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The findings don't prove that warfarin reduces the risk of cancer, cautioned lead researcher James Lorens.
"This is an observational study using data on more than 1.25 million people 50 and older from Norwegian national registries, and cannot prove a cause-and-effect relationship," said Lorens, a professor of biomedicine at the University of Bergen in Norway.
Among adults taking warfarin, however, fewer developed cancer compared with those not taking the drug, Lorens said.
This study suggests there is something about warfarin that might reduce the risk of cancer, said Dr. Len Lichtenfeld, deputy chief medical officer at the American Cancer Society.
However, "the study does not suggest that we should be prescribing warfarin to reduce cancer risk," he said. "No one should be taking warfarin as a cancer prevention measure."
As many as 10 percent of adults in Western countries take warfarin, according to background notes with the study. As a blood thinner, warfarin works by blocking vitamin K, which is essential for clotting. But the drug is difficult to regulate, and frequent blood tests are needed to ensure the dose is high enough to prevent clotting, but not so high as to cause major bleeding.
Because these new drugs have a different mechanism of action, "we do not expect the same cancer protective effect as warfarin," Lorens noted.
Rolf Brekken is a professor of surgery at the Center for Therapeutic Oncology Research at the University of Texas Southwestern Medical Center in Dallas. He hopes to see a trial testing warfarin in patients who have had cancer.
"The next step is to demonstrate that a low dose of warfarin is safe and effective in preventing a return of cancer," he said.
For the latest study, Lorens and his colleagues collected data on warfarin use and cancer among Norwegians born between 1924 and 1954.
Specifically, the researchers looked at prescriptions for warfarin between 2004 and 2012 and any new cases of prostate, lung, breast and colon cancer between 2006 and 2012.
Among 1.25 million people, nearly 93,000 were taking warfarin.
Warfarin's anti-cancer effect was particularly strong among patients taking it for atrial fibrillation, Lorens said.
The study had some limitations. Because Lorens' team did not collect information on other medications or risk factors, "new" cancers may have been recurrences of previous ones.
Also, warfarin prescriptions may be a marker for other factors that can help prevent cancer, they added.
The report was published online Nov. 6 in the journal JAMA Internal Medicine.
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SOURCES: James Lorens, Ph.D., professor, biomedicine, University of Bergen, Norway; Rolf Brekken, Ph.D., professor, surgery, principal investigator, Hamon Center for Therapeutic Oncology Research, University of Texas, Southwestern Medical Center, Dallas; Len Lichtenfeld, M.D., deputy chief medical officer, American Cancer Society; Nov. 6, 2017, JAMA Internal Medicine, online