What are the side effects of blood doping? It has been in the news that endurance athletes such as cyclists sometimes increase their red blood cell count by blood doping. I don't understand how you can just pump more blood into yourself, there must be some bad side effects to doing this, right?
Basically, the intended effect is to increase the number of red blood cells in your system, which in turn gives you increased oxygen carrying capacity to the working muscles. This extra oxygen allows your muscles to work longer and harder.

In the early days of "blood packing", circa the 70's and 80's, it was basically taking a couple of pints of blood out of the body, storing it and letting the body naturally replace it, then transfusing it back into the athlete before their events.

Erythropoietin has actually been around since the early 1900's, and in the 60's, they found that it stimulated blood marrow to produce extra red blood cells. In the late 80's, a synthetic EPO agent was created, and this is what led to the EPO use as an athletic enhancing agent, other than its legitimate medical uses (Anemia, and other diseases).

The biggest effect from extra blood cells (Other than the risk factors of homologous blood {Blood from an outside donor}) is that the blood becomes thicker and more viscous. This can lead to blood clots, strokes, heart attacks and pulmonary embolism. This becomes more likely at rest/sleep, as the heart rate slows quite a bit.