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Muscle and Physical Function

A claim check on peptides for muscle growth and physical function: what trials of growth hormone, MK-677, tesamorelin and GLP-1 drugs found, and why hormone levels are not strength.

Content last updated . This page has not been medically reviewed.

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Quick answer
No peptide has been shown in the human trials we read to improve strength or physical performance in healthy people. Some products marketed for muscle, such as CJC-1295, ipamorelin, sermorelin and GHRP-6, raise growth hormone or IGF-1 levels in human studies, but a higher hormone level is a biomarker, not stronger muscle or better function. The trials that measured muscle are mostly of growth hormone itself or of MK-677, an oral compound that is not a peptide, and they do not transfer automatically to peptide products. Evidence label for peptide products promoted for muscle growth in healthy people: insufficient evidence, with marketing claim for most of the promotion.

Keep the molecules separate

  • Growth hormone is a large protein hormone, not a releasing peptide. Releasing peptides and mimics (sermorelin, CJC-1295, tesamorelin, ipamorelin, GHRP-6) act on the body's own growth hormone release. See peptides vs growth hormone.
  • MK-677 (ibutamoren) is an oral ghrelin mimic that is not a peptide. See MK-677 is not a peptide.
  • Tesamorelin is an approved growth hormone-releasing factor analogue with a narrow US label: reducing excess abdominal fat in HIV-infected adults with lipodystrophy. It is not indicated for weight loss, and its label says long-term cardiovascular safety has not been established. [1]

Evidence, one pairing at a time

PairingPopulation and designSize, comparator, lengthOutcome vs biomarkerAdverse events and limitsFunding
Growth hormone, healthy older adultsSystematic review of randomized trials [2]31 articles from 18 study populations; 220 treated people; mean 27 weeks [2]Lean mass up about 2.1 kg and fat down about 2.1 kg vs no treatment; bone density did not change [2]More swelling, joint pain, carpal tunnel and gynecomastia; small studies; authors said it cannot be recommended as an antiaging therapy [2]Not stated in abstract
Growth hormone, young fit peopleSystematic review of randomized trials [3]44 articles, 27 study samples, 303 treated people; mean 20 days [3]Lean mass up about 2.1 kg, but strength and exercise capacity did not seem to improve [3]More swelling and fatigue; few studies measured performance [3]Not stated in abstract
MK-677 (not a peptide), older adults2-year double-blind placebo-controlled modified-crossover trial [4]65 healthy adults aged 60 to 81 [4]Fat-free mass +1.1 kg vs -0.5 kg; the increase did not change strength or function [4]Blood glucose rose and insulin sensitivity fell; not powered for function [4]Not stated in abstract
CJC-1295 (long-acting GHRH analogue), healthy adultsTwo randomized placebo-controlled ascending-dose trials [5]Healthy people aged 21 to 61; 28 and 49 days [5]Biomarkers only: GH rose 2 to 10-fold and IGF-1 1.5 to 3-fold; no muscle or strength outcome [5]No serious adverse reactions reported in this short study [5]Not stated in abstract
Ipamorelin, surgery patients (not muscle)Phase 2 randomized, double-blind [6]117 enrolled; twice daily for up to 7 days [6]Time to tolerate a meal not significantly different from placebo [6]Small proof-of-concept study [6]Not stated in abstract

The CJC-1295 study is a useful example of why biomarkers mislead. The hormone rose, but the paper did not test whether muscle or function changed. [5] Higher GH or IGF-1 is not inherently better; see biomarker changes vs meaningful health outcomes.

What reviews say about promoted injectable peptides

A 2026 narrative review in the American Journal of Sports Medicine found that CJC-1295 combined with ipamorelin improved a muscle-force measure in mice with glucocorticoid-induced muscle loss, and that this finding was limited to animals. It also found that tesamorelin has no supporting orthopaedic evidence, and that data on indications, dosing, frequency and duration remain unknown. [7] A 2026 Sports Medicine review likewise said many unapproved peptides look favorable in animal models but rigorous human safety data are scarce. [8] We found no randomized trial of ipamorelin, CJC-1295, sermorelin, GHRP-6 or hexarelin for muscle or strength in a PubMed search on 2026-10-22; the one trial-type hit was a 1998 review of mechanism. [9] Evidence label: proper evidence not found for human muscle outcomes.

