epo is what makes your body produce more red blood cells. red blood cells are what carry oxygen - with more of them, you can work harder before you start breathing harder.
it used to be impossible to test for since your body makes its own, but now they have a test.
and now there is stuff out there that does the same, but there is no test for... yet.
but they are all dangerous in that if you have more red blood cells running around, then your blood is thicker. if it is thicker then you get more swirling aroudn and red cells bumping into each other and the arterial walls. you body might mistake this for trauma and induce clotting there. the clot can then end up in your heart or your brain where upon you have either a heart attack or a stroke.
because of that, many pro cyclists when on it don't even go a full night with sleep, when you are in one position, that encourages blood pooling and that increases the clotting chance.
so the coaches will frequently wake them up and have them walk around, then go back to bed. doing this every few hours.
also on planes they won't sit for very long since the knees, ass, and feet pool blood, whereas walking around gets it all going again.
test, anavar, and deca - and likely more - all raise your red cell (and platelet count - platelets are what are used in clotting) count