House finds it surprising that Jeff admitted he is taking illegal enhancement drugs. He agreed to blood doping, which is common among cyclists, to better one's athletic performance. Because red blood cells carry oxygen between the lungs and the muscles, a higher number of red blood cells can increase an athlete's endurance.
House was surprised that Jeff admitted to using illegal performance-enhancing drugs, specifically blood doping, which is a common practice among cyclists to improve athletic performance. This is because blood doping involves increasing the number of red blood cells, which carry oxygen between the lungs and muscles, leading to increased endurance.