FST-315, FS315, follistatin (circulating form)
The form of follistatin that circulates in blood, and the one a genuine vial of either “follistatin-315” or “follistatin-344” holds: the 315-amino-acid chain left after a cell cuts the short signal piece off the 344-amino-acid precursor. It mops up myostatin and activin, two signals that hold muscle growth back.
None of these uses is approved. Uses tested in people come first, then claims that rest on animal studies or user reports.
Trial: tested in people, with the result. Animal: cell or animal studies only. No study: nothing published supports it.
Every compound’s uses, by body system: evidence map, and by condition: conditions A to Z.
Category 2 is FDA’s list of bulk substances that may present significant safety risks in compounding. The 503A bulks list adds substances that pharmacies may compound with. Advisory committee votes are advice; FDA makes the final decision. Sport status follows the WADA Prohibited List in force since January 1, 2026; athletes should confirm with their anti-doping organization. Checked October 5, 2026. Status outside the US checked October 7, 2026 against EMA, MHRA, Health Canada and TGA records. By country: EU, UK, Canada, Australia. Status of every compound.
Follistatin wraps around myostatin and activin, two signals that hold back muscle growth, and stops them reaching their receptors. This 315-amino-acid form is the one that circulates: its tail end lowers how tightly it sticks to heparin-like sugars on cell surfaces, so it travels in blood instead of staying put, which is why the human gene-therapy trials chose the gene that makes it. In mice and monkeys, more follistatin in muscle meant larger, stronger muscle.
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.
No dose has been tested in any person. One public guide quotes 50 to 200 mcg a day under the skin for this form; others give no figure, and the numbers circulating for follistatin-344 run from 50 to 300 mcg. None cites a study.
Mendell 2015 and 2017
Gene therapy, not protein: one set of injections into both thighs of 3 × 10¹¹ to 6 × 10¹¹ vg/kg per leg of an AAV1 virus carrying the FS344 gene, which cells then turn into this protein.
No set cycle, shown over 26 weeks
The ways Follistatin-315 is most often run, each tagged with where it comes from.
Not tested in any study. The figures quoted under this name and under follistatin-344 disagree, and none of the guides cites a source.
Six patients, with an exercise program alongside. The gene makes this protein inside muscle; nobody was injected with the protein itself.
Six patients in two dose groups. Four walked 29 to 125 m further on a 6-minute walk test, two didn’t change, and no adverse effects were reported.
The trials injected a gene, not the protein, into the quadriceps of both legs in hospital. Community use is injection under the skin with rotating sites, or into the muscle someone wants to grow; neither 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 and 2 did not change. No adverse effects were reported.
TrialMuscle size and strength rose and stayed up, with no abnormal changes in major organs. The 2009 report carries an erratum published in August 2026.
AnimalNothing is known. No published study has given follistatin protein to a person, so there are no timelines and no outcomes to report.
CommunityUnknown in people: the protein has never been injected in a published study
The gene-therapy trials say nothing about injecting the protein, which leaves the blood of rats within hours.
Lower FSH and knock-on effects on reproductive hormones
Expected from blocking activin: follistatin was discovered as an FSH-lowering protein. 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.
No 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 this protein.
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.
The side effects people ask about most, answered one by one. Each answer says where it comes from.
Trial: reported in human studies. Precaution: a concern that follows from how it works or from a related drug, not measured. Animal: seen only in animal studies. No data: no human safety data. A side effect that hasn’t been reported can still happen, especially with long or high-dose use. Any of these across every compound: the side-effect finder.
No product label covers follistatin-315, so these come from human studies and from precautions based on how it works. Most important first.
Trial: seen in human studies, or a group those studies left out. 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. Every documented interaction, by medicine: drug interactions.
What’s known about pregnancy, breastfeeding, kidney and liver problems, age and surgery. Where nothing has been studied, the row says so.
Trial: from human studies. Precaution: not studied, so the caution follows from how it works or from a related drug. Ages are given as approved or studied; this site never gives doses for children. Across every compound: pregnancy, breastfeeding, kidney disease, liver disease, older adults and surgery.
Not a stack: the 344-amino-acid chain is the precursor of this 315-amino-acid protein, so genuine vials of either hold the same mature protein. Nothing has been tested together.
