Officials associated with international track and field competitions have noted an increase in the number of athletes who use a synthetic form of erythropoietin. Athletes use this drug because it__.

Athletes use this drug because it enhances their performance.

Explanation:

Erythropoietin is expressed principally in the cortical interstitial cells of the kidney, where its transcription is modulated in response to low oxygen levels (hypoxia).

The use of artificial or recombinant erythropoietin stimulates an increase in the production of red blood cells.

This in turn improves the oxygen carrying capacity of blood and enhances the aerobic performance of athletes.  

The side effect of using artificial erythropoietin would be excess production of red blood cells (polycythaemia) leading to blood clots and possible heart attack.