What is Follistatin?
Follistatin is a protein made by the body that binds activins and related signaling molecules. It was characterized as a regulator of follicle-stimulating hormone, linking it to reproductive physiology as well as muscle biology.[1][4]
Its muscle connection comes partly from binding myostatin, a signal that restrains muscle growth. Reducing that signal can increase muscle size in experimental systems, which is why follistatin has attracted interest in muscle-wasting disorders.[4]
FST344 and FST317 describe precursor forms made through alternative gene splicing. Their processed proteins include FS315 and FS288, which differ in tissue binding and distribution; a label reading “344” does not make a preparation equivalent to circulating FS315.[1][4]
Gene therapy delivers genetic instructions that make cells produce follistatin. Recombinant protein, viral gene-transfer vectors and engineered follistatin-based fusion proteins are separate interventions, with different exposure patterns and biological effects.[5][4] For more context, read the follistatin identity explainer.
What Follistatin is investigated for
Follistatin evidence is grouped by practical use case and injectable, intravenous, subcutaneous, and intraperitoneal route context. Each use case separates confidence, human evidence, animal or mechanistic support, and the practical takeaway.
Muscle function in neuromuscular disease
Mixed research context
Muscle function in neuromuscular disease
Mixed research context
Evidence snapshot
Overall confidence
Follistatin biology is well characterized. Clinical effects depend on the preparation, and a standalone protein-injection regimen remains unestablished.[4][5][6]
Overall confidence is a page-level composite, not an average; it weighs evidence quality, route/molecule match, and practical limitations.
Human evidence
Gene-transfer findings suggest possible benefits in neuromuscular disease; standalone protein effects remain uncertain.[5][6]
Animal / preclinical
Preclinical findings support activin/myostatin binding and muscle-growth effects.[1][2][3][4][5][6][7][8]
Mechanism support
Follistatin captures activin-family proteins before they activate their receptors. Binding myostatin can release part of the restraint on muscle growth, while binding activin also affects reproductive and other tissue signals.[4]
Forms & administration
Follistatin has recombinant-protein, gene-based, fusion-protein and cell-based formulations. Experimental animal delivery includes injections and osmotic pumps. Sustained protein delivery and gene-based formulations produce different exposure patterns from repeated injections.[4][5][6][45][19][40]
Dosing & protocols
These ranges are general reference points, not personal dosing instructions.
Typical Range
A standalone human follistatin-protein dose range is not established. Gene-transfer doses use vector units rather than protein mass.[5][6]
Frequency
One-time gene-transfer interventions and experimental protein dosing have different exposure patterns; they do not share a repeat frequency.[5][45]
Timing Considerations
Human evidence does not establish meal, exercise or bedtime timing for standalone follistatin protein.[5][4]
Cycle Length
A repeatable human protein course is not established; long gene-therapy follow-up does not describe a course of protein injections.[5][6]
Safety profile
Reduced vision and retinal fluid accumulation were reported in eleven bodybuilders after products labeled follistatin-344. This uncontrolled series cannot establish causation or an adverse-event rate, and product identity is a separate concern.[7][8]
Cautions
What we don't know
Reproductive-hormone effects and longer-term safety require product-specific human study; follistatin binds more than one growth-regulating signal.[4]
Regulatory status
United States
Follistatin is not an FDA-approved drug name. Availability of a supplement or research preparation does not establish approval for a medical use.[53]
International
Medicinal-product authorization outside the United States remains unconfirmed for the named preparation; status depends on the exact product and country.[53]
Sports & competition
Follistatin is named in WADA’s 2026 Prohibited List under S4.3 as a myostatin-binding protein, prohibited at all times.
How it works
Follistatin captures activin-family proteins before they activate their receptors. Binding myostatin can release part of the restraint on muscle growth, while binding activin also affects reproductive and other tissue signals.[4]
The effect depends on the protein form and where it is produced. FS288 binds strongly to cell-surface components, while FS315 is more readily found in circulation. Sustained muscle production after gene transfer cannot be reproduced simply by borrowing the vector’s dose or schedule for an injected protein.[4][5]
Research gaps & open questions
What the current literature has not yet settled about Follistatin:
Controlled trials need to separate muscle size from useful strength and mobility, and distinguish the effect of gene transfer from that of administered protein.[5][6]
A characterized protein product needs absorption, distribution and dose studies in people before any injected amount can be linked to an expected effect or compared with FS315/FS288 biology.[4][8]
The reported retinal complications after products labeled follistatin-344 require independent investigation of product identity, causation and recurrence risk; the case series cannot establish an adverse-event rate.[7][8]
Common questions
Does follistatin gene therapy prove protein injections work?
History & discovery
Follistatin entered the literature through reproductive-hormone research before its muscle-growth applications developed. Its history spans a natural protein, distinct isoforms and later gene-transfer interventions.[1][4]
Molecular work identified alternatively spliced human precursors, establishing that follistatin is a family of related protein forms rather than one interchangeable product.[1]
A six-patient Becker muscular dystrophy study delivered an AAV vector to thigh muscles. Its preliminary walking and tissue findings concerned gene transfer, not a purified follistatin injection.[5]
60 citations
Follistatin research covers muscle growth, activin signaling and neuromuscular function. Human findings are preliminary and depend on the intervention: gene transfer, engineered proteins and standalone follistatin have different exposure and safety profiles.[5][6][8][7][4][54][56][58][59][29][31][19][40][34][35][47]
