Follistatin 315 vs 344: Which Version Actually Works Better?

They are not the same molecule. Follistatin 315 and follistatin 344 differ by only 29 amino acids, but those 29 change where the protein goes, how long it lasts, and what it binds.

The marketing usually skips this. The decision should not.

Follistatin 315 vs 344 — quick comparison

Start with the table. This is the version most pages bury under three paragraphs of background.

That table is the whole answer most readers came for. The rest of the article explains why the rows read this way.

For background on the broader category, read the follistatin overview and the follistatin peptide article. For dose specifics, use the follistatin dosage guide.

What the two isoforms actually are

One gene, two transcripts. The FST gene produces two main transcripts through alternative splicing: a long precursor that becomes FS-344, and a shorter transcript that becomes FS-315.

FS-344 carries an extra 29 amino acids on its C-terminal tail. That tail contains a heparin-binding domain — a short, sticky stretch that grabs onto proteoglycans on cell surfaces.

After secretion, FS-344 is enzymatically cleaved. The signal peptide and tail are removed, leaving FS-315 floating freely in blood and extracellular fluid.

So in a strict sense, FS-344 is the parent form and FS-315 is the circulating mature form. Calling them "competing peptides" is misleading.

Why the heparin tail matters

This is the single most important biological difference. The heparin-binding tail on FS-344 acts like a magnet for cell-surface sugars.

That magnetism pulls FS-344 out of the bloodstream and onto local cell surfaces near the injection site. So FS-344 acts mostly where it was put down.

FS-315 lacks the tail. It stays in solution, drifts around the body, and neutralizes activin and myostatin further from where it was secreted.

In gene-therapy work, an AAV-delivered FS-344 transcript transcribes locally inside muscle fibers, then matures to FS-315 in serum. That two-step pattern is the reason the sIBM trial chose FS-344: local production, systemic clearance, low off-target endocrine effect.

Half-life and clearance in practice

The numbers most often cited are research-model figures, not human pharmacokinetic studies.

• FS-344 in research preparations: reported active duration of about 24–48 hours before tail cleavage.
• FS-315 after cleavage: longer circulating half-life, up to about 72 hours, partly because it does not stick to cell surfaces.
• Real human pharmacokinetic data for either isoform as an injected peptide are essentially absent.

That gap matters. The dosing protocols circulating in online communities — 100 mcg daily or every other day — were not built from formal human PK studies. They were extrapolated from animal work and lifter forums.

Off-target binding — the FSH question

This is where the safety conversation actually lives. Follistatin binds activins; activins drive FSH release from the pituitary.

Heavy systemic follistatin can lower FSH and disrupt fertility. A 2025 antibody study found a 75% FSH drop within a week of strong myostatin-pathway suppression and impaired ovulation in females.

That is why the AAV-FS344 trial in sIBM was watched so carefully — and why no FSH, LH, testosterone, or estrogen disturbances were seen at the 6 × 10¹¹ vg/kg quadriceps dose. The FS-344 isoform was specifically chosen to keep the action local.

FS-315 has lower affinity for activins than the cell-bound FS-288 form, but its systemic distribution can still affect the HPG axis at high doses. Anyone running long, high-dose cycles of either isoform should track FSH and LH at baseline and at follow-up.

Hypertrophy magnitude — what the data suggest

The peptide market makes both isoforms sound interchangeable. The available data say otherwise.

One comparative model reported by isoform researchers:

• Local FS-315 gene transfer produced about 41% hypertrophy in the injected muscle, with no effect on distant muscles.
• Local FS-344 gene transfer produced about 23% hypertrophy in the injected muscle and 14–18% in non-injected muscles.

The mouse muscle-mass numbers cited in older reviews — 100–150% increases at high doses in normal mice and roughly 34% fiber-diameter gains in dystrophic models — were generated by AAV-mediated transgenic overexpression, not peptide injection. They overstate what a peptide cycle can deliver.

Human results are far smaller. A 2019 lean-mass study in hypogonadal males reported 8.2% lean mass gain over 12 weeks vs 2.1% placebo, and human case reports show single-digit-percent lean changes paired with strength gains.

The peptide market reality

This is the part most "315 vs 344" articles skip. The bigger problem with both isoforms is what is actually in the vial.

A WADA-affiliated forensic study analyzed 17 black-market follistatin products. Only 9 of 17 contained any follistatin at all. The rest contained other peptides like MGF or GHRP-2.

Of the 9 that did contain follistatin, all carried a His-tag and visible protein clumping — not characteristics of native human follistatin. The Real Peptides team reports that human-grade follistatin costs over $4,500 per milligram, which is why adulteration is so widespread.

The reasonable read:

1. The "is 315 better or worse than 344" question only matters if the vial contains real follistatin.
2. Most lifters comparing 315 vs 344 in forums are comparing two products that may share the same purity problems.
3. Third-party testing, COA review, and a reputable supplier matter more than picking an isoform.

For supplier diligence basics, read the where to buy follistatin guide.

Which isoform fits which goal

If purity is sorted, the isoform choice maps to your goal.

For lifters with no specific local target, FS-315's longer half-life and systemic distribution is the more common pick. For people running it post-tear or for a single muscle group, FS-344 is the more common pick.

Neither isoform should be confused with gene therapy, which is a fundamentally different intervention.

Side effect profile by isoform

Both share the same documented side-effect list. The differences are mostly in distribution.

• Injection-site reactions (15–25%): roughly equal across isoforms.
• Headaches (8–15%) and fatigue (5–12%): more often reported with FS-315 because of its systemic spread.
• FSH suppression: a temporary 15–30% FSH drop has been documented at therapeutic peptide doses; greater theoretical risk with FS-315.
• Tendon weakness: a theoretical concern with both, based on myostatin-deficient mouse tendon data.
• Central serous chorioretinopathy: documented in 11 male bodybuilders using 1 mg FS-344 injections — roughly 10× a normal "research" dose. Most cases resolved in 2–3 months; multi-injection cases recurred.
• Liver enzyme elevation: about 17% of subjects in a 16-week FS-315 protocol showed mild ALT rises (1.5–2× upper limit), resolving 4–6 weeks after stopping.

The bigger picture is the same for both: long-term safety beyond 16 weeks is essentially unstudied, and chronic high-dose use of either isoform is not supported by human data. For the full safety review, see the follistatin side effects article.

How to think about the choice

Practical decision flow. Do not start with the isoform; start with the prerequisites.

1. Is the product third-party tested with a recent COA?
2. Does the vial show no visible clumping after reconstitution?
3. Have you already optimized training, sleep, protein, and creatine?
4. Do you have baseline labs (CBC, CMP, lipid panel, FSH/LH) you can compare against?
5. Is the cycle short (≤4 weeks), not chronic?

If those answers are yes, then the isoform choice is straightforward: FS-315 for systemic goals, FS-344 for localized goals, and follow-up labs to confirm nothing drifted.

If those answers are no, no isoform choice rescues the run.

Sources and notes

• Long-term enhancement of skeletal muscle mass and strength by single gene administration (Kota / Mendell 2009)
• Follistatin gene therapy for sporadic inclusion body myositis (Mendell 2017)
• SteroidWiki — Follistatin 315 vs 344: What's the Difference?
• Behemoth Labz — Follistatin 315 vs Follistatin 344 comparison
• Beyond Health Lab — Follistatin 344 mechanism and FS-315 distinction
• Real Peptides — What Is FST-344
• Stacktrax follistatin guide — WADA forensics
• PeptideInsight — Follistatin-315 profile
• PeptideInsight — Follistatin-344 profile

Frequently Asked Questions