Follistatin-344 Dosage: Gene Therapy vs. Injectable Use
Follistatin-344 is a natural human protein that binds and neutralizes myostatin, the hormone that limits muscle growth, along with activin and other TGF-beta family growth factors. Unlike most peptides on this site, it has a real, FDA-regulated human clinical trial behind it: a gene-therapy study in Becker muscular dystrophy patients. But that trial delivered follistatin using a viral vector injected into muscle, not the synthetic protein vendors sell for direct injection, and a separate, documented case series links high-dose injections of that vendor product to a specific eye condition.
Reviewed by the PepGuard team · Last reviewed Jul 28, 2026
344-amino-acid precursor isoform of follistatin, a myostatin- and activin-binding antagonist (the FS344 construct used in AAV gene-therapy trials and sold by vendors as a recombinant protein) · also labeled FS344, FST-344
Photo: Fayette Reynolds / UnsplashHow blocking myostatin is supposed to build muscle.
Myostatin is a growth factor whose entire job is to put a ceiling on skeletal muscle growth. Animals and rare humans born without functional myostatin develop dramatically more muscle mass, which is why blocking it became an obvious drug target. Follistatin is one of the body's own myostatin antagonists: it binds myostatin (and several related TGF-beta family growth factors, including activin) and neutralizes it. Follistatin-344 is a specific 344-amino-acid isoform of that natural protein.
The idea has a real, if narrow, human proof-of-concept behind it: a Phase 1/2a gene-therapy trial delivered follistatin-344 directly into the muscle of Becker muscular dystrophy patients, using a viral vector to make their own cells produce it. That's a genuinely different thing, technically and regulatorily, from injecting the synthetic protein itself, which is what research-peptide vendors actually sell. The rest of this page keeps those two apart on purpose.
A gene-therapy dose that doesn’t translate into a vial.
| What's cited | Range |
|---|---|
| Gene-therapy trial dose (Mendell et al. 2015, n=6, one-time bilateral intramuscular injection) | 3×10¹¹ or 6×10¹¹ vector genomes/kg per leg; not comparable to a repeatable protein dose |
| Commonly cited loading protocol (vendor/community, injectable protein, no trial behind it) | 100 mcg/day subcutaneous for 10 days, then 100 mcg every other day for 20 days |
| Commonly cited pre-workout protocol (vendor/community) | 50 mcg intramuscular, 30 minutes before training, training days only |
| Doses linked to a documented eye-injury case series (see side effects) | full 1 mg vials injected subcutaneously in a single dose; 8 of 11 documented cases followed just one such injection |
There is no dose here that's both clinically validated and available to buyers, because the trial and the vendor product use entirely different delivery technologies. The gene-therapy dose is measured in viral genome copies per kilogram, a one-time intramuscular injection, not something that converts into a repeatable microgram protocol. Every microgram-based figure below comes from vendor and community sources, not a clinical trial, and one of those high-dose patterns is directly linked to a documented injury case series (see side effects).
Mixed results, and a field that mostly hasn’t held up.
In the six-patient Becker muscular dystrophy trial, four patients improved on a 6-minute walk test by 29 to 125 meters, with muscle biopsies showing reduced fibrosis and fiber hypertrophy at the higher dose. Two patients showed no improvement at all. That's a real signal, but a small and mixed one, not a clean win.
It's also consistent with the broader field. A 2020 review of myostatin-inhibitor trials for muscular dystrophy found a recurring pattern: modest gains in muscle mass repeatedly failed to translate into proportional gains in strength, among other mismatches between the animal models that looked promising and how the pathway actually behaves in human dystrophic muscle. Myostatin inhibition as a therapeutic strategy has underdelivered relative to its preclinical promise, and follistatin-based approaches are part of that pattern, not an exception to it.
