(Reuters) - U.S. health regulators approved on Friday the stroke preventer Xarelto from Bayer AG and Johnson & Johnson for people with a common heart rhythm disorder known as atrial fibrillation.

The U.S. Food and Drug Administration approval for Xarelto as a first-line treatment countered expectations it would only be available for patients who were not able to take older therapies.

The FDA also added a restrictive "black box" to the drug's label, warning people against discontinuing use of Xarelto without consulting their doctors, as stopping abruptly may increase risk of stroke.

Xarelto, clinically known as rivaroxaban, is already approved in Europe for this use. It had been approved in the United States for the narrower indication of anti-clotting after knee and hip surgery.

The once daily anti-clotting pill is now among several drugs angling to replace decades-old warfarin, which helps prevent stroke in people with atrial fibrillation, a market estimated to be worth more than $10 billion.

The drug is the first once daily anticoagulant pill that does not require routine blood monitoring, according to J&J. For Bayer, Xarelto is its biggest pipeline drug.

More than 2 million Americans have atrial fibrillation, meaning irregular heartbeats that can cause blood to pool and increase their risk of blood clots and strokes. But many are unwilling to take warfarin, which requires regular blood tests, or cannot tolerate it.

Bayer has said it expects more than 2 billion euros ($2.7 billion) in peak annual sales from the product.

For Johnson & Johnson, senior Wells Fargo analyst Larry Biegelsen forecasts U.S. sales of $779 million in 2015.

"(This approval) represents the largest commercial opportunity for Xarelto," Biegelsen said in a research note, also noting the FDA did not require another study before approving it, as analysts expected.

SECOND-PLACE?

However, the FDA said there is not enough data to show how Xarelto compares with warfarin when the older drug is used properly. An FDA panel recommended Xarelto, but the reviewers called for the agency to delay its approval because it was not proven to be at least as effective as warfarin.

Echoing some panelists' concerns, the U.S. consumer group Public Citizen said the drug's label should not be expanded because it may cause a rebound occurrence of strokes when the medication is discontinued. The group also said clinical trials of the drug were conducted in a manner that favored Xarelto.

Bristol-Myers Squibb Co and Pfizer Inc's Eliquis may now emerge as the leader of the pack, ISI Group analyst Mark Schoenebaum said in a note.

Eliquis has so far shown the best clinical data, especially for reducing the risk of major bleeding.

Tim Race, an analyst at Deutsche Bank in London, estimated that Xarelto could gain 10 percent of the warfarin-replacement market.

Boehringer Ingelheim, a privately held German company, was the first to market a warfarin replacement drug after the FDA approved its anti-clotting pill Pradaxa in October 2010.

But the company has said the pill was linked to about 50 deaths from bleeding across the world since its market launch and European regulators said last month that patients taking Pradaxa should have their kidneys checked.

(Editing by Gerald E. McCormick, Bernard Orr; editing by Andre Grenon)