Peptides for Muscle Growth: What the Human Trials Show

The short answer

Muscle is built by training, food and time. The compounds sold as "muscle peptides" fall into three groups: growth hormone secretagogues, which make the pituitary release more of its own growth hormone; IGF-1 and its analogues; and blockers of myostatin, the protein that limits muscle growth. None is approved to build muscle in healthy people, and none has been shown in a controlled trial to make healthy people stronger.

The strongest test of the whole idea is growth hormone itself. A systematic review of randomised trials in young, fit adults found that growth hormone, given at an average of 36 micrograms per kilogram a day for an average of 20 days, added 2.1 kg of lean body mass, but strength and exercise capacity did not improve, and recipients had more soft-tissue swelling and fatigue.¹ In healthy older people, trials found about 2 kg more lean mass and 2 kg less fat, again with more swelling, joint pain and carpal tunnel syndrome.² Lean mass, as these studies measure it, includes water, and growth hormone makes the body retain fluid. If the hormone itself does not add strength, compounds sold to raise it have a high bar to clear, and none has been tested against it.

Growth hormone secretagogues: hormone levels, not muscle

These compounds raise growth hormone and IGF-1. That is what their human studies measured; none measured muscle strength in healthy people.

CJC-1295. A long-acting analogue of growth-hormone-releasing hormone. In healthy adults a single injection under the skin raised growth hormone two- to ten-fold for six days or more and IGF-1 1.5- to three-fold for 9 to 11 days, with an estimated half-life of 5.8 to 8.1 days, and the trial found it best tolerated at 30 or 60 micrograms per kilogram.³ FDA reports having identified serious adverse events associated with it, including increased heart rate and a systemic vasodilatory reaction, and notes that clinical data are limited.⁴ No trial has measured muscle, strength or performance.

Ipamorelin. A ghrelin mimetic. Its one controlled human trial was in 114 patients recovering from bowel surgery: time to the first tolerated meal was 25.3 hours on ipamorelin and 32.6 hours on placebo, a difference that was not significant, and no other efficacy measure differed.⁵ There is no trial of ipamorelin for muscle, and none of its popular pairing with CJC-1295.

MK-677 (ibutamoren). An oral ghrelin mimetic and a small molecule, not a peptide, though it is sold beside them. It has the longest trial in this group: 65 healthy adults aged 60 to 81 took it or placebo for up to two years. Growth hormone and IGF-1 rose to young-adult levels, and fat-free mass rose by 1.1 kg against a 0.5 kg fall on placebo, but strength and function did not change. On 25 mg by mouth once a day, body weight rose 2.7 kg, limb fat increased, fasting glucose rose by 0.3 mmol/L and insulin sensitivity fell; appetite, leg swelling and muscle pain were the commonest side effects.⁶ A trial in older adults recovering from hip fracture was stopped early over a signal of congestive heart failure.⁷

Tesamorelin. The one approved drug in the group, and not for muscle: it is approved to reduce excess abdominal fat in adults with HIV. In two phase 3 trials of 806 patients, 2 mg injected under the skin daily cut visceral fat by 15.4% more than placebo at 26 weeks.⁸ The lean-mass question has an answer too: a 2026 meta-analysis of five randomised trials in people with HIV found tesamorelin increased lean mass by about 1.4 kg while reducing visceral, trunk and liver fat, with no change in BMI or in fat under the skin.⁹ That is a real gain, but it was measured in a specific patient group treated for abdominal fat, by body-composition scans rather than strength tests, and the label itself says the drug is weight-neutral and not for weight management; nothing in it shows that healthy people who want bigger muscles would see the same effect. Our tesamorelin guide covers the monitoring it needs, including IGF-1 checks. Its label warns of fluid retention, joint pain, carpal tunnel syndrome and glucose intolerance.¹⁰ Its current labels give 1.4 mg a day for EGRIFTA SV and 1.28 mg a day for EGRIFTA WR.^10,11

IGF-1 and its analogues

Growth hormone works partly through IGF-1, so IGF-1 itself is sold for muscle, usually as the modified forms IGF-1 LR3 and IGF-1 DES. The only approved IGF-1 drug is mecasermin (Increlex), licensed in the US since August 30, 2005, for children with severe primary IGF-1 deficiency, a rare growth disorder.¹² IGF-1 lowers blood sugar, which is why that treatment is started and monitored by specialists.