GLP-1 drugs and lean mass

Approved weight-loss peptides can also change muscle. A 2026 systematic review and meta-analysis of GLP-1 receptor agonists (60 articles, 46 randomized trials) found a consistent decrease in lean or fat-free mass, mainly with liraglutide and semaglutide, with low certainty and changes that appeared largely related to weight loss. Whether this harms muscle function is uncertain. [10] In the BMJ network meta-analysis, tirzepatide reduced fat mass by about 25.7% and lean mass by about 8.3%. [11] See the obesity evidence page and the Wellness guides on sarcopenia and muscle loss and activity, strength and muscle.

Frequently asked questions

Do growth hormone peptides build muscle?

Proper evidence not found for peptide products in healthy people. Growth hormone itself raised lean mass in trials, but strength did not clearly improve and side effects rose. [2][3]

Is a higher growth hormone or IGF-1 level a good sign?

Not by itself. In the CJC-1295 trials the levels rose, but nobody measured whether muscle or function improved. [5]

Is tesamorelin a muscle drug?

No. The US label is for excess abdominal fat in HIV-infected adults with lipodystrophy and it is not indicated for weight loss. [1]

Can approved weight-loss peptides cost muscle?

Meta-analysis shows lean mass falls with weight loss on GLP-1 drugs, with uncertain effect on function. [10]

Are these compounds allowed in sport?

The 2026 WADA list prohibits many of them. [12]

More details: evidence, limits and cautions

The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.

How to read the evidence labels

Labels used on this page: approved medicine (regulator and use named), established, mixed, insufficient evidence, preclinical (cells or animals only), no good evidence of benefit and proper evidence not found.

Sport rules and safety
The WADA 2026 Prohibited List names growth hormone releasing hormone analogues (including CJC-1295, sermorelin and tesamorelin), secretagogues (including ipamorelin and ibutamoren), and GH-releasing peptides (including GHRP-2, GHRP-6 and examorelin, also called hexarelin) under class S2. [12] Anti-doping status does not tell you whether a product is effective or approved. See anti-doping rules for peptides. The FDA category 2 page lists safety concerns for several of these compounds, including immunogenicity risk and, for CJC-1295, serious adverse events including increased heart rate and a systemic vasodilatory reaction. [13] Protein and strength training have their own evidence; see protein for muscle, training and recovery.
References
  1. U.S. National Library of Medicine, DailyMed. EGRIFTA WR (tesamorelin) for injection, FDA prescribing information (DailyMed listing, revised 3/2025). Read 2026-10-22.
  2. Systematic review: the safety and efficacy of growth hormone in the healthy elderly. Ann Intern Med 2007;146:104-115. Read 2026-10-22.
  3. Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med 2008;148:747-758. Read 2026-10-22.
  4. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med 2008;149:601-611. Read 2026-10-22.
  5. 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 2006;91:799-805. Read 2026-10-22.
  6. 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 2014;29:1527-1534. Read 2026-10-22.
  7. Injectable Peptide Therapy: A Primer for Orthopaedic and Sports Medicine Physicians. Am J Sports Med 2026;54:223-229. Read 2026-10-22.
  8. Mendias CL, Awan TM. Safety and Efficacy of Approved and Unapproved Peptide Therapies for Musculoskeletal Injuries and Athletic Performance. Sports Med 2026;56:1921-1935. Read 2026-10-22.
  9. U.S. National Library of Medicine. [PubMed search: (ipamorelin OR CJC-1295 OR sermorelin OR GHRP-6 OR hexarelin) AND (muscle OR sarcopenia OR strength) AND (randomized controlled trial[pt])](https://pubmed.ncbi.nlm.nih.gov/?term=%28ipamorelin%20OR%20CJC-1295%20OR%20sermorelin%20OR%20GHRP-6%20OR%20hexarelin%29%20AND%20%28muscle%20OR%20sarcopenia%20OR%20strength%29%20AND%20%28randomized%20controlled%20trial%5Bpt%5D%29). Search run 2026-10-22; result titles read.
  10. GLP-1 Receptor Agonists and Musculoskeletal Outcomes: A Systematic Literature Review and Meta-Analysis. Drugs 2026;86:1661-1691. Read 2026-10-22.
  11. Comparative effects of drugs for adults with overweight or obesity: systematic review and network meta-analysis. BMJ 2026;394:e372161. Read 2026-10-22.
  12. World Anti-Doping Agency. 2026 Prohibited List (in effect 1 January 2026). Read 2026-10-22.
  13. U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks (content current as of 04/22/2026). Read 2026-10-22.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.