Human chorionic gonadotrophin is a strange hormone. Its only found in the placenta of pregnant women. For women it has fairly little use if any however, but to the male athlete it has one interesting property. It can mimic the action of luteinizing hormone (LH) in the body. LH is a pituitary hormone that is released and signals the manufacture of testosterone in the testicles. The sex hormones in the body work via a negative feedback system, where too much sex hormone (like anabolic androgenic steroids and estrogens) causes a signal to the brain to stop the release of LH. During long duration cycles, if natural test stays suppressed for considerable time, a male user will begin to note an atrophy in his testicles, meaning they will visibly shrink purely out of disuse. By administering an LH-mimicking agent, one can bring back the function of the testicles and let them regain their size. This is the main use of HCG.
Since it forms testosterone in the body to some extent, it can impart certain performance enhancing properties, but usually these are not major. The side-effects accompanied with HCG use (usually androgenic such as extreme acne), its low rate of effect, the cost compared to more effective steroids and so on will mostly keep athletes from using it for that purpose. Moreover it can be tested for in athletic competitions, so most will stay clear of it. But to the steroid user HCG is an almost essential part of a cycle. Because of its effect on bringing testicle size back it can promote the return of natural testosterone, since the first natural signals can immediately deliver a higher yield of testosterone in the body. And getting natural testosterone back online after a cycle is crucial, especially if you intend to keep most of your hard-earned gains. Without adequate natural endocrine response you will not be able to maintain a mass that was higher than before.
The downside is that HCG too is suppressive of natural testosterone. Because it takes the place of LH. LH is not the first step in the chain of command, instead its manufactured in the pituitary under the response of Gonadotropin releasing hormone (GnRH) which is secreted from the hypothalamus. And since an LH mimicking agent is supplied exogenously, the negative feedback signal to the hypothalamus will still tell it to stop making GnRH, and so no natural LH is produced. This is why the product is always used in conjunction with a potent estrogen receptor antagonist like clomid or Nolvadex. When the androgen level in the body has dropped, these antagonists will lower estrogenic response creating a steroid deficit that signals the Hypothalamus to start making GnRH. When it does, after HCG therapy, testicle size is up again and shortly thereafter natural testosterone manufacture should return to normal. But therefore its crucial that users note that though HCG is essential after long cycles, it shouldn't be used without clomid or Nolvadex AND HCG should be discontinued at least two weeks before coming off Clomid or Nolvadex or else it will suppress natural testosterone itself.
Also important to take into account : using HCG for too long a period of time or in doses that are excessively high, can desensitize the testicles to the effect of LH and would put your right back where you started from. Basically that would mean you spent money to no avail. In terms of side-effects one should expect some androgenic signs such as acne and there is a risk for hair loss or prostate hypertrophy, but in most cases this compound will be used for 3-4 weeks, so these should not manifest themselves to any serious degree. There will also be some estrogen build-up, but since the user HAS to be on clomid or Nolvadex, this should not become apparent either. Next to this, HCG being a fertility drug, one should be aware that increased blood pressure and blood clotting can occur. HCG is clinically used to make women ovulate, or to invoke birth in pregnant women.
You would normally opt to use HCG after you've done a long cycle, usually 8 weeks or more. Note that almost all proper cycles are 8 weeks or more in length, its just that some beginners have a phobia of needles and opt to waste their time with an all oral stack first, in which case the cycle wouldn't be longer than 6-7 weeks. In these cases too HCG can have a use, but most of the time testicular atrophy will not have progressed to such a stage that it is an absolute necessity. In any case, you should run it about 3 weeks, totaling about 4 shots. One every 5-6 days. Start off with one shot of 3000 IU somewhere in the last week of your stack, then another 3000 5 days later, then drop to 1500 5 days later and a last shot of 1500 6 days after that. Sometime after the second or third shot, therapy with Nolvadex or clomid should be commenced and continued for 4-5 weeks. How to do this, I refer you to the Nolva/clomid profile.
In any case, I'll repeat it again, since it is important. HCG IS and always will be an important part of post-cycle recovery, but it should never be run too long or at too high a dose and should always be accompanied by the use of either Clomid or Nolvadex. The use of Clomid or Nolvadex should also be continued at least 2 weeks after HCG is discontinued to avoid the HCG causing problems.