FRANKFURT (Reuters) - Bayer (>> Bayer AG) raised its annual core earnings forecast on Thursday after beating expectations with a 15 percent quarterly advance helped by plastics unit Covestro and sales of stroke-prevention pill Xarelto in Europe and Japan.

Analysts had expected a worse result for Xarelto, a key drug for Bayer, after partner Johnson & Johnson (>> Johnson & Johnson), which owns the U.S. rights, reported a near 10 percent drop in sales for the drug hit by discounting.

Bayer's quarterly sales of Xarelto, prescribed to prevent potentially deadly blood clotting, climbed by about 20 percent to 751 million euros (£636.42 million), Bayer said.

"Our key growth products were once again especially successful," Bayer said in a statement, also citing contraceptive Mirena, eye medicine Eylea and cancer drugs Xofigo and Stivarga.

Bayer shares rose 3 percent to an 11-month high, topping the German blue-chip DAX <.GDAXI>, which was flat.

On top of rivalry from competing blood thinners, J&J's U.S. sales were dragged down by discounts granted to patients to offset reimbursement gaps in the U.S. government's Medicare programme. J&J pays up to 30 percent in royalties to Bayer.

Xarelto competes mainly with Eliquis from Pfizer (>> Pfizer Inc.) and Bristol-Myers Squibb (>> Bristol-Myers Squibb Co) as well as with Pradaxa from Boehringer Ingelheim.

"Xarelto was better than many had expected following weak U.S. number from J&J last week," Bernstein analysts wrote in a note, reiterating their "outperform" rating on the stock. "Bayer beat expectations for all divisions."

Adjusted group earnings before interest, taxes, depreciation and amortisation (EBITDA) rose 15 percent to 3.9 billion euros (£3.30 billion), topping the 3.6 billion forecast by analysts in a Reuters poll.

Bayer said it now expected group EBITDA before special items to improve by a "low-teens percentage", up from a medium single-digit percentage gain seen previously.

It pointed to a brighter outlook for its plastics and chemicals subsidiary Covestro, which reported earnings on Tuesday.

Bayer also announced that its Chief Financial Officer Johannes Dietsch will leave the company at his own request in May 2018, after helping the German drugs and pesticides maker conclude its planned $66 billion acquisition of U.S. seeds giant Monsanto (>> Monsanto Company) by the end of this year.

(Editing by Georgina Prodhan and Jason Neely)

By Ludwig Burger