FRANKFURT (Reuters) - German drugmaker Bayer said on Tuesday it planned to collaborate with U.S. peer Portola Pharmaceuticals to study several drugs that reverse the effects of its blood-thinner Xarelto.

Bayer said it expected to complete the study in the second half of this year.

Dangerous bleeding is one of the main risks of bloodthinners such as Xarelto, which are used against thrombosis or heart attacks, and drugs can be applied more widely if they have so-called "antidotes".

U.S. regulators last June rejected the use of Xarelto to prevent heart attacks and strokes in patients with acute coronary syndrome (ACS), citing the risk of bleeding.

Bayer said in September its development partner Johnson & Johnson had filed a response to the regulators' concerns.

(Reporting By Peter Dinkloh; Editing by David Cowell)