NOT FDA-APPROVED

FST-344 (Follistatin-344)

A lab-made version of a natural protein that blocks myostatin — the body's built-in "brake" on muscle growth. It's one of the most-talked-about compounds in muscle research, and also one of the most experimental in humans.

The 30-second read

FST-344 is a synthetic form of follistatin, a protein your body already makes. Its claim to fame is that it grabs onto myostatin — the molecule that tells your muscles to stop growing — and holds it back, effectively taking a brake off muscle growth. In animals, blocking myostatin reliably produces bigger, stronger muscles. The catch: almost none of that has been tested with the injectable FST-344 peptide in people. The most impressive human results actually came from a different technology (gene therapy), not the peptide being sold in vials. It's not FDA-approved, and the human safety picture is close to blank. Genuinely interesting biology, genuinely thin human evidence.

Why this peptide is on people's radar

Every muscle in your body has a built-in governor called myostatin. It exists to keep muscle from growing without limit, and it works well — which is exactly why people trying to build muscle find it so frustrating. Follistatin is one of the body's natural ways of turning that governor down. So the idea behind FST-344 is simple and, on paper, exciting: give the body more follistatin, quiet the myostatin brake, and let muscle grow more freely.

The excitement has real roots. There are famous cases of animals — and a few rare humans — born with naturally low myostatin who are strikingly, visibly muscular. Lab studies where myostatin is blocked show muscle gains of roughly 15–30%, sometimes without any exercise at all. That kind of headline travels fast through bodybuilding forums, longevity circles, and "muscle-preservation while aging" conversations.

The honest backdrop, and it's a big one: the strongest human data on follistatin didn't come from injecting the FST-344 peptide. It came from gene therapy trials in muscular-dystrophy patients, where researchers delivered the follistatin gene directly into muscle. That's a very different thing from the peptide in a vial, and it matters enormously for what we can and can't claim.

What people are usually trying to do with it

People looking into FST-344 are usually focused on:

Building muscle mass beyond what training alone seems to allow
Holding onto muscle while dieting or in a calorie deficit
Preserving muscle with age (the "sarcopenia" conversation)
Exploring the muscle-wasting research behind conditions like muscular dystrophy
Adding a myostatin-focused angle to a broader body-composition plan

What the science actually shows

The animal and mechanism data is strong. The human data almost all comes from gene therapy, not the injectable peptide. Plain-English summary:

Blocks myostatin and activin

Follistatin binds tightly to myostatin and a related molecule called activin, both of which normally limit muscle growth. Removing that limit is a well-established way to increase muscle size in the lab.1

Large muscle gains in animals

A widely cited 2009 mouse study using follistatin gene therapy reported roughly a 15% increase in muscle-fiber size in four weeks with no training, rising toward ~35% when paired with resistance exercise.2

Human gene-therapy trials (not the peptide)

Small Phase 1/2a trials delivered the follistatin gene into the muscles of Becker muscular dystrophy and inclusion-body myositis patients, and reported improved walking distance and healthier muscle tissue.3 4

No human trials of injectable FST-344

To date, no published human trial has tested the injectable FST-344 peptide for either safety or muscle-building effect. This is the single most important gap to understand.

Product quality is a real question

Independent testing of research-chemical peptides in this class has turned up wrong identity, contamination, and inconsistent amounts — so what's in a vial labeled "FST-344" isn't guaranteed.5

The honest read

What's solid:

The core biology. Follistatin really does inhibit myostatin, and blocking myostatin really does grow muscle in animals. The gene-therapy trials in dystrophy patients are a genuine scientific milestone.

What's still unproven:

Whether the injectable FST-344 peptide does anything meaningful in a healthy human. There are no human trials of the peptide, no established dose, and no human safety data. The dramatic results people cite come from gene therapy — a different delivery method that puts the follistatin gene inside muscle cells, not a peptide you inject under the skin.

What's hyped beyond the evidence:

The leap from "myostatin blockade builds muscle in mice" to "inject this vial and get jacked." Follistatin touches many pathways beyond muscle, so blanket, long-term inhibition raises real (if theoretical) concerns about reproduction and cell growth. "Irreversible gains" claims run far ahead of anything that's been demonstrated in people.

Things to know if you're looking into it

How it's studied: in research settings the peptide is a subcutaneous injection. There is no validated human protocol — community routines are experiments, not established medicine.
Two isoforms exist: FST-344 is the longer form; FST-315 is the shorter form that circulates more in the bloodstream. Most research-peptide products are labeled 344.
The gene-therapy distinction: the impressive human trials used the follistatin gene delivered by a viral vector, not the injectable peptide. Results from one don't transfer to the other.
Broad biology, broad unknowns: follistatin acts on more than muscle, so off-target effects are hard to predict and haven't been mapped in humans.
Regulatory status: not FDA-approved. Not on the FDA Category 2 list as of July 2026. Sold only as a research chemical, which is not regulated for human use.
Product uncertainty: because it's unregulated, purity and identity vary between sources; third-party testing is the only way to know what's actually in a vial.
Healthcare provider involvement: strongly recommended before considering anything this experimental, especially alongside other compounds.
Mechanism, half-life, dosing detail, and full references: all in the "Want to go deeper?" section below.