IGF-1 LR3 was designed as a laboratory reagent for cell culture. A PubMed search of clinical-trial records on September 30, 2026 found no trial of it in people; the single record returned was a rat cell study. Nothing is known about its effects, dose or safety in humans, and our IGF-1 LR3 entry lists what the laboratory work shows.

The myostatin pathway: where muscle drugs are actually tested

Myostatin is a signal that limits muscle growth, working through activin type II receptors. Blocking that pathway is the most serious effort to build muscle with a drug, and it is being pursued with antibodies and gene therapy, not with the peptides sold online.

Follistatin. Follistatin binds myostatin. Its only human trial was gene therapy: a virus carrying the FS344 form of follistatin was injected into the thigh muscles of six men with Becker muscular dystrophy. Four walked further on a six-minute walk test afterwards, by 58, 125, 108 and 29 metres, and two did not change.¹³ "Follistatin-344" sold as an injectable peptide is a different thing and has never been tested in people.

Bimagrumab. An antibody that blocks activin type II receptors. In a phase 2 trial in 75 adults with type 2 diabetes and obesity, 48 weeks of infusions every 4 weeks, at 10 mg per kilogram up to 1,200 mg, reduced fat mass by 20.5% and increased lean mass by 3.6%, against almost no change on placebo, and lowered HbA_1c by 0.76 percentage points.¹⁴ It is investigational, and its main interest now is preserving muscle during weight loss rather than adding it in healthy people. Our encyclopedia also covers the investigational antibodies trevogrumab and garetosmab and the myostatin-binding protein taldefgrobep alfa.

Legal status and sport

CJC-1295, ipamorelin, IGF-1 LR3 and follistatin peptides are not FDA-approved drugs, and our regulatory tracker follows FDA's decisions on which peptides pharmacies may compound. MK-677 is not an approved drug either. Growth hormone, tesamorelin and mecasermin are prescription medicines for specific diseases.

In sport, all of them are prohibited at all times. The World Anti-Doping Agency's 2026 list names CJC-1295, ipamorelin and ibutamoren (MK-677) under S2.2.4, growth-hormone-releasing factors; growth hormone under S2.2.3; IGF-1 and its analogues under S2.3; and follistatin and bimagrumab under S4.3, agents that prevent activin receptor IIB activation.¹⁵

What this means for building muscle

The interventions with trial evidence behind them are the ordinary ones. A meta-analysis of 49 trials with 1,863 participants found that adding protein during months of resistance training increased gains in one-repetition-maximum strength by 2.49 kg and in fat-free mass by 0.30 kg, with no further gain in fat-free mass above a total protein intake of about 1.62 g per kilogram of body weight a day.¹⁶ Those are modest numbers, and they are larger than anything a muscle peptide has shown in a controlled trial of healthy people.

For the peptides themselves, the evidence stops at hormone levels. The products are sold without the manufacturing controls of a medicine, so identity and purity are unknown until tested; our verification guide explains what a certificate of analysis can and cannot show. Anyone losing muscle because of illness, age or a medicine should be assessed by a clinician, because several of those conditions have treatments.

Frequently asked questions

What is the best peptide for muscle growth?

None has been shown in a controlled trial to make healthy people stronger, and none is approved for building muscle. Even growth hormone itself added lean mass without improving strength or exercise capacity in trials of young, fit adults.

Does MK-677 build muscle?

In a two-year trial in healthy adults aged 60 to 81, it added 1.1 kg of fat-free mass in a year with no change in strength or function, raised fasting glucose and reduced insulin sensitivity. It is a small molecule, not a peptide.

Do CJC-1295 and ipamorelin build muscle?

Nobody knows, because no trial has measured it. CJC-1295's studies measured growth hormone and IGF-1 levels, and ipamorelin's one controlled trial, in bowel surgery, found no benefit.

Is follistatin-344 tested in humans?

Only as gene therapy: six men with Becker muscular dystrophy received a virus carrying follistatin into their thigh muscles, and four walked further afterwards. Injectable follistatin peptide has never been tested in people.

Are muscle peptides allowed in sport?

No. WADA's 2026 list prohibits CJC-1295, ipamorelin and MK-677 (S2.2.4), growth hormone (S2.2.3), IGF-1 and its analogues (S2.3), and follistatin and myostatin-pathway blockers (S4.3) at all times.

