HGH Fragment 176-191: what it is, how it differs from AOD-9604 and what the research shows

Human growth hormone is a 191-amino-acid protein, and the last few dozen amino acids of its chain, the C-terminal region, were noticed in the 1970s to carry some of the hormone’s effects on sugar and fat metabolism. Researchers at Monash University in Melbourne synthesised a series of short peptides copied from this region, including the sequences 172-191, 176-191, 177-191 and shorter pieces, and tested them in rats ^[1]. HGH Fragment 176-191 is the 16-amino-acid member of that series: it is the 177-191 fragment with one extra phenylalanine at position 176 ^[1]. Like the intact hormone, it contains two cysteines joined by a disulfide bond, which closes part of the chain into a ring ^[13].

The fragment sold today as HGH Frag 176-191 is often described as “the fat-burning part of growth hormone”. That description comes from the later Monash work on the 177-191 fragment, which was given the code AOD9401, and on a modified version with a tyrosine added to its start, AOD9604 ^{[4] [5]}. The version that went into human trials was AOD-9604, not 176-191, and the two are not interchangeable in the published record. Almost everything written online about HGH Fragment 176-191 benefits is borrowed from studies of those two related peptides.

It is worth being clear about what the fragment is not. It is not a growth hormone secretagogue like ipamorelin or CJC-1295, which make the pituitary release more growth hormone, and it is not HGH itself. Its only relationship to growth hormone is that its sequence is copied from one end of the hormone. See what research peptides are for how products like this are sold.

The two peptides differ by a single amino acid at the start of the chain. AOD-9604 is amino acids 177 to 191 with an added tyrosine in front, giving 16 amino acids; HGH Fragment 176-191 is the same 177-191 core with its natural neighbour phenylalanine in front, also 16 amino acids ^{[1] [6]}. Tyrosine and phenylalanine differ only by one hydroxyl group, which is why the two peptides have nearly identical molecular weights (about 1815 g/mol for AOD-9604 and roughly 1799 g/mol for the free 176-191 peptide) ^[13].

The difference in evidence is much larger than the difference in structure. AOD-9604 was developed by Metabolic Pharmaceuticals as an oral anti-obesity drug and went through six randomised human trials, all reviewed by the FDA in 2024 ^[7]. HGH Fragment 176-191 has no human trials at all, and only one published animal experiment, from 1978, in which it was tested for its effect on blood sugar rather than fat ^[1]. When sellers describe HGH Frag 176-191 as “clinically tested”, they are describing AOD-9604’s programme. Even a 2026 clinical review refers to AOD9604 as “hGH 176-191”, which shows how commonly the two names are run together ^[11].

Whether the extra phenylalanine changes how the fragment behaves in the body has not been studied. Ng and Bornstein’s 1978 rat experiments found that 172-191, 176-191, 177-191 and 178-191 were all active in raising blood glucose, while 179-191 and 180-191 were inert, so the 176 residue is not required for that activity ^[1]. No study has compared 176-191 and AOD-9604 for fat loss.

The proposed mechanism is that the C-terminal region of growth hormone carries its effects on fat tissue: increasing the breakdown of stored fat (lipolysis) and reducing the building of new fat (lipogenesis), without the growth-promoting and blood-sugar-raising effects of the whole hormone ^{[3] [4]}. In isolated rat fat tissue, the 177-191 fragment stimulated hormone-sensitive lipase, the enzyme that releases fat from fat cells, and inhibited acetyl-CoA carboxylase, an enzyme needed to make fat, in a similar way to intact growth hormone ^[4].

The evidence is not uniform. In the 1993 study that first reported the fragment’s antilipogenic action, 177-191 reduced fat synthesis in the same way as whole growth hormone but had no significant lipolytic effect, measured by glycerol release from fat pads of treated rats; the authors concluded that the main effect on fat was on lipogenesis rather than lipolysis ^[3]. Later papers reported both effects ^{[4] [5]}. The molecular target has never been identified. The FDA noted in its 2024 review of AOD-9604 that the fragment does not act through the growth hormone receptor and that its mechanism remains unknown ^[7].

The earliest work on this part of the molecule found something different. The 1978 rat experiments showed that a single dose of 176-191 or its relatives produced a short-lived rise in blood glucose and a more sustained rise in insulin, and that peptides containing the 178-191 sequence reduced insulin sensitivity in intravenous insulin tolerance tests ^[1]. A 1982 follow-up located this insulin-antagonistic core within residues 178 to 190 ^[2], and a 1983 study found that 177-191 inactivated glycogen synthase in rat muscle ^[12]. The later obesity studies reported that chronic 177-191 and AOD-9604 did not cause insulin resistance ^{[4] [6]}, so the picture is that acute and chronic effects on glucose may differ, and the question has not been examined for 176-191 in any study since 1978.

