Peptides for Muscle Growth: What Human Trials Show for MK-677, CJC-1295 and More

No peptide has been shown in human trials to make healthy people stronger. The peptides sold for muscle mostly work by raising growth hormone, and in controlled trials growth hormone and the growth hormone pill MK-677 added 1 to 2 kg of lean mass without improving strength or physical function.^[1][3] MK-677’s largest patient trial was stopped early over a heart-failure signal.^[4] CJC-1295 and ipamorelin have been tested for hormone levels but not for muscle, IGF-1 LR3 has no published trial in people, and follistatin is not available as an approved drug at all.^[6][10] None is approved for building muscle, and all are banned in sport.^[15]

The evidence at a glance

Growth hormone: the benchmark the peptides are chasing

Most muscle peptides are sold on the promise that they raise growth hormone, so the hormone’s own record sets the ceiling. A systematic review of 27 study groups of healthy, fit young adults found growth hormone added about 2.1 kg of lean body mass, but strength and exercise capacity did not seem to improve, and users more often had fluid retention and fatigue.^[1] In healthy older adults, a second review found the same lean-mass gain alongside more swelling, joint pain, carpal tunnel syndrome and breast enlargement in men, and concluded growth hormone could not be recommended as an anti-aging therapy.^[2]

Federal law makes it a crime to distribute growth hormone for anything other than an authorized medical use ordered by a physician.^[17] That is part of why peptides that release it are marketed as an alternative. The comparison is covered in MK-677 vs HGH.

MK-677 (ibutamoren): more mass, no more strength, a heart-failure signal

MK-677 is the most studied compound in this group, and it is a pill: a non-peptide molecule that acts on the ghrelin receptor.^[18] In a two-year randomized trial in 65 healthy adults aged 60 to 81, it raised growth hormone and IGF-1 to young-adult levels. Fat-free mass rose 1.1 kg while it fell 0.5 kg on placebo, but the gain did not change strength or function. Body weight rose 2.7 kg, fasting blood sugar went up and insulin sensitivity went down, and the most common side effects were increased appetite, mild leg swelling and muscle pain.^[3]

In 123 older adults recovering from hip fracture, IGF-1 rose but most measures of physical function did not improve, and the trial was stopped early because of a safety signal of congestive heart failure in some patients. The authors judged the safety profile unfavorable in that population.^[4] FDA cites that trial in listing ibutamoren as a compounding safety risk.^[5] A 2026 sports-medicine review of six popular recovery peptides singled out MK-677 for significant risks including heart failure.^[13]

CJC-1295 and ipamorelin: hormone levels, not muscle

CJC-1295 is a long-acting copy of the hormone that tells the pituitary to release growth hormone. In healthy adults, a single injection raised growth hormone 2- to 10-fold for six days or more and IGF-1 1.5- to 3-fold for nine to 11 days, with no serious adverse reactions reported.^[6] That trial measured hormones. It did not measure muscle, strength or body composition. FDA’s safety-risk page records serious adverse events with CJC-1295, including an increased heart rate and a body-wide vasodilatory reaction, and calls the clinical data limited.^[5]

Ipamorelin copies ghrelin’s effect on growth hormone release. It has no muscle trial in people. FDA records serious adverse events, including death, in a study that gave it intravenously to improve gut motility, and says it has no safety information for other injection routes.^[5] The GHRP-2 and GHRP-6 peptides sold alongside it also sit on FDA’s safety-risk list, with recorded effects on cortisol, blood sugar and insulin needs.^[5] Clinical reviewers list raised prolactin and cortisol, blood-sugar problems, fluid retention, joint and muscle pain and injection-site reactions among the effects reported with this whole family.^[14] The pair is compared in CJC-1295 vs ipamorelin.

Tesamorelin: approved, but not for muscle

Tesamorelin is the one growth hormone-releasing peptide FDA has approved, as Egrifta WR, and only for reducing excess abdominal fat in adults with HIV-related lipodystrophy. Its label says it is not indicated for weight loss because its effect on weight is neutral.^[8] In the pivotal trial of 412 patients, visceral fat fell 15.2% over 26 weeks against a 5.0% rise on placebo, and IGF-1 rose 81%.^[7] Muscle growth was not an approved claim. The approved peptide and its unapproved cousin are compared in sermorelin vs tesamorelin.

IGF-1 LR3 and mechano growth factor: no human trials

IGF-1 LR3 is a lab-modified version of IGF-1, the growth factor that carries much of growth hormone’s signal to tissue. It has no published clinical trial in people. The approved IGF-1 drug, mecasermin (Increlex), is for children with severe primary IGF-1 deficiency. Its label warns of severe low blood sugar leading to seizures and rules it out for children with cancer or a history of it.^[9] For pegylated mechano growth factor, FDA says it has found no human exposure data by any route.^[5]

Follistatin: a gene-therapy experiment and a black-market label

Follistatin binds myostatin, a signal that limits muscle growth. No approved follistatin drug exists.^[10] The human data come from gene therapy: in a small trial, six men with Becker muscular dystrophy had a virus carrying the follistatin gene injected into their thigh muscles, and four walked farther afterward, with no adverse effects reported.^[11] That is a one-time genetic treatment for a muscle-wasting disease, not a vial of peptide. When doping-control chemists tested 17 black-market products labeled follistatin, only nine contained it, and some of the rest held growth-promoting peptides such as GHRP-2 or MGF instead.^[10] Myostatin-blocking drugs are being tested properly in trials; one, given with tirzepatide, is covered in a drug to protect muscle.

What else is in the vial

Because none of these products is approved for muscle, they are sold as research chemicals, and what is inside is uncertain. Doping laboratories have found modified versions of GHRP-2, GHRP-6 and ipamorelin, each with an extra amino acid, in black-market products.^[12] A 2026 scoping review found that two-thirds of the published research on six popular recovery peptides came from animal models, and that the few human studies mostly lacked proper controls.^[13] The broader quality and safety record is in are peptides safe, and the human tendon and joint evidence for BPC-157 is in BPC-157 for joint and tendon pain.

Legal status and sport

None of these peptides is among the six covered by the HHS policy of October 28, 2026, under which HHS does not intend to take enforcement action against 503A pharmacies that compound them (BPC-157, TB-500, Epitalon, Semax, KPV and MOTS-c).^[16] Tesamorelin is available only as the approved drug, for its approved use.^[8] The World Anti-Doping Agency bans growth hormone, the GHRH analogs including CJC-1295 and tesamorelin, the secretagogues including MK-677 and ipamorelin, GHRP-2 and GHRP-6, IGF-1 and its analogs, and mechano growth factors under S2, and follistatin under S4, at all times.^[15]