HCG is produced by placental trophoblast cells during pregnancy and also by a number of different types of tumour cell. Its major physiological role is stimulation of the corpus luteum, to maintain synthesis and secretion of the hormone progesterone during pregnancy. However, when injected into males, HCG also stimulates the Leydig cells of the testis to produce testosterone and epitestosterone, and so it can mimic the natural stimulation of testicular hormone production produced by luteinizing hormone (LH).

The Leydig cells of the testis possesess receptors which when stimulated by LH or HCG bring about activation of testosterone synthesis. This increase in synthesis is rapid, a 50% increase in plasma testosterone concentration has been measured 2 hours after intramuscular injection of 6000IU of HCG.

To stimulate ovulation, HCG is used in conjunction with FSH (follicle stimulating hormone) in infertile women. Occasionally, HCG is used to stimulate testicular hormone production when puberty is delayed.

The increasingly successful identification of anabolic steroid abusers by various IOC approved tests has led abusers to switch to testosterone abuse, which itself may be detected by measurement of the testosterone:epitestosterone ratio. Abuse of HCG has become popular because it stimulates the secretion from the testes of both testosterone and epitestosterone, resulting in a urinary excretion ratio of less than 6:1, below the limit set by the IOC. This led to the banning of HCG by the IOC in 1987.

A standard doping regime for HCG has been described (Brooks et al., 1989) in which the abuser firstly injects testosterone. Apart from any gains in strenght or competitiveness, the testosterone causes inhibition of LH secretion from the pituitary. When testosterone is withdrawn before competition (to avoid detection) the athlete would be at a disadvantage with lower that normal plasma testosterone levels. However, administration of HCG stimulates testicular testosterone secretion and also that of epitestosterone, so that apparent compliance with IOC regulations occurs. In a small, elgant experiment, Kicman, Brooks and Cowan (1991) have reproduced this situation in three normal men and shown that HCG can stimulate the testosterone substitution claimed by abusers and retain the testosterone/epitestosterone ratio within the IOC limits. In all three cases, the HCG could be detected in the urine by radioimmunosassay as long as plasma testosterone levels were raised. Despite the availability of a highly specific radioimmunosassay for HCG, it does not yet reproduce the discriminating power of gas chromatography and mass spectrometry and so it is as yet unaccepted by the IOC.

The side-effects of HCG will be similar to those described for anabolic steroids. However, the incidence of gynaecomastia may be greater as HCG also stimulates oestradiol production by the Leydig cells.