Ninety-six healthy men were randomized into the following three groups : warfarin [ international normalized ratio ( INR ) 2.0-3 .0 ] transitioned to rivaroxaban 20mg once daily ( od ; group A ) ; warfarin ( INR 2.0-3 .0 ) followed by placebo od ( group B ) ; and rivaroxaban alone 20mg od ( group C ) for 4 days .