Oxytocin

HCG is a hormone produced in pregnancy that happens to be an almost perfect mimic of luteinizing hormone. That single fact is why it has two completely unrelated uses: it is an approved fertility medicine, and it is the standard tool for keeping the testicles working during testosterone therapy. It is also the basis of the discredited HCG diet, which is a separate story and not supported by evidence.

MOTS-c is a 16-amino-acid peptide your own mitochondria make, encoded inside the 12S rRNA region of mitochondrial DNA and discovered in 2015. It is studied as a metabolic regulator and a so-called exercise mimetic, because its levels rise when you work out and it improves insulin sensitivity in animals. The catch: the impressive results are almost entirely in mice, with no completed published human efficacy trials.

Rusfertide (brand name Mimrylo) is an injectable peptide that mimics hepcidin, the body's master hormone for controlling iron, and it is used to manage polycythemia vera (PV), a blood cancer that makes too many red blood cells. By limiting iron availability, it throttles red blood cell production and reduces the need for repeated blood draws (phlebotomy). It is not a research-only or gray-market peptide: the FDA approved it on August 28, 2026 for adults with PV, after a positive phase 3 trial.

Cetrorelix is a GnRH antagonist: a modified 10-amino-acid version of the brain's gonadotropin-releasing hormone that blocks its receptor instead of activating it. Sold as Cetrotide and FDA-approved since August 2000, its job in IVF is narrow and important: it stops the body's own LH surge from releasing eggs too early while the ovaries are being stimulated. It is a well-established fertility drug with decades of use, and it is not a tool for anything else people sometimes imagine it for.

Leuprolide (brand name Lupron) is a synthetic GnRH agonist, a modified peptide far more potent than natural GnRH, and it is FDA-approved for advanced prostate cancer, endometriosis, uterine fibroids, and central precocious puberty. The clever part is counterintuitive: it works by overstimulating the GnRH system so hard that the pituitary shuts down sex hormone production. It is a real, long-established prescription drug (first US approval 1985), not a research peptide, and it carries meaningful side effects from the low-hormone state it creates.

Desmopressin (dDAVP) is a synthetic tweak of the natural hormone vasopressin, redesigned to keep the water-retaining effect while dropping most of the blood-pressure effect. First approved in the US in 1978, it is used for central diabetes insipidus, bedwetting, nocturia, and two bleeding disorders: hemophilia A and type 1 von Willebrand disease. This is settled medicine, not an experimental peptide, and its main real-world danger is straightforward: it can drop your blood sodium dangerously low if you drink too much fluid on it.