Short answer: no peptide has been shown to build usable muscle in a healthy adult. MK-677 is the only one in this group with a two-year randomised human trial — it added about 1.1 kg of fat-free mass, but produced no gain in strength or physical function, and it raised fasting glucose while lowering insulin sensitivity. Everything else on the usual “best peptides” lists rests on hormone measurements, animal work, or nothing at all.
“Best” here means most studied and most discussed — not best proven. Below, each of six compounds gets the same treatment: what the trials actually measured, the dose and schedule those trials used, how long they ran, the side effects that were recorded, and an evidence tier so the gap between a human randomised trial, a mouse study and an anecdote stays visible.
This is a research-use-only educational reference. It is not medical advice, not a recommendation to use, take or buy any compound, and none of the peptides discussed is approved to build muscle in healthy people.
Comparison at a glance
| Peptide | What it’s studied for (re: muscle) | Evidence tier | Key caveat |
|---|---|---|---|
| MK-677 (ibutamoren) | Ghrelin-receptor agonist; raises GH/IGF-1; increased fat-free mass in a 2-year RCT | Moderate (human RCT) | No gain in strength or function; raised fasting glucose, lowered insulin sensitivity |
| Sermorelin | GHRH analog; raises IGF-1; small lean-mass rise (men only) in a related-analog trial | Weak human | Tiny studies; no effect in women; muscle outcome minimal |
| CJC-1295 | Long-acting GHRH analog; sustained rise in GH/IGF-1 in healthy adults | Indirect only | Human trial measured hormones, not muscle mass or strength |
| Ipamorelin | Selective GH secretagogue (MK-677-like mechanism) | Preclinical / none | No human muscle-growth trial; human use studied mainly for gut motility |
| IGF-1 LR3 | Long-acting IGF-1 analog; muscle hypertrophy in animal overexpression models | Animal / preclinical | No human trials; systemic IGF-1 signaling carries theoretical risk |
| Follistatin-344 | Myostatin antagonist; large muscle gains in animals | Preclinical peptide; human data only via gene therapy | Human benefit shown only with AAV gene therapy in disease, not the injectable peptide |
MK-677 (ibutamoren)
MK-677 is an orally active ghrelin-receptor agonist that raises growth hormone (GH) and IGF-1, and it has the strongest human data of this group. In a 2-year randomized, placebo-controlled trial of 65 healthy older adults, 25 mg daily raised GH and IGF-1 into the young-adult range and increased fat-free mass by about 1.1 kg versus a 0.5 kg loss on placebo. Crucially, the authors reported that this extra fat-free mass did not translate into any gain in strength or physical function, and MK-677 also increased appetite, raised fasting glucose, and reduced insulin sensitivity (Nass et al., 2008). So the honest read is “measurable body-composition change, unproven performance benefit, real metabolic trade-offs.” For how it is characterized in research settings, see the MK-677 dosage reference.
Sermorelin
Sermorelin is a growth-hormone-releasing hormone (GHRH 1-29) analog that prompts the pituitary to release its own GH. In a small 16-week randomized trial of a closely related GHRH analog in older adults, treatment raised IGF-1 and increased lean body mass — but only in men, not women, and the sample was tiny (Khorram et al., 1997). A separate retrospective chart review of a GH-secretagogue/sermorelin combination raised average IGF-1 from roughly 160 to 239 ng/mL, but measured hormones rather than muscle (Sigalos et al., 2017). The evidence linking sermorelin to actual muscle growth is therefore weak and indirect. See the sermorelin dosage reference.
CJC-1295
CJC-1295 is a long-acting GHRH analog. In healthy adults, subcutaneous CJC-1295 produced sustained, dose-dependent increases in GH (roughly 2- to 10-fold) and IGF-1 for days to weeks (Teichman et al., 2006). The important caveat: that trial measured hormone levels and safety, not muscle mass, strength, or body composition — so there is no direct human evidence that CJC-1295 builds muscle. It is a pharmacology result, not a muscle-growth result. See the CJC-1295 dosage reference.
Ipamorelin
Ipamorelin is a selective GH secretagogue in the same mechanistic family as MK-677, but it lacks MK-677’s body-composition trial. Its documented human development was largely in gastrointestinal motility (for example, post-operative ileus), and its GH-releasing effect is characterized mainly in preclinical work. No controlled human trial has tested ipamorelin for muscle growth, so any muscle claim is extrapolation from mechanism and anecdote, not clinical evidence. See the ipamorelin dosage reference.
