An experimental anti-clotting drug that Merck & Co. is developing increased risk of bleeding internally in a study, the second time it's shown that dangerous side effect.

Merck says the top-line results from a study called TRA-2P show
Vorapaxar, when added to standard anticlotting therapy, reduces
risk of heart attack, stroke, death from heart disease or emergency
heart surgery.

But the company says there was a significant increase in
bleeding, including bleeding inside the skull. That intracranial
bleeding was less common in patients who had not previously had a
stroke.

A year ago, Merck halted another study because of the same
bleeding problem.

Merck says it will present detailed results from the latest
study at a March medical conference.