Warfarin with Influenza Virus Vaccine Interaction Details
Brand Names Associated with Warfarin
- Coumadin®
- Jantoven®
- Warfarin
Medical Content Editor Dr. Brian Staiger, PharmD
Last updated
Nov 07, 2023
Interaction Effect
An increased risk of bleeding
Interaction Summary
A fatal intracranial bleed may have been attributed to the administration of an inactivated influenza vaccine in a patient receiving warfarin therapy. In addition, there were case reports of serious bleeding episodes after administration of influenza vaccine in 2 other patients who were maintained on warfarin. However, a number of investigators have not been able to reproduce this interaction and present data suggests this interaction does not occur in most patients .
Severity
Major
Onset
Delayed
Evidence
Probable
How To Manage Interaction
While significant effect on the anticoagulant response to warfarin in patients receiving the influenza vaccine is unlikely, it may be prudent to monitor the prothrombin time ratio or INR more frequently during the 4 to 6 weeks following the vaccine. Further vigilance would seem unnecessary except in cases where the clotting times differed significantly from the baseline.
Mechanism Of Interaction
Unknown
Literature Reports
A) A 64-year-old man on warfarin therapy (5 mg every other day alternating with 2.5 mg) experienced a fatal bleeding episode 4.5 weeks after administration of an inactivated influenza vaccine. The patient was receiving warfarin for stroke prophylaxis secondary to atrial fibrillation, had a significant medical history, and was receiving 9 other medications for various conditions. His INR, stable for the previous 6 months at 1.4 to 4.7, increased from 2 at the time of vaccination to 15 upon admission. The family reported a 2-day history of rectal bleeding and denied any recent changes to the patient's health, medication regimen, or usage. Laboratory examination revealed low hemoglobin (8.3 g/dL), hematocrit (24%), and serum albumin (3.1 g/dL) and a non-contrast CT scan showed a large parenchymal hemorrhagic infarction involving the left temporal, parietal, and occipital lobes. Despite interventions, the patient died approximately 17 hours after admission .
B) Several authors have reported serious bleeding episodes in patients well-controlled on warfarin following administration of influenza vaccine . The mechanism of this interaction is unknown. Other studies have not found a significant effect on warfarin metabolism when influenza vaccine is administered .
C) No evidence of an effect of influenza vaccine on inhibition of theophylline metabolism or enhancement of warfarin induced-hypoprothrombinemia has been reported in elderly patients . Vaccination in the elderly receiving theophylline or warfarin with influenza vaccine does not appear to require dosing adjustments of either drug. The authors emphasize that previous reports demonstrating an interaction were based on invalid methodological and statistical designs. These negative results were confirmed in an additional study of elderly patients receiving warfarin .
D) In a study involving 24 evaluable patients who had received warfarin for at least 6 weeks prior to administration of influenza vaccine (types A & B vaccines), prothrombin time values following vaccination were unchanged except for a significant decrease in prothrombin time values during the first 2 weeks postvaccination .
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.
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Definitions
Severity Categories
Contraindicated
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.
Major
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.
Moderate
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.
Minor
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.
Onset
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.
Evidence
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.
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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.