My dad has been on coumidin for 30+ years, and yes they can become bleeders. Most of the time, because they do regular clotting tests, they can modify their dosage level and greatly reduce that kind of risk. Dad has had several successful surgeries during the 30+ year period, and he has been fine. My hope is that your dad fairs as well. Oh, and by the way, the less these guys do, the worse they get.