Reconstitution & dose calculator

Highly experimental — no validated human dose exists. There are no human trials of injectable FST-344, so there is no established, safety-tested dose. The numbers below reflect small community-reported research ranges (typically around 100 mcg per day in short cycles) and exist only to help you do the reconstitution math — they are not a recommendation to use it. This is an educational reference, not dosing guidance.

Common start

100 mcg/day

The figure most often cited in research-community reports

Reported range

100–300 mcg/day

Wide variation; none of it clinically validated

Cycle

10–30 days

Short courses, not continuous use

Vial sizes

0.5 & 1 mg

The two sizes this peptide usually ships in

Vial size

Bacteriostatic water

mL

1 mL into a 1 mg vial gives 1 mg/mL (1000 mcg/mL), so a 100 mcg dose is a small, easy-to-read 10-unit draw. Because the vials are tiny, 1 mL of water keeps the syringe numbers comfortable across the whole reported dose range.

Daily dose (during cycle)

mcg

Subcutaneous injection, once daily during a short cycle (commonly 10–30 days), then off. Remember: this is a research-community range, not a tested human dose.

Concentration

1.0 mg/mL

Per dose

0.10 mL

10 units on insulin syringe

Doses per vial

~10

~10 days of daily dosing

How to think about FST-344 dosing — a special case

There is no proven human dose. Unlike most peptides on this site, FST-344 has never been through a human dosing trial as an injectable. Everything below describes what the research community reports doing, not what evidence supports. Treat the whole topic as experimental.

The two vial sizes make the math simple. At 1 mg/mL:

1 mg vial + 1 mL water → 100 mcg is a 10-unit draw; the vial covers ~10 daily doses
0.5 mg vial + 1 mL water → 100 mcg is a 20-unit draw; the vial covers ~5 daily doses
Using less water raises the concentration and shrinks the draw; using more does the opposite

Short cycles, not continuous use. Research-community routines describe running it daily for a short course (often 10–30 days) rather than indefinitely. Given how little is known about long-term follistatin inhibition, open-ended use is where the theoretical risks pile up fastest.

The gene-therapy results don't apply here. It bears repeating: the impressive human muscle results came from delivering the follistatin gene into muscle, not from injecting this peptide. Don't borrow those outcomes to justify a peptide protocol.

Watch for: because there's no human safety data for the peptide, there's no reliable side-effect list to give. Theoretical concerns center on follistatin's broad activity — effects on reproductive hormones and on cell growth — which is exactly why a licensed provider should be in the loop.

The honest read. The biology is real and the animal data is genuinely strong. But the distance between "myostatin blockade grows muscle in mice" and "this vial does something predictable and safe in me" is enormous, and it hasn't been closed by any human study of the peptide. If you're weighing it, weigh it as an experiment with unknown odds, not an established tool.

For educational and research purposes only. This is not medical advice. FST-344 is not FDA-approved and no injectable follistatin product is approved for human use; it is sold only as an unregulated research chemical. No human trial has established a safe or effective dose. Consult a licensed healthcare provider before any health decision.

What people often ask

Does FST-344 actually build muscle in people?

Nobody knows, because it hasn't been tested. The muscle-growth evidence is from animals and from human gene-therapy trials that used a totally different delivery method. There's no published human study of the injectable peptide, so confident "yes it works" claims are running ahead of the data.

What's the difference between FST-344 and the gene therapy I've read about?

Gene therapy puts the follistatin gene inside your muscle cells using a viral vector, so the muscle itself makes follistatin continuously. FST-344 is the follistatin protein, injected from outside. Same molecule at the end, but very different delivery — and only the gene-therapy version has real human results.

What is myostatin, in plain terms?

Think of it as your muscles' built-in speed limiter. It stops muscle from growing endlessly. Follistatin blocks myostatin, which is like easing off that limiter — at least in animal studies.

FST-344 or FST-315 — what's the difference?

They're two natural forms of follistatin. The 315 version circulates more in the blood; the 344 version tends to stay bound to tissue. Most research peptides are sold as 344, but neither has validated human dosing.

Are the muscle gains permanent?

There's no human evidence to answer this. "Permanent gains" is a marketing claim, not a finding. Muscle built through any means generally needs continued stimulus to maintain, and we simply don't have human data on what happens after an FST-344 cycle.

Why isn't it FDA-approved?

No company has taken the injectable peptide through the trials the FDA requires. The only FDA involvement is clearance of an investigational gene-therapy application for dystrophy research — which is not approval of follistatin protein for general use.

Is it safe?

