FST344, FS344, follistatin
A protein the body makes that mops up myostatin and activin, two signals that hold muscle growth back. Blocking them builds muscle in animals.
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Follistatin is a protein the body makes that binds myostatin and activin, two signals that hold back muscle growth, and stops them reaching their receptors. In mice, follistatin made in muscle produced large muscle gains. It was first identified as a protein that lowers FSH, a pituitary hormone needed for fertility. Human trials delivered the FS344 gene, not the protein, into thigh muscle with a virus.
AnimalMainly from cell and animal studies; not shown in people.
Whether injected follistatin protein reaches muscle at all is unknown: in rats, injected follistatin left the blood within hours, mostly taken up by the liver. Its effects on human reproductive hormones are also unmeasured.
About 100 mcg once daily for 10–30 days is the figure that recurs, but public guides range from 50 to 300 mcg a day and none of them cite a study. No dose of the protein has been tested in people.
Mendell 2015 and 2017
Gene therapy, not protein: a single set of injections of 2 × 10¹¹ to 6 × 10¹¹ vg/kg per thigh of an AAV1 vector carrying FS344.
No set cycle, shown over 26 weeks
The ways Follistatin-344 is most often run, each tagged with where it comes from.
Not tested in any study. The public guides disagree by sixfold and none of them cite a source for the numbers.
Six patients, with an exercise program alongside. Walking distance improved against 8 untreated matched people (Mendell 2017).
Six patients in two dose groups. Four walked further, two did not change, and no adverse effects were reported (Mendell 2015).
The trials used multiple direct injections into the quadriceps muscle of both legs, in hospital. Community use is subcutaneous injection with rotating sites, or injection into the muscle someone wants to grow, neither of which has been studied.
No published guidance. Skip the missed day and carry on; don’t double up.
In inclusion body myositis, walking distance changed by +56.0 m a year against −25.8 m a year in 8 untreated matched people (p=0.01).
TrialFour of 6 people walked 29 to 125 m further on the 6-minute walk test and 2 did not change. No adverse effects were reported.
TrialNothing is known. No published study has given follistatin protein to a person, so there are no timelines and no outcomes to report.
CommunityNo adverse effects were reported in the gene-therapy trials (15 people, safety followed 2 years)
That is about a virus delivering a gene into muscle, not about injecting the protein, which no study has done.
Lower FSH and knock-on effects on reproductive hormones
Expected from blocking activin, which is why the trials chose the FS344 form. Blood tests can check FSH, LH and sex hormones.
Immune reaction to a protein grown in cells
Cell-grown proteins can carry other cell proteins and unfamiliar sugar patterns. Stop at hives, swelling or wheeze and get help.
Injection-site pain, redness or infection (unregulated vials)
Use sterile technique. Spreading redness, heat or pus needs medical care.
Hives, swelling of the face or throat, or trouble breathing after an injection: stop and get medical help. A hot, spreading red injection site with fever needs urgent care.
No product label covers follistatin-344, so these come from human studies and from precautions based on how it works. Most important first.
Precaution: follows from how it works or from a related drug, and hasn’t been tested. This isn’t a complete list: a pharmacist or doctor can check a specific medicine or condition.
Commonly talked about together as muscle-growth compounds. Neither has been injected in a published human study, and the pair has never been tested together.
2 key published human studies, with how many people took part. Animal studies aren’t listed here.
Walking distance changed by +56.0 m a year against −25.8 m a year in 8 untreated matched people (p=0.01); 4 of 6 improved 58 to 153 m and 2 improved 5 to 23 m, with less fibrosis and more regeneration on biopsy.
Mendell et al., Mol Ther 2017, PMID 28279643
On the 6-minute walk test two people gained 58 m and 125 m in the lower-dose group and two gained 108 m and 29 m in the higher-dose group, while 2 showed no change. No adverse effects were encountered.
Mendell et al., Mol Ther 2015, PMID 25322757
No published study has given follistatin protein itself to a person. The human data are gene-therapy trials in 15 people with Becker muscular dystrophy or inclusion body myositis, with no placebo group; most walked further. ACE-083, an engineered follistatin-based drug, grew the injected muscle in a placebo-controlled study of 58 healthy women but didn’t make it stronger, and its Phase 2 trials were terminated. Injecting follistatin under the skin is untested for dose, effect and safety.
| Vial | Water | Common dose | Draw | Strength | Action |
|---|---|---|---|---|---|
| 1 mg | 1 mL | 100 mcg | 10 units (0.10 mL) | 1 mg/mL |
Vials are usually 1 mg. With 1 mL of bacteriostatic water, 100 mcg is 10 units on a U-100 syringe. This is a glycoprotein, not a short peptide: research-grade follistatin is made up with a carrier protein such as albumin, because without it the protein sticks to the glass and is lost.