IGF-1 LR3
IGF-1 LR3 is a long-acting laboratory analog of IGF-1. There are no human trials of IGF-1 LR3 for muscle growth; it is primarily used as a cell-culture reagent. The supporting science is preclinical: viral overexpression of IGF-1 in mouse muscle produced hypertrophy and greater strength, partly through satellite-cell activation (Barton-Davis et al., 1999). Animal hypertrophy does not establish that injected IGF-1 LR3 is effective or safe in people, and broad systemic IGF-1 signaling carries theoretical risks. See the IGF-1 LR3 dosage reference.
Follistatin (Follistatin-344)
Follistatin antagonizes myostatin, a natural brake on muscle growth, and in animals follistatin can dramatically increase muscle mass. The only human muscle data comes from gene therapy, not the injectable peptide: an AAV1-delivered follistatin (FS344) trial in sporadic inclusion-body myositis improved 6-minute walk distance (+56 m/year versus −25.8 m/year in untreated matched patients) in a small, non-blinded study (Mendell et al., 2017). Reviews also note that several anti-myostatin drugs showed limited efficacy in trials (Mariot et al., 2017). There is essentially no controlled human evidence that injectable follistatin peptide builds muscle in healthy people. See the follistatin-344 dosage reference.
What doses and schedules the studies actually used
This is the question readers ask first, so it is worth being exact. The figures below are the doses used in the published trials cited on this page. They describe how the research was conducted — they are not a protocol for anyone to follow.
| Compound | Dose used in the study | Route & schedule | Study length |
|---|---|---|---|
| MK-677 (ibutamoren) | 25 mg | Oral, once daily | 2 years; primary endpoints at 12 months |
| GHRH(1-29) analog (sermorelin class) | 10 µg/kg | Subcutaneous, self-injected nightly | 16 weeks, after 4 weeks of placebo |
| CJC-1295 | 30–60 µg/kg were the best-tolerated doses | Subcutaneous; single dose, then weekly or biweekly | 28 and 49 days (two trials) |
| Ipamorelin | No human muscle-growth trial exists | ||
| IGF-1 LR3 | No human trial exists; used as a cell-culture reagent | ||
| Follistatin | Human data only from AAV1-delivered gene therapy, not the injectable peptide | ||
Half that table is empty, and that is the finding. For three of the six compounds most often recommended for muscle, there is no human dose to report because there has been no human muscle study. Reference figures for how each compound is expressed and reconstituted in research settings are on the individual dosage pages linked in each section above.
How long before anything changed?
Where change was measured at all, it was slow and modest.
- MK-677: GH and IGF-1 rose quickly, but body composition was assessed at six-month intervals — the fat-free-mass difference is a 12-month result, confirmed at two years. Body weight rose 2.7 kg on MK-677 versus 0.8 kg on placebo, and part of that was limb fat, which increased more on MK-677 than on placebo.
- GHRH analog: IGF-1 rose within two weeks; the lean-mass change, in men only, was measured at 16 weeks.
- CJC-1295: a single injection raised GH for six days or more and IGF-1 for 9–11 days. Nothing about muscle was measured at any timepoint.
The pattern is consistent across the three: hormones move within days, body composition moves over months if it moves at all, and function did not improve in the one trial that bothered to measure it.
Side effects recorded in these trials
These are effects that were observed and reported in the studies cited on this page. This is not a complete safety profile, and it is not a list of everything that could happen.
- MK-677: increased appetite, which subsided over a few months; transient, mild swelling of the lower legs; and muscle pain. Fasting blood glucose rose by roughly 5 mg/dL and insulin sensitivity fell. Cortisol also rose. Those are metabolic trade-offs, not cosmetic ones.
- GHRH(1-29) analog: the only adverse effect reported was transient hyperlipidemia, which resolved by the end of the study. Skin thickness increased in both men and women.
- CJC-1295: no serious adverse reactions over 28–49 days, and the authors described 30 and 60 µg/kg as relatively well tolerated. Long-term safety was not assessed.
- Ipamorelin, IGF-1 LR3, injectable follistatin: no controlled human safety data for muscle-growth use. Nothing appears here because nothing was studied — absence of reported side effects is not evidence of safety.
What about stacking them?
Combination protocols — usually a GHRH analog paired with a GH secretagogue — are widely discussed, and pre-mixed blends are sold as single vials. The honest position is short: no published trial has tested any of these combinations for muscle growth in humans. The rationale is mechanistic, and it is reasonable pharmacology: a GHRH analog raises the pituitary’s readiness to release growth hormone, and a secretagogue triggers the pulse. Reasonable is not the same as demonstrated. Stacking also stacks the unknowns — two compounds with thin individual safety data have no combined safety data at all.
What the evidence actually supports
Honest bottom line: none of these six has robust clinical proof that it builds functional muscle in healthy adults.
- Best-evidenced (still limited): MK-677 is the only one with a multi-year human RCT showing a fat-free-mass increase — but that gain did not improve strength or function, and it worsened glucose control.