A documented eye condition, tied to the exact product sold.
| Signal | Frequency | Context |
|---|---|---|
| Central serous chorioretinopathy (subretinal fluid buildup causing blurred or distorted vision) | rarely reported | A published 11-patient case series (all male bodybuilders, mean age 36.8) linked this specifically to high-dose follistatin-344 injection. Subretinal fluid resolved in an average of 2.3 months after single-injection cases, but recurred in patients with repeated injections. True population incidence is unknown since this is a case series, not a controlled trial, but it's a real, repeated, documented signal, not an isolated report. |
| Injection site reaction | commonly reported | General to subcutaneous or intramuscular injection, not specific to follistatin-344's mechanism. |
| Serious adverse events in the published gene-therapy trial | rarely reported | The 6-patient Mendell trial reported no serious adverse events from the AAV1.CMV.FS344 injection itself, but a 6-person trial cannot establish population-level safety. |
| Long-term or off-target effects of chronic myostatin/activin blockade | unknown / not studied | Activin and other TGF-beta family members that follistatin also binds play roles in reproductive hormone regulation and other tissues. No long-term human safety trial of sustained follistatin-344 exposure exists for either delivery method. |
| Efficacy and safety of the specific injectable protein product sold to buyers | unknown / not studied | The only human efficacy and safety trial used a one-time gene-therapy vector, not the synthetic or recombinant protein most buyers inject repeatedly. No comparable trial exists for that product or dosing pattern. |
Not FDA-approved as a drug. The only regulated human trial used gene therapy, not the injectable protein sold by vendors. Not found on any current FDA 503A compounding list.
No follistatin-344 product, gene therapy or injectable protein, has been approved by the FDA for any use. The one human clinical trial conducted under FDA oversight used AAV1.CMV.FS344, a viral vector engineered to make a patient's own muscle cells produce follistatin-344, injected directly into the quadriceps of six Becker muscular dystrophy patients (NCT02354781). That's a fundamentally different regulatory and safety category from the synthetic or recombinant follistatin-344 protein research-peptide vendors sell for direct subcutaneous or intramuscular injection: one went through FDA's gene-therapy/biologics review pathway as an investigational product, the other is an unapproved research chemical that has never been tested in a comparable human trial. Unlike most other peptides on this site, we checked directly against FDA's current 503A Category 1, Category 2, and withdrawn-nomination bulk drug substance documents and could not find follistatin-344 listed on any of them, meaning it hasn't gone through that specific compounding-review pathway at all, for reasons the public record doesn't make clear.
Every claim above, traced back.
The gene-therapy trial and its registry entry, a documented ophthalmology case series, a review of why the broader approach has underperformed, and current FDA compounding records. Click through and check them yourself.
Follistatin-344 questions, answered.
No. The only human trial conducted under FDA oversight was a gene-therapy study (AAV1.CMV.FS344) in Becker muscular dystrophy patients, an investigational product, not an approved drug. The injectable protein sold by research-peptide vendors is a different product entirely and has never gone through an FDA review of any kind, including the 503A compounding-list process most other peptides on this site have at least been nominated for.
The evidence is genuinely mixed. In the one published gene-therapy trial, 4 of 6 Becker muscular dystrophy patients improved on a 6-minute walk test (by 29 to 125 meters), with muscle biopsies showing reduced fibrosis and fiber hypertrophy at the higher dose, while 2 patients showed no change. More broadly, a 2020 review of myostatin-inhibitor trials for muscular dystrophy found that modest muscle mass gains repeatedly failed to translate into proportional strength gains, among other translational problems between animal models and human muscle.
There's a specific, documented risk worth knowing about: a case series published in International Ophthalmology linked high-dose follistatin-344 injection to central serous chorioretinopathy, a fluid buildup under the retina causing blurred or distorted vision, in 11 male bodybuilders. Most cases resolved within a few months, but recurred in those who kept injecting. That's a real, named risk tied to the specific injectable product sold to buyers, not the gene-therapy version studied in the muscular dystrophy trial.
The trial used a viral vector that reprograms a patient's own muscle cells to produce follistatin-344 internally, delivered as a single injection. Vendors sell a synthetic or recombinant follistatin-344 protein meant to be injected directly and repeatedly. They share a name and a target (myostatin), but they're different technologies with different risk profiles, and only one of them has ever been tested in a human trial.
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