UPDATE 1-Bayer's Xarelto pill cuts risk in long-term study

* Relative blood-clotting risk 82 pct lower vs placebo

* Study focused on 12-mth prevention in thrombosis patients

* Risk of side effects described as low (Adds details, background)

FRANKFURT, Nov 19 (Reuters) - Bayer (BAYGn.DE) and J&J's JNJ.NO blood thinner Xarelto was shown to prevent blood clots in thrombosis-prone patients in a long-term drug trial, adding to positive study results for the potential blockbuster.

The pill, based on the active ingredient rivaroxaban, led to an 82 percent lower risk that a dangerous venous blood clot would form again, compared with a control group on placebo, according to the abstract of a study published on the Website of the American Society of Hematology (ASH) on Thursday.

Participants in the late-stage study had already suffered a thrombosis -- a blood clot in the veins that can travel to the lungs and cause deadly clogging -- and had received six to 12 months standard treatment afterwards.

Subsequently, they were either given the once-daily pill Xarelto or a placebo for an additional six to 12 months to test the effectiveness and safety of Bayer's drug.

Bayer described the results as "positive" in a brief statement, and declined to comment further.

The rate of major bleeding, the main danger associated with anti-blood-clotting treatments, was described as low, with a risk of 0.7 percent.

More detailed results will be published at an ASH conference on Dec. 8 in New Orleans.

In May, the U.S. Food and Drug Administration (FDA) declined to approve Xarelto for its first use in patients temporarily bedridden after hip and knee replacement surgery, asking for more information but stopping short of demanding new tests on humans. [ID:nLR959606]

The drug has already been approved in Europe for use after orthopaedic surgery.

Bayer and J&J are also targeting the mass market of stroke prevention in patients suffering from a common form of irregular heartbeat. Market launch for that indication is not seen before 2011.

Bayer says the anticoagulant drug could eventually generate more than 2 billion euros ($2.98 billion) in annual sales. ($1=.6722 Euro) (Reporting by Ludwig Burger; Editing by Jon Loades-Carter)