Results Summary

What was the research about?

Each year, more than 100,000 people in the United States die from blood clots that form in their veins and travel to their lungs. Many people who survive a clot will have another one within 10 years. Blood thinners can prevent future clots but may cause problems, such as severe bleeding.

In this study, the research team compared ways to prevent blood clots among people who have already had one. The team compared:

  • People who took warfarin, an older blood thinner, versus newer blood thinners called direct acting oral anticoagulants, or DOACs, for more than six months. Examples of DOACs are apixaban, rivaroxaban, and dabigatran.
  • People who took blood thinners for only three months versus more than three months.

What were the results?

Compared with people who took warfarin, people who took DOACs for more than six months had:

  • A lower risk of another blood clot
  • Higher satisfaction with treatment

People who took warfarin and people who took DOACs didn’t differ in:

  • The risk of a hospital stay for bleeding
  • Death from any cause
  • Quality of life

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

  • Higher risk of another blood clot
  • Higher risk of a hospital stay for bleeding
  • Lower risk of death from any cause

The higher risk of another blood clot in people who took blood thinners for more than three months was a surprising finding. Blood thinners probably weren’t the reason for the clots; more research may help explain this finding.

What did the research team do?

The research team reviewed health records for 38,062 adults who had a blood clot between 2010 and 2018. Of these adults, 73 percent were White, 15 percent were Black or African American, 5 percent were Asian or Pacific Islander, and 7 percent were other or unknown race; 17 percent were Hispanic. The average age was 64, and 50 percent were women. All took a blood thinner for at least three months and received care at Kaiser Permanente in California. The team looked at the risk of another blood clot, a hospital stay for bleeding, and death from any cause.

The research team also surveyed a subset of 5,017 patients who took blood thinners for a blood clot between 2015 and 2018. Surveys asked about treatment satisfaction and quality of life.

People with blood clots, family members, doctors, and health system administrators gave input on the study.

What were the limits of the study?

The research team didn’t assign patients by chance to take warfarin or DOACs or tell them how long to take blood thinners. People who stayed on blood thinners longer may have had a higher risk for future blood clots that the team wasn’t able to take into account.

Future research could take into account more factors that affect patients’ risk of blood clots.

How can people use the results?

People who have had a blood clot and their doctors can use the results when considering ways to prevent another blood clot.

Final Research Report

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

Peer-Review Summary

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

Conflict of Interest Disclosures

Project Information

Margaret Fang, MD, MPH
University of California, San Francisco
The Comparative Effectiveness of Warfarin and New Oral Anticoagulants for the Extended Treatment of Venous Thromboembolism

Key Dates

July 2016
September 2022

Study Registration Information


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Last updated: January 18, 2023