Follistatin-315 Reconstitution Calculator

Follistatin-315 (FS-315) is one of the natural isoforms of follistatin, a protein that binds and neutralizes myostatin — the body's main "brake" on muscle growth — as well as activin. FS-315 specifically is the freely circulating, systemic form: it has only weak binding to cell surfaces, so it travels through the bloodstream rather than staying anchored in local tissue. By mopping up myostatin and activin in circulation, it's studied as a way to release the natural limit on muscle growth.

In research and anecdotal protocols, follistatin is referenced in micrograms — commonly around 100 mcg per day by subcutaneous injection, often run in short blocks of a few weeks. Because the protein clears from the blood within hours, daily dosing is typical. These are research-reference figures, not an approved regimen, and the right dose, length, and whether it's appropriate at all are clinical decisions.

Add bacteriostatic water to the dry vial slowly and swirl gently — never shake, as proteins are fragile. For example, a 1 mg vial + 1 mL of water gives 1 mg/mL (1,000 mcg/mL); a 100 mcg dose is then 0.1 mL, which is 10 units on a U-100 insulin syringe. Use the calculator above to match your actual vial size (1 mg is common) and target dose. Doses are in micrograms, so draw carefully.

They're closely related: Follistatin-344 is the longer precursor protein, and the body processes it into the mature circulating forms — Follistatin-315 being the main one. So FS-315 is essentially the systemic, free-floating version that FS-344 becomes, with weak cell-surface binding and broad circulation. Practically, research that wants systemic, whole-body myostatin and activin neutralization leans toward the circulating FS-315 profile, whereas FS-344 is the commonly sold "starting" form; the third isoform, FS-288, is the opposite extreme — it sticks to cell surfaces and acts locally.

Here the reputation runs far ahead of the evidence: the famous "double-muscled" results — myostatin-knockout cattle and mice with 50–115% more muscle — come from genetic modification and gene therapy, not from injecting follistatin protein, and there are essentially no human studies showing the injected peptide reproduces them. Because follistatin also blocks activin (a regulator of FSH), there are theoretical effects on reproductive hormones, and strong myostatin inhibition raises questions about tendon and cardiac tissue. It is not FDA-approved, is sold for research only, and is banned by WADA as a myostatin inhibitor. Store the dry vial refrigerated and out of light; once reconstituted, keep it cold and use within a few weeks.