interaction detail pages contain references specific to that interaction. You can click on the reference number within brackets '[]' to see what reference was utilized.

The information posted is fact-checked by HelloPharmacist clinicians and reviewed quarterly.