[...] two sources told ESPN's T.J. Quinn and Mark Fainaru-Wada that the drug used by Ramirez is HCG -- human chorionic gonadotropin. HCG is a women's fertility drug typically used by steroid users to restart their body's natural testosterone production as they come off a steroid cycle. It is similar to Clomid, the drug Bonds, Giambi and others used as clients of BALCO. [...]
A source with intimate knowledge of steroids told ESPN that a male athlete usually uses HCG after a cycle of steroids because steroids often shut down the testosterone-making ability of the testicles. HCG restores their capacity to make testosterone. The source said that some males may use HCG in lieu of steroids also. HCG by itself can provide a substantial boost in the body's own testosterone, and this may provide some performance-enhancement benefits.
And from an updated story at Yahoo! Sports:
Two sources said Ramirez tested positive for a gonadotropin. Major League baseball's list of banned substances includes the gonadotropins LH and HCG, which are most commonly used by women as fertility drugs. They also can be used to trigger testosterone production. Testosterone is depleted by steroid use, which can cause sexual dysfunction.
"Testosterone and similar drugs are effective for erectile dysfunction in that they jazz up your sex drive, said Charles Yesalis, a professor at Penn State who has testified before Congress on issues of performance-enhancing drugs. "But far more clinicians accept that affect with Viagra and Cialis. It's hard for me to understand if it was erectile dysfunction why they would use it."