Let’s begin by examining EPO – a drug that has plagued the Tour de France.

The Biology of Blood Doping
When you exercise, your muscles demand oxygen. The less oxygen they have, the more tired they get. Simple enough, right? Luckily, your body has a great way to keep up with their demand; it delivers extra blood to them. Your muscles suck up the oxygen in red blood cells, and the de-oxygenated blood then flows to your lungs to re-oxygenate itself.

When your muscles fatigue, it’s often because that this cycle isn’t working quick enough, therefore your muscles aren’t getting enough oxygen.

But what if you could increase the amount of oxygen available to your muscles throughout the cycle? This is where doping comes in. “Blood doping gives your body more red blood cells. More red blood cells mean more oxygen, which means greater athletic endurance,” explains Gary Wadler, M.D., former chair of the World Anti-Doping Agency’s Prohibited List Committee.

Doping Type #1: Blood Transfusion
“A few weeks before a race these guys take about 14 ounces of blood out of their body and then freeze it. After this blood is extracted, the body senses that it’s missing and produces more blood to replace it,” says Dr. Wadler. “When it’s time to compete the body has replaced the missing blood, so then they put the blood that they extracted back into their body. They now have extra blood. Extra blood equals extra red blood cells.”

Doping Type #2: EPO
Athletes can also inject a drug called erythropoietin (EPO). “EPO stimulates your bone marrow to produce more red blood cells,” explains Dr. Wadler. “Typically your blood is about 40 to 45 percent red blood cells. EPO can increase that percentage to 50 percent or higher.”

But come drug-testing time, EPO use can be easy to detect because it increases the ratio of red blood cells to the rest of the cells in the blood, such as plasma and white cells. To get around that, dopers will usually take smaller doses over time to avoid quick spikes in blood composition.

Doping by transfusion is the most difficult to detect because it increases the amount of red blood cells and other cells, so the ratios are the same as a non-doper.

Why Doping is Banned
During transfusion tainted blood can be put back into the body, and often the people transfusing the blood aren’t trained doctors. Putting tainted blood into your body or letting your friend transfuse your blood rank pretty high on the list of best ways to make it to the emergency room.

The EPO method is no better. Once the percentage of red blood cells rises above 50 percent, the blood becomes thick and viscous like molasses. This can lead to a heart attack or stroke. In fact, after EPO was developed in the late 1980s, 20 European endurance athletes died in a span of 4 years, and most experts suspect that they died of heart failure due to thick blood.

Boost Your Endurance the Natural (Legal!) Way
Plain old cardio will increase your body’s production of red cells. Because your muscles need more oxygen when you exercise regularly, your body increases its production of red blood cells as a way to adapt to the stress placed on it. Any exercise that makes you breathe heavily will cause this effect.

A more extreme version of this method is to train at high altitudes, as many pro cyclists and long-distance runners do. Oxygen levels decrease with altitude, so training in a low oxygen environment triggers the body to pump out more red blood cells, although not enough to make your blood too thick.