Peptides for Muscle Growth: What the Evidence Actually Shows

Key Takeaways

• “Peptides for muscle growth” covers two very different product groups: oral collagen peptides, described in an early trial as designated foodstuff [1], and injectable signalling peptides such as growth hormone–releasing peptides (GHRPs), CJC-1295 and BPC-157.
• In a 2024 meta-analysis of randomized trials, collagen peptides combined with training increased fat-free mass versus placebo (SMD 0.48), with moderate certainty of evidence for body composition and low certainty for muscle architecture and strength [2].
• Fat-free mass is not the same as contractile muscle: in one trial, collagen raised fat-free mass while type II muscle fibre cross-sectional area increased in both groups without a significant between-group difference [3], and in the two head-to-head trials that measured muscle protein synthesis or muscle thickness, whey protein outperformed collagen [4][5].
• The CJC-1295 trial in healthy adults measured hormone levels, not muscle [6]. Growth hormone itself increased lean body mass in healthy young adults without improving strength, and the authors of one meta-analysis considered fluid retention a likely major contributor to that lean-mass gain [7][8].
• In a 2025 systematic review, 35 of 36 BPC-157 studies were preclinical, and no human clinical safety data were found [9].
• In my clinical experience, peptides come up only occasionally: athletes mostly use whey protein and creatine, injectable peptides have not come up in my own clinical work, and I am not aware of a medical indication for using them to build muscle. I see overall protein intake, rather than peptides, as the more useful focus.

Introduction: Why “Peptides for Muscle Growth” Means Two Different Things

Search for peptides for muscle growth and you will find two very different things sold under the same word.

On one side are collagen peptides: hydrolysed collagen powders, which an early trial described as designated foodstuff [1]. On the other side are injectable signalling peptides. These include growth hormone–releasing peptides, which are short peptides of 3–6 amino acids with GH-releasing properties [10], long-acting analogues of growth hormone–releasing hormone such as CJC-1295 [6], and BPC-157, a peptide that has been studied for musculoskeletal injury healing [9].

The two groups differ in how they are supposed to work, in their legal and anti-doping status, and above all in how much human evidence exists. This article goes through the human research on each group of peptides for muscle growth: what was measured, in whom, and what the results do and do not show.

In my clinical experience, peptides come up only occasionally, both in sports circles and at the clinic. The people who raise the topic have usually looked into it on their own. When patients do use peptides, it is most often collagen peptides taken as a dietary supplement, while injectable peptides rarely come up in my consultations. From a clinical perspective, this mirrors the two product groups described below.

Collagen Peptides for Muscle Growth: The Best-Studied Option

An early trial, described by its authors as the first controlled study of collagen peptides for fat-free mass and muscle strength, was a 12-week randomized, double-blind study in which 60 older men were randomized and 53 (mean age 72.2 years) completed the trial; all had class I or II sarcopenia [1]. All participants did supervised resistance training three times a week; one group took 15 g of collagen peptides daily and the other a silica placebo. Measured by DXA, fat-free mass increased by 4.2 kg with collagen versus 2.9 kg with placebo, isokinetic quadriceps strength increased by 16.5 Nm versus 7.3 Nm, and fat mass decreased by 5.4 kg versus 3.5 kg, all significant between-group differences [1].

Three details matter when reading this trial. The placebo provided no calories, so the collagen group also received extra energy; the authors acknowledged this but argued that extra calories alone were unlikely to explain the result [1]. The placebo group also started with relatively more fat-free mass and less fat mass, a non-significant baseline difference whose influence the authors could not fully rule out [1]. And a 2025 correction clarified that the supplement manufacturer, Gelita AG, provided the study product and paid for participant compensation and sample analysis, and that one author is named as co-inventor on a patent application relating to the tested substance [11].

A follow-up trial in 57 young, recreationally active men used the same 15 g/day dose during 12 weeks of resistance training [3]. Fat-free mass, measured by bioelectrical impedance, showed a significant group-by-time interaction favouring collagen: it rose from 70.1 to 72.1 kg in the collagen group and from 69.4 to 70.1 kg with placebo, although post-intervention values did not differ significantly between groups [3]. Strength increased in both groups without a significant between-group difference [3].

The most informative finding came from muscle biopsies in a subset of 21 participants: type II muscle fibre cross-sectional area increased in both groups, with no difference between them [3]. The authors therefore proposed that the extra fat-free mass with collagen may reflect connective-tissue adaptation rather than additional growth of contractile muscle fibres, a hypothesis they noted would need confirmation with direct measurement of these tissues [3]. This study was financially supported by the Collagen Research Institute, and Gelita provided the supplement [3].

