The U.S. Food and Drug Administration has recommended against an immediate approval for Bayer AG and Johnson & Johnson’s anti-clotting drug, Xarelto, as a treatment to prevent strokes in patients with atrial fibrillation.

The agency said data from a late-stage study of more than 14,000 patients, known by the acronym ROCKET, doesn't make it clear how safe Xarelto is, or whether it's as effective as the widely used warfarin.

Johnson & Johnson and partner Bayer Healthcare of Germany developed the drug, which is chemically known as rivaroxaban.

Xarelto was approved in July for reducing the risk of deadly blood clots in patients getting knee and hip replacements, a small part of the potential patient pool. That approval followed a delay of roughly two years due to FDA concerns about the internal bleeding risk.

An additional patient study could be done, which would cause a long delay before the drug-makers could again seek approval.

Bayer Healthcare already markets rivaroxaban in 110 countries around the world.

The daily pill works by blocking a clotting protein called factor Xa. Older blood thinners, including warfarin, work by preventing platelets from sticking together.

Warfarin, sold as Coumadin and other brands, has its own drawbacks. It's notoriously difficult to get the dose of the drug correct, so patients need frequent blood tests because too much warfarin can cause dangerous internal bleeding.

Xarelto is one of several experimental drugs being developed as a replacement for warfarin, a medicine originally derived from rat poison that has been used safely and effectively for seven decades to prevent strokes.

The newer drugs are all meant to be more convenient than warfarin, which must have its dose adjusted using frequent blood tests, and, potentially, to cause less bleeding. Of the three new drugs, only Xarelto is given once a day, with the other drugs requiring twice-a-day dosing.

The review is a big blow to the drug giants. Analysts largely expected Xarelto to generate peak sales of $1 billion or more.