HCG (human chorionic gonadotropin) is provided as a glycoprotein powder to be diluted with water and taken by injection, either intramuscularly or subcutaneously. It acts in the body like luteinizing hormone (LH), stimulating the testes to produce testosterone even when natural LH is not present or is deficient. It therefore is useful for maintaining testosterone production and/or testicle size during a steroid cycle. Additionally, outside of or in-between steroid cycles, it can be very useful for increasing testosterone production. The success of this depends on the ability of the testes to actually produce greater amounts of testosterone with increased stimulation. Where the testes themselves are the limiting factor, HCG cannot overcome this.
Including HCG as part of a hormone replacement therapy (HRT) program is superior to relying on testosterone alone if maintenance of sperm production and/or normal testicle size is considered important. Use of testosterone alone can result in infertility or reduced fertility, as normal testicular function depends on higher intratesticular testosterone levels than results from such use.