The broadest summary of collagen peptides for muscle growth is a 2024 systematic review and meta-analysis in Sports Medicine covering 19 randomized trials and 768 participants [2]. In the eight trials (418 participants) that measured fat-free mass, collagen peptides combined with training produced a significant pooled effect (standardized mean difference 0.48), and certainty of evidence for body composition was graded moderate [2]. Effects on muscle architecture (SMD 0.39) and maximal strength (SMD 0.19) were statistically significant but rated low certainty, and muscle soreness after damaging exercise was not significantly affected [2].

Two methodological points from the same review are worth knowing. Six of the eight fat-free-mass trials used bioelectrical impedance rather than DXA, and nine of the 19 trials used a silica placebo that was not matched for calories [2]. The most common dose was 15 g per day, used in 12 trials [2].

Why Collagen Peptides Are Not a Like-for-Like Protein Swap

Like other dietary protein, collagen peptides are a source of amino acids. They are produced by hydrolysis of collagen [1]. What sets them apart for muscle is their amino acid profile: collagen peptides are generally considered low in essential amino acids and are often categorized as a low-quality protein [2], and the collagen peptides used in one trial provided only about 0.4 g of leucine per 15 g [12]. Three head-to-head trials show why this matters for muscle specifically.

In 22 healthy older women (mean age 69) who took 30 g of either whey or collagen twice daily for six days, whey increased both acute and longer-term muscle protein synthesis. With collagen, the longer-term rate was not significantly elevated above baseline in either the rested or the exercised leg, and longer-term synthesis was greater with whey in both conditions [4].

In a 10-week training study in 22 untrained young adults, 35 g of whey (containing 3.0 g of leucine) was compared with 35 g of collagen matched for leucine by adding 2.0 g of free leucine [5]. Whey produced larger increases in vastus lateralis thickness (8.4% vs 5.6%) and biceps brachii thickness (10.1% vs 6.0%), while strength and power improved in both groups with no significant between-group difference [5].

A three-arm trial in 97 untrained middle-aged men compared 15 g per day of collagen peptides, whey protein or placebo during 12 weeks of resistance training [12]. Fat-free mass, measured by DXA, increased by 3.42 kg with collagen, 2.27 kg with whey and 1.83 kg with placebo. Collagen differed significantly from placebo, whereas in the exploratory comparisons whey did not differ significantly from placebo and collagen did not differ significantly from whey [12]. The authors suggested that the 15 g whey dose may have been insufficient to induce optimal effects, although the trial did not test different whey doses; the study was partly funded by Gelita, which also provided the collagen peptides [12].

Put together, the collagen data describe something narrower than the marketing. In placebo-controlled trials, adding collagen to training tends to increase fat-free mass [2]. But fat-free mass includes passive tissue as well as contractile muscle [3]. In the biopsy subset of one trial (21 participants), collagen did not produce a significantly greater increase in muscle fibre cross-sectional area than placebo [3], and in head-to-head comparisons with whey, collagen was either inferior or not significantly different [4][5][12].

In my view, collagen peptides are essentially a protein supplement. They supply amino acids that muscles and other cells use as building material, and I do not see collagen-derived peptides as fundamentally different from the amino acids found in ordinary dietary protein. Whey protein may even have a more favourable composition for muscle growth, and in the head-to-head trials above, collagen did not appear superior to whey. For that reason, I do not see a clear reason to choose collagen peptides over whey protein, which is also the more commonly used option. In practice, choosing collagen peptides for muscle growth seems to me more of a refinement for its own sake than a meaningful advantage. For someone who has a problem with milk protein, collagen can be one alternative, but adequate protein intake can also be reached by eating ordinary protein from a variety of food sources.

Growth Hormone–Releasing Peptides for Muscle Growth: Hormone Changes, Little Muscle Data

Growth hormone–releasing peptides and CJC-1295, two types of injectable peptides for muscle growth, work differently. Rather than supplying amino acids, they are intended to increase the body’s own growth hormone secretion [10][6]. CJC-1295, for example, is a long-acting analogue of growth hormone–releasing hormone [6].

The hormonal effect itself is real. In two randomized, placebo-controlled trials in healthy adults aged 21–61, a single CJC-1295 injection raised mean GH concentrations 2- to 10-fold for six days or more, and IGF-I 1.5- to 3-fold for 9–11 days [6]. The main outcome measures, however, were GH and IGF-I concentrations and pharmacokinetics, not muscle mass or strength [6].