- Weak / indirect human data: sermorelin and CJC-1295 reliably raise GH and IGF-1 in people, but raising a hormone is not the same as growing muscle; the muscle-specific outcomes are small, inconsistent, or simply not measured.
- Preclinical or anecdotal only: ipamorelin, IGF-1 LR3, and injectable follistatin have no controlled human muscle-growth trials. Their reputation rests on mechanism, animal studies, or gene-therapy work in disease — not on evidence that they work, or are safe, for physique goals.
Important limitations
- Every compound here is sold and discussed for research use only. Most are not approved by any regulator for muscle growth, and several are prohibited in competitive sport.
- “Most studied” is not “proven for you.” Individual results, long-term safety, product purity, and interactions are unknown, and some carry documented downsides (for example, MK-677’s effect on blood glucose).
- This overview summarizes published research; it is not medical advice and not a recommendation to use, take, or buy anything. Decisions about your health belong with a qualified healthcare professional who knows your history.
- If you are comparing figures for study or reference purposes only, our peptide dosage calculator is an educational tool, not a prescription.
FAQ
What is the single best peptide for muscle growth?
There isn’t one with strong proof. Of the commonly named options, MK-677 has the most human data, but even its randomized trial found increased fat-free mass without any measured gain in strength or function — so “best studied” still falls short of “proven to build usable muscle.”
Do peptides that raise GH or IGF-1 automatically build muscle?
No. Compounds like CJC-1295 and sermorelin clearly raise GH and IGF-1 in humans, but the trials that show those hormone changes did not demonstrate meaningful muscle growth. Elevated hormones are a mechanism, not a guaranteed outcome.
Are any of these approved or safe for bodybuilding?
No. None is approved for muscle building in healthy people, several are banned in competition, and safety data for this purpose are limited or absent. They are research compounds, and any decision about them is a matter for a qualified clinician, not a website.
How much did the studies use, and for how long?
Only three of the six have any human dosing to report: MK-677 at 25 mg orally once daily for up to two years; a sermorelin-class GHRH analog at 10 µg/kg injected nightly for 16 weeks; and CJC-1295 at 30–60 µg/kg subcutaneously in trials lasting 28 to 49 days. Those are study conditions, not instructions.
What side effects showed up in the trials?
The clearest signal is metabolic. MK-677 raised fasting glucose and reduced insulin sensitivity, alongside increased appetite, mild leg swelling and muscle pain. The GHRH-analog trial reported only transient hyperlipidemia, which resolved. For ipamorelin, IGF-1 LR3 and injectable follistatin there is no controlled human safety data for this purpose at all.
How long would it take to see anything?
In the one trial that measured body composition, the fat-free-mass difference was a 12-month result — and it arrived with no improvement in strength or function. Hormone levels change within days; body composition, where it changed, took months.
Do peptide stacks work better than a single compound?
Unknown. No published human trial has tested a GHRH-analog-plus-secretagogue stack for muscle growth. The combination is mechanistically plausible and entirely unproven, and it compounds the safety unknowns of each part.
Sources
Concrete claims above are drawn from peer-reviewed literature indexed in PubMed:
- Nass R, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults. Ann Intern Med. 2008. doi:10.7326/0003-4819-149-9-200811040-00003 (PMID 18981485)
- Teichman SL, et al. Prolonged stimulation of GH and IGF-1 secretion by CJC-1295. J Clin Endocrinol Metab. 2006. doi:10.1210/jc.2005-1536 (PMID 16352683)
- Khorram O, et al. Endocrine and metabolic effects of long-term administration of a GHRH analog in age-advanced men and women. J Clin Endocrinol Metab. 1997. doi:10.1210/jcem.82.5.3943 (PMID 9141536)
- Sigalos JT, et al. Growth hormone secretagogue treatment in hypogonadal men raises serum IGF-1 levels. Am J Mens Health. 2017. doi:10.1177/1557988317718662 (PMID 28830317)
- Barton-Davis ER, Shoturma DI, Sweeney HL. Contribution of satellite cells to IGF-I induced hypertrophy of skeletal muscle. Acta Physiol Scand. 1999. doi:10.1046/j.1365-201x.1999.00618.x (PMID 10632630)
- Mendell JR, et al. Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes. Mol Ther. 2017. doi:10.1016/j.ymthe.2017.02.015 (PMID 28279643)
- Mariot V, et al. Downregulation of myostatin pathway in neuromuscular diseases may explain challenges of anti-myostatin therapeutic approaches. Nat Commun. 2017. doi:10.1038/s41467-017-01486-4 (PMID 29192144)
- Cardaci TD, et al. LGD-4033 and MK-677 use impacts body composition and biomarkers: a case report. Exp Physiol. 2022. doi:10.1113/EP090741 (PMID 36303408)
