In clinical settings, it's used to treat infertility in women by stimulating the ovaries to release eggs. In prepubescent boys, it's used to encourage normal sexual development and the natural lowering of the testicles into the scrotum.
In men, it's used to bump up testosterone levels -- it tells the testes to release testosterone.
Here's a good overview, from WebMD, of the hormone's more traditional medical uses.
As Dr. Gary I. Wadler, a sports medicine expert and a key official of the World Anti-Doping Agency, says in today's L.A. Times story Baseball's anti-doping program faces renewed scrutiny in light of Ramirez case: "In the legitimate world, HCG is used for infertility [by women] or for delayed puberty. . . . But for a man of his age  with a testosterone deficiency, there are much better drugs."
If you're wondering about the ways to treat that condition, known as male hypogonadism, think patches, gels and, of course, the efficient injection. Oral medicines, convenient though they are, have some particularly unpleasant side effects.
Here are two good descriptions of treatment, one from the Mayo Clinic and one from the Urology Channel. The latter, as a bonus, includes a quiz via which you can check yourself for symptoms of low testosterone. (Do you have low sex drive? Loss of energy? A decrease in strength or endurance?)
But perhaps we digress. (To be fair -- perhaps not.) HCG also has other uses, hence the brouhaha.
As anabolicsteroidsguide.com notes: "In a man the luteinizing hormone stimulates the Leydigs cells in the testes; this in turn stimulates production of androgenic hormones (testosterone). For this reason athletes use injectable hCG to increase the testosterone production. hCG is often used in combination with anabolic/androgenic steroids during or after treatment."
And from steroid.com: "HCG is no doubt one of the most misused, misunderstood and underutilized tools in bodybuilding pharmacology we have available."
-- Tami Dennis
Photo: Dodger Manny Ramirez warms up before a game in April.
Credit: Christian Petersen/AFP/Getty Images