There is no published study of HGH Fragment 176-191 and fat loss. The animal evidence that is usually cited belongs to the 177-191 fragment and to AOD-9604. In genetically obese ob/ob mice, chronic treatment with synthetic 177-191 reduced cumulative weight gain and fat mass and inhibited lipogenesis in fat tissue ^[5]. In obese Zucker rats, 20 days of the 177-191 fragment (AOD9401) reduced weight gain and shrank the average fat cell from 110 to 80 micrometres in diameter, without the insulin resistance or glucose intolerance seen with growth hormone ^[4]. Given by mouth to ob/ob mice for 30 days, AOD9401 slowed weight gain from day 16 onwards without changing food intake, and increased lipolysis in isolated fat tissue from obese rodents and from humans ^[6].

AOD-9604, the tyrosine-modified version, produced similar results in obese rats and mice: about half the weight gain of controls over 19 days of oral dosing at 500 µg/kg, more fat breakdown in fat tissue, and no worsening of insulin sensitivity on euglycaemic clamp testing ^{[8] [9] [10]}. These are the studies behind claims that HGH Frag 176-191 results in fat loss, and none of them used the 176-191 sequence.

The wider list of HGH Fragment 176-191 benefits found on vendor pages, such as improved sleep, cartilage repair, bone density and anti-ageing, has no published support for this fragment. The FDA found AOD-9604 marketed for a similar list of conditions and noted that its only human trials were in obesity ^[7]. A 2026 review of peptides sold in sports medicine likewise found rigorous human safety and efficacy data scarce across this class ^[14].

We found no clinical trial, case series or pharmacokinetic study of HGH Fragment 176-191 in humans, and no entry for it on ClinicalTrials.gov. The only human data in this family are for AOD-9604, and it is important to report them precisely because they are so often attributed to the 176-191 fragment.

Metabolic Pharmaceuticals ran six randomised, double-blind, placebo-controlled trials of AOD-9604 between roughly 2000 and 2007, using intravenous single doses in small groups of men and oral tablets in larger groups. The largest, a 24-week phase 2b study that enrolled 536 adults with obesity, found no significant difference in weight loss between AOD-9604 and placebo at the 12-week primary end point, and the company ended development for obesity in February 2007 ^[7]. The FDA summarised all six trials in 2024 and concluded that there was a lack of evidence that AOD-9604 is effective for obesity by any route, and that no study of subcutaneous injection, the route now sold, had been published ^[7]. Our AOD-9604 guide covers each trial in detail.

Two things follow. First, the trials tested a different peptide from the one sold as HGH Frag 176-191, so they cannot show that the fragment works. Second, even the tested peptide did not produce meaningful weight loss in people. Together these leave the claim that HGH Fragment 176-191 causes weight loss without human support from either direction.

Searches for HGH Fragment 176-191 weight loss results and HGH Fragment 176-191 before and after pictures return forum posts and vendor testimonials rather than data. These cannot show what a vial contained: Belgian authorities have seized unknown preparations that only laboratory analysis identified as AOD9604 ^[15]. Diet, training and other drugs usually change at the same time. The only controlled results for any related peptide come from rodents, and the only human trials, of AOD-9604, were negative ^[7].

For how long HGH Fragment 176-191 takes to work, there is no human answer. In obese rodents given the 177-191 fragment, reduced weight gain appeared after about two to three weeks of daily dosing ^{[4] [6]}. Those timelines describe rats and mice fed by mouth, not people injecting the 176-191 fragment.

Because 176-191 has no published dosing studies beyond a single 1978 rat experiment, in which peptides were given at 5 nmol/kg to test their effect on insulin sensitivity ^[1], the doses discussed online are extrapolated from the related peptides. In rodents, 177-191 and AOD-9604 were given orally at around 500 µg/kg per day for 19 to 30 days ^{[6] [8]}. In the human AOD-9604 trials, doses were 25 to 400 µg/kg intravenously as single doses, and 0.25 to 30 mg a day by mouth for 12 or 24 weeks ^[7]. None of these are HGH Fragment 176-191 doses, none used injection under the skin, and none are recommendations.

The HGH Fragment 176-191 dosage protocols found on vendor sites, typically a few hundred micrograms injected once or twice a day before meals or exercise, are not drawn from any trial. The FDA recorded the same “inject before eating” advice on AOD-9604 clinic websites as a marketing claim rather than a study finding ^[7]. If you are working out what a vial contains once mixed, the arithmetic is simple: a 5 mg vial dissolved in 2 mL of bacteriostatic water gives 2.5 mg per mL, so each unit on a U-100 insulin syringe holds 25 µg. Our peptide calculator and the HGH Fragment 176-191 calculator page do this sum for other vial sizes.