That raises an indirect question: if a peptide does raise growth hormone, what does more growth hormone do for muscle in healthy people? Here the evidence is better, and it is sobering.

A systematic review of randomized trials in healthy, physically fit participants aged 13–45, in which 303 participants received GH, found that lean body mass increased by 2.1 kg but strength and exercise capacity did not appear to improve; GH-treated participants more often experienced soft-tissue oedema and fatigue [7]. A later meta-analysis of 11 placebo-controlled trials in healthy young adults found a lean body mass increase of 2.86 kg alongside a 1.77 kg increase in extracellular water, with no improvement in muscle strength or maximal oxygen uptake; the only study that assessed anaerobic exercise capacity reported an improvement [8]. Its authors considered it likely that fluid retention accounts for a major share of the lean-mass increase seen in GH studies of healthy people [8]. Both reviews note that the doses studied may differ from those used in sport [7][8].

Longer-term controlled data exist for MK-677 (ibutamoren), an oral ghrelin mimetic [13] that the anti-doping literature describes as a non-peptidyl growth hormone secretagogue developed to stimulate GH release [14]. In a two-year randomized, placebo-controlled trial in 65 healthy adults aged 60–81, 25 mg daily for 12 months increased fat-free mass by 1.1 kg, compared with a 0.5 kg decrease on placebo, but the gain did not translate into changes in strength or function [13]. Body weight rose by 2.7 kg (0.8 kg with placebo), fasting glucose rose by an average of 0.3 mmol/L, insulin sensitivity decreased, and the most frequent side effects were increased appetite and transient, mild lower-limb oedema and muscle pain [13]. The authors noted that the study was not powered to evaluate functional outcomes [13].

Injectable peptides, by contrast, have not come up in my own clinical work, and patients rarely mention them. In my experience, injected substances for muscle growth are discussed mainly in connection with anabolic steroids and other anabolic agents, which are prohibited in sport. I am not currently aware of medical indications for using these peptides to build muscle. In primary care they are rarely, if ever, seen, and they were hardly discussed during my medical training.

Laboratory tests related to the anabolic and growth hormone axis are also measured fairly rarely in practice. In endocrinology, IGF-1 and related tests may sometimes be ordered, but in primary care and occupational health I seldom use them, because I do not see a good clinical reason to measure them outside specific endocrine contexts. In my clinical experience, I have not come across a good medical reason for anyone to start injecting these substances on their own.

BPC-157: A Healing Peptide With Almost No Human Data

In the orthopaedic sports medicine literature, BPC-157 has been studied for injury healing rather than muscle growth. A 2025 systematic review in HSS Journal identified 36 relevant studies published between 1993 and 2024: 35 were preclinical and one was a clinical study [9]. In animal models, BPC-157 improved functional, structural and biomechanical outcomes after muscle, tendon, ligament and bone injuries [9].

The single human study reported that 7 of 12 people with chronic knee pain felt relief lasting more than six months after one BPC-157 knee injection [9]. Since that was the only clinical study, no study in the review examined muscle growth in humans. The review reported a half-life of less than 30 minutes, no adverse effects in preclinical safety studies, and no clinical safety data [9]. In a pharmacokinetic study in rats and dogs, the elimination half-life of BPC-157 was under 30 minutes after intravenous or intramuscular dosing; that study included no human data [15]. The review also notes that, at the time of writing, BPC-157 was not FDA-approved and was banned in some sports, and the authors classified the underlying evidence as level IV and level V [9].

Product Quality and Anti-Doping Status

BPC-157, for example, was not approved by the US Food and Drug Administration at the time of the 2025 review [9], and anti-doping laboratories have documented problems with the contents of black-market products [16][10].

One case report described a black-market injection vial containing His-tagged Long-R3-IGF-I, a form usually produced for biochemical research, whose effects in humans have not been described; the authors also cite earlier reports in which black-market products did not contain the declared substances [16]. A 2018 analysis of growth-promoting black-market products identified a modified form of growth hormone carrying an extra amino acid, as well as glycine-extended analogues of GHRP-6, GHRP-2 and ipamorelin [10].

For tested athletes, there is a second issue. GHRPs and growth hormone secretagogues, including ipamorelin and ibutamoren, have been listed on the World Anti-Doping Agency Prohibited List [14], and growth hormone itself has been prohibited at all times, in and out of competition [8]. Because the list is updated regularly, the current WADA Prohibited List is the authoritative source for the status of any individual substance.

