MONDAY, Nov. 5 (HealthDay News) -- About a quarter of people who experience the dangerous blood clots in the legs or lungs known as venous thromboembolisms (VTEs) develop them for no discernable reason, and most will receive a powerful anti-clotting drug such as warfarin in the months after the clot forms.

But what about longer-term care, to ward off a recurrent clot, or events such as heart attack or stroke? A new study suggests that patients who go on low-dose daily aspirin after they are weaned off more powerful anticoagulants can derive real benefit.

While this study alone could not show a significant effect for aspirin therapy in preventing a recurrent clot for these patients, it did show a significant lowering of overall cardiovascular risk for such complications as heart attack, stroke, major bleeding or death from any cause.

The study was presented Sunday at the American Heart Association's annual meeting in Los Angeles, and was published simultaneously in the New England Journal of Medicine.

The study authors said they saw a "non-significant" trend hinting at aspirin's ability to prevent a second VTE, but the study group was simply too small to push that to statistical significance.

However, they added, when the findings from this study were combined with those of a similar trial called WARFASA, the combined data didshow that daily aspirin could prevent recurrent blood clots.

"The essential message of the study that we are presenting today, and combining that with the WARFASA study that was published earlier this year in the NEJM, is that aspirin does have a benefit: about a 30 percent risk reduction of recurrent vein thrombosis, and also about a similar effect in reducing other major vascular events stroke, myocardial infarction [heart attack] and cardiovascular death," said study author Timothy Brighton, a consultant hematologist in Sydney, Australia.

VTEs include sudden, potentially life-threatening clots in the legs (known as deep vein thrombosis, or DVT) and clots that travel to the lungs, called pulmonary embolisms.

As Brighton explained, in about 75 percent of cases, these clots have a known cause injury, surgery or underlying illness, for example. But for the other quarter of patients, the cause is unknown.

More than 800 people with a VTE of uncertain cause were included in the new study. Approximately half were given low-dose (100 milligrams/day) daily aspirin, while the other half received a placebo, after they had completed their initial regimen of a more powerful anticoagulant. The patients were then followed for an average of more than three years.

Brighton noted that patients often cannot continue on anticoagulants such as warfarin indefinitely, because of bleeding risks and inconveniences associated with these drugs.

"The question comes, though, after [discontinuation of initial anti-coagulation therapy], what do you do?" he said.

The answer, from the new study as well as WARFASA, seems to be low-dose daily aspirin. Brighton's group also pointed out that incidents of bleeding were similar for those on low-dose aspirin and placebo, suggesting that aspirin does not raise bleeding risk.

One cardiologist said the study is encouraging for patients. Dr. Stephen Green, associate chairman of the department of cardiology at North Shore University Hospital in Manhasset, N.Y., said that while it's true the trial (on its own) did not find a significant effect of aspirin in reducing VTE recurrence, "the study was positive enough that physicians should consider adding aspirin when stopping warfarin in patients who are reasonable candidates for long-term aspirin."

But he also noted that since the time of the trial, many patients with VTE have now moved from initial warfarin therapy to treatment with newer blood-thinning agents such as Xarelto and Pradaxa.

More study might be needed to assess aspirin's effectiveness in that clinical scenario, he said.

More information

Find out more about deep vein thrombosis at the U.S. National Library of Medicine.