Related on Grey Peptides

• CJC-1295 encyclopedia entry
• Ipamorelin encyclopedia entry
• MK-677 encyclopedia entry
• CJC-1295 complete guide
• Peptide side effects
• Regulatory tracker

Sources

1. Liu, H. et al. (2008). Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med, 148(10), 747-58. PMID: 18347346 · doi:10.7326/0003-4819-148-10-200805200-00215
2. Liu, H. et al. (2007). Systematic review: the safety and efficacy of growth hormone in the healthy elderly. Ann Intern Med, 146(2), 104-15. PMID: 17227934 · doi:10.7326/0003-4819-146-2-200701160-00005
3. Teichman, S. L. et al. (2006). Prolonged stimulation of growth hormone (GH) and insulin-like growth factor I secretion by CJC-1295, a long-acting analog of GH-releasing hormone, in healthy adults. J Clin Endocrinol Metab, 91(3), 799-805. PMID: 16352683
4. U.S. Food and Drug Administration. Certain bulk drug substances for use in compounding that may present significant safety risks: CJC-1295 (serious adverse events including increased heart rate and systemic vasodilatory reaction; clinical data limited). Content current as of April 22, 2026; read September 30, 2026. FDA
5. Beck, D. E. et al. (2014). Prospective, randomized, controlled, proof-of-concept study of the Ghrelin mimetic ipamorelin for the management of postoperative ileus in bowel resection patients. Int J Colorectal Dis, 29(12), 1527-34. PMID: 25331030
6. Nass, R. et al. (2008). Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med, 149(9), 601-11. PMID: 18981485
7. Adunsky, A. et al. (2011). MK-0677 (ibutamoren mesylate) for the treatment of patients recovering from hip fracture: a multicenter, randomized, placebo-controlled phase IIb study. Arch Gerontol Geriatr, 53(2), 183-9. PMID: 21067829
8. Falutz, J. et al. (2010). Effects of tesamorelin (TH9507), a growth hormone-releasing factor analog, in human immunodeficiency virus-infected patients with excess abdominal fat: a pooled analysis of two multicenter, double-blind placebo-controlled phase 3 trials with safety extension data. J Clin Endocrinol Metab, 95(9), 4291-304. PMID: 20554713 · doi:10.1210/jc.2010-0490
9. Badran, A. S., et al. (2026). Body composition, hepatic fat, metabolic, and safety outcomes of Tesamorelin, a GHRH analogue, in HIV-associated lipodystrophy: A meta-analysis of randomized controlled trials. Obes Res Clin Pract, 20(1), 2-12. PMID: 41545261
10. Theratechnologies Inc. EGRIFTA SV (tesamorelin) for injection, US prescribing information: section 5 (fluid retention with edema, arthralgia and carpal tunnel syndrome; glucose intolerance, HbA1c 6.5% or higher in 5% vs 1%; hypersensitivity; injection site reactions 25% vs 14% over 26 weeks; evaluate glucose before and during therapy). DailyMed version of July 31, 2026, read September 30, 2026.
11. Theratechnologies. EGRIFTA WR (tesamorelin) for injection, US prescribing information: section 2 (1.28 mg under the skin once daily; applies only to the 11.6 mg vial). DailyMed version 2, effective July 29, 2026, read September 30, 2026.
12. U.S. Food and Drug Administration. Drugs@FDA: Increlex (mecasermin), BLA 021839 (Ipsen), original approval August 30, 2005; marketing status prescription. Read September 30, 2026.
13. Mendell, J. R. et al. (2015). A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy. Mol Ther, 23(1), 192-201. PMID: 25322757 · doi:10.1038/mt.2014.200
14. Heymsfield, S. B. et al. (2021). Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity: A Phase 2 Randomized Clinical Trial. JAMA Netw Open, 4(1), e2033457. PMID: 33439265 · doi:10.1001/jamanetworkopen.2020.33457
15. World Anti-Doping Agency. The 2026 Prohibited List, in force January 1, 2026: S2.2.3 (growth hormone), S2.2.4 (growth hormone releasing factors, including CJC-1295, ipamorelin and ibutamoren), S2.3 (IGF-1 and its analogues), S4.3 (agents preventing activin receptor IIB activation, including follistatin and bimagrumab). Read September 30, 2026. WADA
16. Morton, R. W. et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med, 52(6), 376-384. PMID: 28698222

Educational information, not medical advice. Each dose in this guide names its source, an approved label or a published study. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Loss of muscle from illness, age or medicines should be assessed by a clinician, and untested compounds are not a substitute for treatment.