Commonly talked about together as muscle-growth compounds. Neither has been injected in a published human study, and the pair has never been tested.
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, with less fibrosis and more regeneration on biopsy. The protein itself was not injected.
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
Registered trials that are still running, or that finished without published results, most important first. Checked on ClinicalTrials.gov, October 7, 2026.
A virus carrying the follistatin gene, with a second gene, in 12 adults 35 and older with age-related muscle loss
Recruiting from June 2026; completion expected June 2028
A follistatin plus klotho gene plasmid in 14 adults, run at a clinic abroad
Completed April 2026; no results posted and no publication
Every compound’s registered trials: trials under way.
No published study has given follistatin protein, in this or any form, 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, where most walked further. The protein evidence is cells, mice and monkeys. Dose, effect, safety and purity for injected follistatin are all untested.
Sold as powder vials of a protein grown in living cells, usually 1 mg, with no regulated label. Vial Guide gives no mixing or dose figures here, because no person has received follistatin protein in a published study and the amounts quoted for the two forms don’t agree. Research-grade follistatin is made up with a carrier protein such as albumin, or it sticks to the glass and is lost.
Every compound side by side: the half-life chart and the storage chart.
Every compound’s numbers: the identifiers table.
Vials hold a protein grown in living cells, usually 1 mg, and rarely say which cells. Public references print weights from 3,470 to 34,998 for a chain that weighs about 34,700 before sugars, so labels may not match the contents.
The form of follistatin that circulates in blood, and the one a genuine vial of either “follistatin-315” or “follistatin-344” holds: the 315-amino-acid chain left after a cell cuts the short signal piece off the 344-amino-acid precursor. It mops up myostatin and activin, two signals that hold muscle growth back.
Follistatin wraps around myostatin and activin, two signals that hold back muscle growth, and stops them reaching their receptors. This 315-amino-acid form is the one that circulates: its tail end lowers how tightly it sticks to heparin-like sugars on cell surfaces, so it travels in blood instead of staying put, which is why the human gene-therapy trials chose the gene that makes it. In mice and monkeys, more follistatin in muscle meant larger, stronger muscle. 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 gene therapy: a 15-person trial that ended in 2017, two small open-label studies of a follistatin plasmid run at a clinic abroad in 2023 and 2026, and a 12-person trial of an AAV follistatin gene in age-related muscle loss that began recruiting in June 2026. ACE-083, an engineered follistatin-based protein injected into muscle, had its phase 2 trials terminated in 2020. Never nominated for pharmacy compounding, so follistatin appears on none of FDA’s 503A or 503B bulk substance category lists (503A list updated May 14, 2026).
From community reports, not established: No dose has been tested in any person. One public guide quotes 50 to 200 mcg a day under the skin for this form; others give no figure, and the numbers circulating for follistatin-344 run from 50 to 300 mcg. None cites a study. From human studies (Mendell 2015 and 2017): Gene therapy, not protein: one set of injections into both thighs of 3 × 10¹¹ to 6 × 10¹¹ vg/kg per leg of an AAV1 virus carrying the FS344 gene, which cells then turn into this protein. These are the amounts sources reference, not recommendations.
No product label covers follistatin-315, 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 discovered 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. The “Who should avoid it” section lists 1 more group.
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 of any form.
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, with less fibrosis and more regeneration on biopsy. The protein itself was not injected.
Trial No human studies of the protein. The gene-therapy trials followed 15 people for up to 2 years after a single treatment.
Trial Never given to people as a protein by any route. The human trials injected a gene into thigh muscle; community use is injection under the skin or into muscle. No oral or nasal form exists.
No study No data. No study has looked at follistatin and alcohol together.
Precaution No study has combined them. GLP-1 drugs cause some muscle loss alongside fat loss, which is why myostatin blockers are being trialed with them, but not this protein.
No data There’s no label, no overdose data and no human dosing study of the protein. Call Poison Help (1-800-222-1222), or emergency services for severe symptoms.
For the protocol details
For how it works, who should avoid it and the limits of the evidence
For uses, the side-effect questions, special situations, trials, identifiers and status outside the US
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.