Unknown, and that's the honest answer. There's no human safety data for the peptide. Follistatin affects more than muscle, so blocking it broadly carries theoretical risks that haven't been mapped in people. That uncertainty is the whole story here.

FDA and regulatory status

Status as of July 14, 2026: Not FDA-approved for any medical indication. No injectable follistatin product is approved for human use. The FDA has cleared an investigational new drug (IND) application for an AAV-delivered follistatin gene therapy (AAV1-FS344) for muscular-dystrophy research only — this is not approval of the FST-344 peptide. FST-344 is sold as an unregulated research chemical and is not on the FDA Category 2 list. No Phase II or III trials of the injectable peptide are registered on ClinicalTrials.gov as of this date. Status updates land here when they happen.

Want to go deeper? Mechanism, isoforms, half-life, studied dosing, gene-therapy trials, side effects, and references. Click to expand.

Background and discovery

Follistatin is a naturally occurring glycoprotein first identified for its ability to suppress follicle-stimulating hormone (hence the name). It was later found to be a high-affinity binding partner and inhibitor of several TGF-β superfamily proteins, most notably myostatin (GDF-8) and activin A. FST-344 refers to the 344-amino-acid isoform; a shorter 315-amino-acid isoform (FST-315) is the predominant circulating form. The 344 isoform has greater affinity for cell-surface heparan sulfate proteoglycans, keeping it more tissue-associated.

Mechanism of action

Myostatin and activin inhibition

Myostatin is a negative regulator of skeletal-muscle mass: it signals through activin type II receptors (ActRIIB) and the Smad pathway to limit muscle-fiber growth. Follistatin binds myostatin (and activin) with high affinity, preventing receptor engagement. The net effect is disinhibition of muscle growth, promoting both hypertrophy (larger fibers) and, in some models, hyperplasia (more fibers).

Breadth of activity

Because follistatin binds multiple TGF-β family ligands, its effects are not confined to muscle. This breadth underlies both its therapeutic interest and the difficulty of predicting off-target consequences of sustained systemic inhibition.

Commonly studied / reported dosing

These are not recommendations, and they are not validated. No human trial has established a safe or effective injectable dose. Always consult a licensed healthcare provider before any clinical decision.

Research-community reports (peptide): roughly 100 mcg subcutaneously per day, run in short cycles of about 10–30 days. Some reports describe up to ~300 mcg/day. None of this reflects controlled human data.

Gene therapy (for context only): the AAV1-FS344 trials delivered the follistatin gene by intramuscular injection of a viral vector at defined vector-genome doses — an entirely different modality that cannot be translated into a peptide dose.

Half-life and pharmacokinetics

Circulating follistatin protein has a relatively short plasma half-life (on the order of hours), which is part of why sustained effects are difficult to achieve with an injected protein and why gene-therapy approaches — which produce follistatin locally and continuously — have been pursued for durable outcomes. Peptide pharmacokinetics for research-grade FST-344 in humans have not been characterized.

Clinical trial context (gene therapy)

Phase 1/2a trials led by researchers at Nationwide Children's Hospital delivered AAV1-FS344 into the muscles of patients with Becker muscular dystrophy and, separately, sporadic inclusion-body myositis. Reported outcomes included improved six-minute-walk-test distance and favorable histological changes (reduced fibrosis, more normal fiber-size distribution). These trials support follistatin's biological relevance in humans but do not evaluate the injectable FST-344 peptide.

Side effects and safety profile

For the injectable peptide, there is no human safety dataset. Considerations flagged in the literature and by independent testing programs include:

Unknown off-target effects from broad TGF-β/activin pathway inhibition, including theoretical effects on reproductive hormones and cell proliferation
No established maximum tolerated dose or long-term safety data in humans
Product-quality risks in unregulated research chemicals: incorrect peptide identity, contamination, and inconsistent content
Standard injection-related risks (site reactions, infection with poor technique)

The gene-therapy trials reported acceptable short-term safety in small, closely monitored patient groups, but those findings apply to a controlled clinical setting and a different modality — not to self-administered research peptide.

References

Lee SJ. (2004). "Regulation of muscle mass by myostatin." Annu Rev Cell Dev Biol, 20, 61–86. PubMed
Winbanks CE, et al. (2012). "Follistatin-mediated skeletal muscle hypertrophy." J Cell Biol, 197(7), 997–1008. PubMed
Mendell JR, et al. (2015). "A Phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy." Mol Ther, 23(1), 192–201. PubMed
Mendell JR, et al. (2017). "Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes." Mol Ther, 25(4), 870–879. PubMed
U.S. Food & Drug Administration. "Research chemicals and unapproved peptides: quality and identity concerns." Consumer guidance. FDA.gov

For educational and research purposes only. This is not medical advice. FST-344 is not FDA-approved, and no injectable follistatin product is approved for human use. The strongest human data comes from gene-therapy research, not the injectable peptide. Consult a licensed healthcare provider before considering any peptide. PeptideLibraryHub is independent and does not sell peptides or accept money from anyone who does.