Identity
HCG — human chorionic gonadotropin — is a large glycoprotein hormone produced in pregnancy by the placenta, and the molecule that home pregnancy tests detect. As a medicine it is supplied either purified from the urine of pregnant women or as a recombinant version (choriogonadotropin alfa). It is a protein hormone, not a short peptide, which matters for how it is made, handled and verified.
Mechanism (as proposed / established)
HCG's structure closely resembles luteinizing hormone (LH), and it acts at the same LH/CG receptor. That makes it an LH-mimetic: it stimulates the gonads directly — in men, driving testicular Leydig cells to produce testosterone; in women, triggering ovulation and supporting the corpus luteum. Because it acts at the gonad rather than the pituitary, it bypasses the brain's control loop, which is exactly why it is used both to trigger ovulation on schedule and, off-label, to keep the testes working when external steroids have shut the natural axis down. This is textbook reproductive endocrinology.
The approved reality, the off-label use, and the debunked fad
Three layers, and honesty requires separating them.
The approved uses are real: triggering ovulation in assisted reproduction, treating certain forms of male hypogonadism, and cryptorchidism (undescended testes) in children. Grade A, under supervision.
The off-label bodybuilding / TRT-adjunct / post-cycle use is mechanistically coherent — HCG genuinely restarts or maintains testicular testosterone production — but it is outside approved indications, carries estrogen-conversion and thromboembolic considerations, and belongs under medical management rather than self-direction.
The "HCG diet" is the debunked layer: multiple controlled trials found HCG no better than placebo for weight loss, and regulators have acted against over-the-counter HCG weight-loss products. Any weight lost comes from the extreme calorie restriction the diet imposes, not from the hormone. And for athletes: HCG is WADA-prohibited in males.