BBC Sport Online examines EPO, or erythropoietin, the latest performance-enhancing drug to hit athletics.
What is it?
Erythropoietin (EPO) is a hormone naturally produced by the kidneys.
However, this hormone can be artificially produced to improve the performance of, for example, athletes or cyclists by injection.
Why would athletes do this?
Its overall effect is to increase endurance and, in athletics, it is used mainly by long distance-runners.
It is injected under the skin and stimulates red blood cell production.
The more red cells there are in your body, the more oxygen that can be delivered to the muscles.
This delays the onset of fatigue, meaning an athlete can run harder and for longer.
How effective is this drug?
Tests in Australia have shown that improvements in an athlete's performance over four weeks would match those expected over several years.
Is its use detectable?
It cannot be traced by conventional drug testing.
New methods have been developed in France where the results of blood sample analysis and urine analysis are compared.
These techniques were introduced ahead of the Sydney Olympics last year, although no positive tests were announced.
The blood test will show if there is an unusual reading of blood cells.
The following urine test will then be able to highlight any difference between the EPO levels produced naturally and synthetically.
Knowing they will be tested, won't athletes just simply stop using it ahead of a competition?
The results will be positive if an athlete has taken it within five to seven days.
However, for the drug to be effective, an athlete would need to take it within five to seven days of running.
Is taking the drug dangerous?
Injecting the drug will thicken the blood. If it is over-used the blood will become so thick, there is a real danger of the heart stopping when the body slows down, for example, when falling asleep.
Increasing the likelihood of suffering blood clots, heart attacks and strokes is a real possibility.