Follistatin (FST-344 / FST-315)
Follistatin 344, Follistatin 315, FS344, FS315
Forms: Black-market vials are mostly labeled 'follistatin 344', a few '315'. FS344 is the longer precursor; trimming its tail gives FS315, the main form in blood.
A natural protein that binds myostatin and activin, signals that limit muscle growth. Sold on the black market as a 'research peptide'; never approved and never tested as an injected protein in people.
Legal status: Not approved anywhere; sold as a 'research' product. Banned in sport at all times (WADA S4.3, myostatin-binding proteins) and in horse racing.
What it gives
What people use it for and what the sources report. Strength is a general category from the literature: mild, clear or strong.
Purpose
Used in the hope of more muscle by blocking myostatin. In medicine it has been studied only as gene therapy for muscular dystrophy, not as an injected protein. Not approved for any use.
What people seek and what studies show
People hope for extra muscle beyond normal limits. Mice bred to overproduce follistatin had 194–327% more muscle. The only human data are gene therapy: the follistatin gene injected into the thigh muscles of 6 men with Becker dystrophy, 4 of whom walked farther afterwards. Injected follistatin protein has never been tested in a human trial, so its effect in healthy people is unknown.
Effects
Evidence: Animal data only — effects in people are unproven
Sought effects by goal (as reported)
Possible side effects
Possible does not mean certain: whether they appear depends on dose, duration, individual response and monitoring. Below — what to watch for and how to notice it early.
4 possible side effects
level · how often it is reportedWhat the levels mean
Level: general category from the literature, not your personal risk
How often
- common
- reported often
- possible
- reported in some people
- rare
- reported occasionally
How to lower the risks
For 1 of 4 possible side effects there are specific ways to lower the chance: everyday measures, supplements and options a doctor may consider. For the rest — general advice. No measure removes a risk completely.
Products obtained without a prescription are often fake, mislabeled or non-sterile, and there is no way to check at home. For legal supplements, look for third-party testing (Informed Sport, NSF).
What is unknown cannot be lowered, only avoided; tell your doctor what you have taken.
Low-level side effects
How each option helps (2)
- Regular blood tests — LH, FSH and testosterone show how deep suppression is and whether natural production has recovered afterwards.
- Working with a doctor — Assesses how deep suppression is and guides recovery afterwards, including fertility testing.
Check-ups and blood tests
Which tests and check-ups to do and when. This is a guide — agree the exact schedule with your doctor.
Which doctors can help
For women
Follistatin was first found in ovarian fluid as a brake on FSH; effects of the injected protein on periods, fertility and pregnancy are unstudied.
Myth & fact
“Follistatin injections give 'double muscle', like myostatin-free animals.”
Those animals were bred or gene-modified. Injected follistatin has never been tested in people, and about half of black-market vials did not contain it at all.
Combinations
No specific combination notes for this substance yet. The stack check still adds up side effects that several substances share.
Sources
Possible side effects by body system, combinations, how to lower the risks and which tests to do — free.
Open the stack check