HGH Fragment 176-191 is sold almost entirely as a freeze-dried powder for injection under the skin, usually in 2 mg, 5 mg or 10 mg vials. This is the one route with no published data even for its relatives: the animal studies of 177-191 and AOD-9604 mostly used oral dosing, the human AOD-9604 trials used oral tablets and intravenous infusion, and the FDA found no published study of subcutaneous AOD-9604 ^[7]. Oral, nasal and cream forms of the fragment also appear for sale, with no data on whether a 16-amino-acid peptide survives the gut or crosses the nasal lining in useful amounts.

Half-life has never been measured for 176-191. For AOD-9604 it was about 3 minutes in pigs after an intravenous dose and about 4 minutes in rat blood, where the peptide was broken down step by step from its starting end ^[7]. A peptide differing by one amino acid at that same starting end is unlikely to behave very differently, but that is inference, not measurement.

No safety data exist for HGH Fragment 176-191 in people. The closest evidence is again for AOD-9604. The company’s summary of its trials described AOD-9604 as well tolerated, with headache the most common complaint on intravenous dosing and headache, diarrhoea and flatulence on oral dosing, no rise in IGF-1 and no effect on glucose tolerance ^[7]. The FDA’s reviewers were more cautious, noting that several cancers were reported in one 12-week oral trial and judged unrelated by the investigators, that the published information was too thin to confirm this, and that no safety data existed for injection under the skin or for long-term use ^[7]. In December 2024, the FDA’s compounding advisory committee voted 12 to 0 against allowing US pharmacies to compound AOD-9604 ^{[7] [16]}.

One consideration applies to 176-191 more than to AOD-9604. The original reason these fragments were synthesised was their effect on blood sugar: in rats, 176-191 caused a transient rise in glucose and a longer rise in insulin, and the 178-190 core reduced insulin sensitivity ^{[1] [2]}. Whether repeated injections of 176-191 affect blood sugar in people has never been examined. Anyone with diabetes or insulin resistance has no evidence to rely on either way.

As with any injected peptide bought outside the medicines system, the product itself is a risk. Certificates supplied with AOD-9604 nominations to the FDA reported purity but no results for impurities, aggregates, microbial limits or endotoxin ^[7], and the same gaps are common for HGH Frag 176-191 listings. A 2026 review of growth hormone-axis peptides sold as research compounds grouped AOD-9604 (which it called hGH 176-191) with products whose composition and dose are uncertain in unregulated supply ^[11].

HGH Fragment 176-191 is not an approved medicine in the US, UK, EU, Canada, Australia or New Zealand, and has never been submitted for approval. In the US, the FDA’s 2024 review and the advisory committee’s vote against compounding concerned AOD-9604 ^{[7] [16]}; the fragment 176-191 has no separate FDA file. See our US legal status page.

In sport, the World Anti-Doping Agency added growth hormone fragments to section S2 of the Prohibited List from the 2018 list, naming AOD-9604 and hGH 176-191 as examples; they are prohibited at all times, in and out of competition ^{[17] [18]}. Anti-doping laboratories screen urine for peptides under 2 kDa including AOD-9604 with detection limits below 1 ng/mL ^[19], and have characterised its breakdown products ^[20]. UK Anti-Doping applies the same list ^[21].

In the UK, the fragment has no marketing authorisation, and regulation 46 of the Human Medicines Regulations 2012 makes it an offence to sell or supply an unauthorised medicinal product ^[22]. It is not a controlled drug under the Misuse of Drugs Act 1971, whose Class C peptide hormone entries cover somatropin itself but not fragments of it ^[23]. Our UK legal status page explains how these rules are applied to research peptides, and the legal status overview covers other countries, including Australia, where AOD-9604 is listed in the Poisons Standard.

HGH Fragment 176-191 is supplied as a freeze-dried powder. Its disulfide bond can be broken and the peptide can aggregate, a point the FDA made about the near-identical AOD-9604 ^[7], so it should be kept cold, dry and away from light, and refrigerated once mixed. See how to store peptides.

PepFinder does not sell HGH Fragment 176-191. You can compare HGH Fragment 176-191 prices, including listings in the UK and the US, and see which vendors have third-party tested batches or published COAs.

Because the fragment and AOD-9604 differ by one amino acid and about 16 g/mol, a certificate of analysis should report a measured mass consistent with the peptide named on the label, around 1799 g/mol for the free 176-191 peptide against 1815 g/mol for AOD-9604 ^[13]. A mass spectrum is the only way to tell them apart. Read how to read a COA, third-party peptide testing and how to spot a fake peptide supplier before relying on a label. If fat loss is the goal, our peptides for weight loss page compares the compounds that have actually been tested in people.