Syringe units for common doses at different water volumes. Select a cell to open it in the calculator.
| Dose | Water added | ||
|---|---|---|---|
| 1 mL | 2 mL | 3 mL | |
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U-100 insulin syringe: 100 units is 1 mL. Draws over 100 units show in mL. Faded values are under 2 units, which is hard to measure. Check that your vial holds the larger volumes.
Every compound side by side: the half-life chart and the storage chart.
A protein the body makes that mops up myostatin and activin, two signals that hold muscle growth back. Blocking them builds muscle in animals.
Follistatin is a protein the body makes that binds myostatin and activin, two signals that hold back muscle growth, and stops them reaching their receptors. In mice, follistatin made in muscle produced large muscle gains. It was first identified as a protein that lowers FSH, a pituitary hormone needed for fertility. Human trials delivered the FS344 gene, not the protein, into thigh muscle with a virus. Whether injected follistatin protein reaches muscle at all is unknown: in rats, injected follistatin left the blood within hours, mostly taken up by the liver. Its effects on human reproductive hormones are also unmeasured.
Not approved as a drug in the US or EU, and no follistatin product is approved anywhere. Human testing has been limited to gene therapy trials that ended in 2017 and to a follistatin-based fusion protein, ACE-083, whose Phase 2 trials in facioscapulohumeral muscular dystrophy and Charcot-Marie-Tooth disease were terminated. Never nominated for pharmacy compounding, so it appears on neither FDA’s Category 1 nor Category 2 bulk substance lists (April 2026 snapshot).
From community reports, not established: About 100 mcg once daily for 10–30 days is the figure that recurs, but public guides range from 50 to 300 mcg a day and none of them cite a study. No dose of the protein has been tested in people. From human studies (Mendell 2015 and 2017): Gene therapy, not protein: a single set of injections of 2 × 10¹¹ to 6 × 10¹¹ vg/kg per thigh of an AAV1 vector carrying FS344. These are the amounts sources reference, not recommendations.
There’s no single right amount, because the water only sets the strength. With 1 mL of bacteriostatic water in a 1 mg vial, the strength is 1 mg/mL, so 100 mcg is 10 units on a U-100 insulin syringe. More water makes small doses easier to measure. The dose chart above shows other volumes.
No product label covers follistatin-344, so these come from human studies and from precautions based on how it works. Pregnancy, breastfeeding or trying to conceive: It blocks activin and was first identified as a protein that lowers FSH, a hormone needed for egg and sperm production. No human safety data exist. Anyone relying on product quality: It is a protein grown in living cells; vials rarely say which cells made it, and nothing checks its identity, sugar coating or amount. Cell-grown proteins can trigger immune reactions. Anyone with active or past cancer: It blocks signals that limit tissue growth. No study has tested whether follistatin affects cancer.
About 4 minutes, then about 2 hours, in rats, measured in animals, not people. After 1 mcg of radioiodine-labeled follistatin into a vein in rats: initial half-life 4.0 minutes and terminal half-life 130.8 minutes, measured as radioactivity, with most of it taken up by the liver. No human measurement exists for injected follistatin.
Before mixing: Freezer (−20 °C or colder) for long-term storage, kept dry and dark. After mixing: Fridge (2–8 °C) for a few weeks, or frozen in single-use portions. Research-grade follistatin is made up with a carrier protein such as albumin. This is common practice; no product label covers it.
Yes. S4.3 agents preventing activin receptor IIB activation: myostatin-binding proteins, with follistatin named. Banned at all times, as a non-Specified substance. Follistatin gene therapy is separately covered by M3 gene and cell doping.
Yes. The human studies section lists 2 published studies. The largest, from 2017, included 6 people. Walking distance changed by +56.0 m a year against −25.8 m a year in 8 untreated matched people (p=0.01); 4 of 6 improved 58 to 153 m and 2 improved 5 to 23 m, with less fibrosis and more regeneration on biopsy.
For the protocol details
For how it works, who should avoid it and the limits of the evidence
Doses tagged Community come from public dosing guides and user reports, checked against at least two of them. Those sites aren’t named here because many of them sell peptides or earn commissions on them.