Results Summary

What was the research about?

Each year, more than 500,000 people in the United States are hospitalized for blood clots that form in their veins. Many people who have a clot will have another within 10 years. Patients can take medicines called blood thinners to treat and prevent new clots. But blood thinners may cause problems, like severe bleeding. Questions remain about how long people should take these medicines to prevent future clots.

In this study, the research team compared ways to prevent blood clots in people who have already had one. The team compared patients who took blood thinners after a clot for 3 months with patients who took them for more than 3 months and up to 12 months. The team also compared the safety of taking three common blood thinners—warfarin, apixaban, and rivaroxaban—for more than 3 and up to 12 months.

The research team looked at risk of a hospital stay for new blood clots or bleeding.

What were the results?

Compared with patients who stopped taking blood thinners after three months, patients who took blood thinners for more than three months had:

  • A lower risk of a hospital stay to treat new blood clots within six months
  • A higher risk of a hospital stay to treat severe bleeding within six months

Statistical tests found that, among patients who took blood thinners for more than three months, patients taking:

  • Apixaban had a lower risk of a hospital stay to treat new blood clots than patients taking warfarin
  • Apixaban and patients taking rivaroxaban had a similar risk of a hospital stay to treat new blood clots
  • Rivaroxaban and patients taking warfarin had a similar risk of a hospital stay to treat new blood clots

The risk of a hospital stay to treat severe bleeding didn’t differ among patients who took warfarin, apixaban, or rivaroxaban.

Who was in the study?

The study included health records from three nationwide health insurance claims databases for 90,921 adult patients. All patients had been in the hospital for a blood clot and took blood thinners after leaving the hospital for at least 90 days. The average age was 70, and 57 percent were women.

What did the research team do?

The research team looked at health records from 2010 to 2021 to find patients who had been hospitalized for a blood clot and who hadn’t used blood thinners before the clot. Then the team tracked hospital stays for patients who filled prescriptions for a blood thinner and took it for at least 90 days.

Patients with prior blood clots and doctors helped design the study.

What were the limits of the study?

The study didn’t assign patients by chance to length of treatment. Differences between patients who extended treatment and who stopped treatment may have affected study results.

Future research could determine the best length of time for patients to take blood thinners to reduce the risk of future blood clots and bleeding.

How can people use the results?

Patients and doctors can use these results when considering treatments for blood clots.

Final Research Report

This project's final research report is expected to be available by November 2023.

Peer-Review Summary

The Peer-Review Summary for this project will be posted here soon.

Conflict of Interest Disclosures

Project Information

Katsiaryna Bykov, PharmD, ScD
Brigham and Women's Hospital
The Dabigatran, Apixaban, Rivaroxaban, Edoxaban, Warfarin Comparative Effectiveness Research Study (The DARE warfarin CER study)

Key Dates

July 2016
November 2023

Study Registration Information


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Populations Populations PCORI is interested in research that seeks to better understand how different clinical and health system options work for different people. These populations are frequently studied in our portfolio or identified as being of interest by our stakeholders. View Glossary
Intervention Strategy Intervention Strategies PCORI funds comparative clinical effectiveness research (CER) studies that compare two or more options or approaches to health care, or that compare different ways of delivering or receiving care. View Glossary
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Last updated: February 1, 2023