FST-344 (Follistatin)

FST-344 is an isoform of the body's own protein follistatin, specifically the alternatively spliced FS344 variant. Follistatin is well known in muscle research because it is a strong antagonist of myostatin (also called GDF-8). Myostatin is the natural brake on muscle growth: without it, muscles grow beyond the normal range. You see this in double-muscled cattle like the Belgian Blue and in mice lacking the myostatin gene.

The idea is simple: if follistatin mops up myostatin, the growth brake should be released. That is why follistatin shows up in a performance context. What matters is cleanly separating where the impressive data come from - and where they do not. More on that below.

This is a freeze-dried peptide in a vial that you dissolve and inject yourself. That is fundamentally different from the gene-therapy studies you are about to read. This information is for research and educational purposes only. No medical recommendations.

Follistatin binds and neutralizes myostatin and related molecules from the TGF-beta family, including activin. Myostatin normally signals through the activin type II receptors and keeps muscle growth in check. If that pathway is inhibited, the brake is theoretically off - in preclinical models this led to markedly more muscle mass.

Notably, in mice completely lacking the myostatin gene, an added follistatin transgene still increased muscle mass further - up to four times the normal value. That suggests follistatin captures not only myostatin but also other growth brakes such as activin.

The key catch: all of this comes from permanent expression - either built in genetically (transgene) or delivered into muscle tissue via gene therapy, where the tissue itself continuously produces follistatin. Injected follistatin peptide, by contrast, has a very short half-life in the blood and is rapidly degraded. Whether a subcutaneous peptide injection captures enough myostatin systemically to trigger muscle growth is simply not established.

Before you start, have everything ready:

Because the vial holds only a tiny amount, careful math matters here especially - small errors in concentration make a large percentage difference.

The powder is dissolved with bacteriostatic water. How much water you use determines the concentration. That in turn decides how many units you later draw on the insulin syringe for your dose. With a 1 mg vial and the usual doses you are working in a very small unit range, so it is worth using a bit more water to make drawing up more accurate.

Use the peptide calculator for exactly this: pick vial size and water amount to see the concentration and units per dose right away.

It is injected subcutaneously, into the fat under the skin. Typical sites are the belly (avoiding the area around the navel), the front and the inner side of the thigh, or the back of the upper arm.

Rotate the injection site every time.

The follistatin-myostatin axis is exceptionally well studied - but almost always in animal models or via genetic engineering:

• Cattle and mice: A myostatin mutation produces double-muscled animals (Belgian Blue, Piedmontese). This shows how powerful myostatin is as a growth brake - preclinical, animal genetics.
• Transgenic mice: Follistatin as a built-in transgene increased muscle mass dramatically. In myostatin-free mice it even rose to four times normal. That is permanent genetic expression, not injection.
• Gene therapy (mouse to monkey): AAV-packaged FS344 delivered into muscle tissue increased muscle size and strength across several species.
• Human: In a phase 1/2a gene-therapy trial, six Becker patients received AAV1-FS344 injected directly into the thigh. Some clearly improved their walking distance. Muscle tissue showed hypertrophy. But: this was a viral vector with permanent follistatin production in the muscle - not the injected peptide.

The common thread: everything convincing requires permanent follistatin production at the site of action. For the freeze-dried peptide you inject subcutaneously, there is simply no comparable human efficacy data. Using FST-344 as a peptide means operating in a largely unproven space.

Because solid human data on the injected peptide is lacking, the side-effect profile is also poorly characterized. What can be inferred from the biology and the gene-therapy data:

• Follistatin originally derives from the ovary and can suppress follicle-stimulating hormone - so in theory it touches the hormone axis. The FS344 isoform was specifically chosen to bypass such off-target effects.
• Local reactions at the injection site, as with any subcutaneous injection.
• Permanently unbraked muscle growth raises long-term safety questions, for example regarding tendons and heart muscle - discussed in a preclinical frame, not studied for the peptide.

Bottom line: the real risk here is not a dramatic acute profile, but that almost nothing is known about longer-term use of this peptide form.

FST-344 is a prime example of how far a fascinating research mechanism and a sold peptide can diverge. Myostatin inhibition is real and impressively documented - but the strong effects depend on permanent expression via genetic engineering. The injected, short-lived peptide is a different thing. The evidence for it is largely missing.

Anyone looking at this in a research context should keep expectations honestly low and not be dazzled by the gene-therapy numbers. This information is for research and educational purposes only. No medical recommendations.

The undissolved powder is robust, normal room temperature (around 20 to 25 degrees) is fine, ideally kept in the dark. Once mixed, it keeps for at least 4 to 6 months in the fridge (2 to 8 °C). Bacteriostatic water brings a preservative that keeps the dissolved form stable for longer. Peptides are sensitive to heat and light - so do not leave them sitting out in the open.