In my experience, athletes mainly use whey protein and creatine, while peptides seem to enter the picture more in bodybuilding circles, where people are always looking for a small edge over others. Precise counting and optimisation of macronutrients also belongs, in my experience, mainly to bodybuilding and various strength sports. In combat sports, where I have my own background, and in more traditional recreational sports, this kind of detailed macronutrient optimisation comes up less often.

How to Read Claims About Peptides for Muscle Growth

The studies above suggest a few questions that separate a meaningful finding from a marketing claim about peptides for muscle growth:

• What was measured? A rise in fat-free mass is not the same as growth of muscle fibres [3], and a rise in GH or IGF-I is not the same as more muscle or strength [6][13].
• What was the comparator? Many collagen trials used a non-caloric placebo [1][2], which is a different question from testing collagen against another protein; against whey, collagen did not match it for muscle thickness [5].
• Who was studied? Results in older men with sarcopenia [1] do not automatically carry over to trained young adults [3].
• Humans or animals? For BPC-157, almost all available evidence comes from preclinical studies [9].
• Who funded it? Several key collagen trials involved the supplement manufacturer [11][3][12].
• Is it permitted in sport? GHRPs and secretagogues have been listed on the WADA Prohibited List [14].

In my experience, the use of collagen peptides is fairly uncommon and rarely comes up in conversation, especially compared with whey protein, which is considerably more common. Training, protein powders and shakers are widely used and familiar to many people, whereas peptides tend to attract a more niche group. From what I have seen, much of what that group knows about peptides comes not from research but mainly from experience-based internet discussion and word of mouth, which is one reason questions like the ones above can be useful.

Conclusion: Peptides for Muscle Growth Are Mostly a Collagen Story

When the research on peptides for muscle growth is sorted by evidence rather than by marketing, it splits cleanly. Collagen peptides have a body of randomized trials showing a moderate-certainty increase in fat-free mass when combined with training [2], but in the biopsy subset of the one trial that measured muscle fibre size directly, collagen did not produce a significantly greater increase in fibre cross-sectional area than placebo [3], although the meta-analysis found a small, low-certainty effect on muscle architecture [2]. In head-to-head trials, collagen fell behind whey for muscle protein synthesis and muscle thickness [4][5].

For the injectable compounds discussed here, direct human evidence on muscle growth is much more limited. CJC-1295 can raise growth hormone and IGF-I [6], yet growth hormone itself did not improve strength in trials of healthy young adults [7][8]. BPC-157 has very little human data [9], and concerns about product composition and the prohibited status of several of these substances in sport add further reasons for caution [16][14]. In my own clinical work, injectable peptides have not come up, and I am not aware of a medical indication for using them to build muscle.

If a patient asked me about peptides for muscle growth, I would explain that peptides do not appear to be any more special as nutrition than other protein, and that, if anything, collagen peptides may be a weaker option than whey protein. In my experience, what matters more is getting enough protein overall and keeping things simple, and for people who can use whey protein, I see it as the more straightforward choice. In my view, whey and ordinary protein from food already have a good amino acid composition, and optimising that composition further does not usually seem to lead to better results. Plant proteins may be an exception: in one analysis of commercially available protein isolates, plant-based proteins contained less leucine on average than animal-based proteins, with considerable variation between sources [17], and in my view they may benefit from added leucine; the evidence on this is discussed in our article on the leucine threshold. Overall, I do not see a good reason to optimise amino acid composition separately by taking peptides; in my experience, the amount of protein is the more useful focus. In my view, nature usually does this job better than we can engineer it.

References

1. https://doi.org/10.1017/S0007114515002810
2. https://doi.org/10.1007/s40279-024-02079-0
3. https://doi.org/10.3390/nu11051154
4. https://doi.org/10.1093/ajcn/nqz332
5. https://doi.org/10.1123/ijsnem.2021-0265
6. https://doi.org/10.1210/jc.2005-1536
7. https://doi.org/10.7326/0003-4819-148-10-200805200-00215
8. https://doi.org/10.1016/j.ghir.2017.05.005
9. https://doi.org/10.1177/15563316251355551
10. https://doi.org/10.1016/j.ghir.2018.05.001
11. https://doi.org/10.1017/S000711452510425X
12. https://doi.org/10.3390/ijerph18094837
13. https://doi.org/10.7326/0003-4819-149-9-200811040-00003
14. https://doi.org/10.5478/MSL.2016.7.3.55
15. https://doi.org/10.3389/fphar.2022.1026182
16. https://doi.org/10.1016/j.ghir.2010.07.001
17. https://doi.org/10.1007/s00726-018-2640-5