Fragment 176-191 (also marketed as AOD-9604) is a 16-amino acid segment of human growth hormone, studied primarily for its ability to promote fat breakdown without the hormonal side effects of full GH.
What it does
Fragment 176-191 is the tail end of the human growth hormone (hGH) molecule, specifically amino acids 176 through 191. It is chemically identical to AOD-9604 — the two names refer to the same compound, one describing its position in the GH sequence, the other being its pharmaceutical development label.
The fragment targets β3-adrenergic receptors on fat cells (adipocytes). When those receptors activate, they trigger a rise in cyclic AMP (cAMP), a cellular signaling molecule that in turn activates hormone-sensitive lipase (HSL) — the enzyme that breaks triglycerides (stored fat) into fatty acids that can be burned for energy. This process is called lipolysis. Separately, the fragment appears to block de novo lipogenesis, meaning it interferes with the cell's ability to synthesize new fatty acids from scratch al. 2001.
Critically, Fragment 176-191 does not activate the GH receptor and does not raise IGF-1 (insulin-like growth factor 1, the downstream hormone responsible for most of GH's growth-promoting effects). That separation matters: full GH can cause insulin resistance, promote cell proliferation (mitogenesis), and over time contribute to acromegaly (abnormal tissue growth). None of those effects have been observed with this fragment al. 2001.
What the evidence shows
Fat loss / lipolysis Moderate rodent evidence; very limited human data
Animal studies showed that Fragment 176-191 reduced body fat in obese mice and demonstrated measurable lipolytic activity comparable to full-length GH, without the accompanying rise in blood glucose or IGF-1 al. 2001. The fragment both accelerated fat breakdown and inhibited new fat synthesis in these models.
Human data is sparse. AOD-9604, the same molecule, completed Phase I and Phase II clinical trials for obesity, but published results from those trials are limited in the open literature. What exists confirms the fragment does not alter IGF-1, glucose, or insulin levels in humans at studied doses al. 2001. Meaningful human efficacy data on body composition changes remains thin — the evidence base is primarily preclinical.
How it's used
In studies and self-reported protocols, doses range from 250 mcg to 1000 mcg per day, administered via subcutaneous injection (SubQ — injected into fat tissue just under the skin). A moderate dose of 500 mcg daily appears most common in self-reported use. The half-life is approximately 30 minutes, so timing is considered relevant: most protocols place the injection in the morning in a fasted state, on the theory that low circulating insulin allows lipolysis to proceed without interference. Daily administration is standard.
Side effects and safety
Reported side effects are mild. Injection site irritation and occasional mild headaches are the most frequently noted. Moderate effects including nausea and drowsiness have been reported less commonly. No severe adverse events have been documented in the available literature or self-report records.
The absence of IGF-1 elevation and GH receptor activation is a meaningful safety distinction from full GH — there is no documented mitogenic (cell-proliferation) risk or diabetogenic effect at studied doses al. 2001. Pregnancy is an absolute contraindication. Long-term safety in humans is genuinely unknown; the clinical trial program for AOD-9604 did not advance to Phase III, so there is no long-duration human safety dataset.
Bottom line
Fragment 176-191 has a plausible and reasonably well-characterized mechanism for promoting fat breakdown, with solid supporting data in animals, but human evidence remains limited and no large trials have been completed. It is a cleaner safety profile than full GH for anyone concerned about IGF-1 or insulin effects, but anyone expecting human-trial-backed efficacy data will find the cupboard mostly bare.