Follistatin 315
Also known as FST-315, FS315
Follistatin 315 is the more abundant circulating isoform of follistatin, lacking the heparin-binding domain present in the 344 variant, which makes it behave more as a freely circulating systemic inhibitor of activin and myostatin. It is studied in research settings for the same underlying rationale as follistatin 344: reducing myostatin-driven suppression of muscle growth. Because it does not bind cell surfaces as readily, its distribution and activity profile in tissue is understood to differ somewhat from the 344 isoform, though comparative human data are minimal. It remains an emerging research compound rather than an established therapeutic.
Reported benefits
- Proposed systemic reduction in myostatin/activin signaling
- Suggested support for muscle mass preservation and growth in research models
- May offer a different distribution profile than follistatin 344 for those studying systemic effects
How it works
Follistatin 315 binds myostatin and activin A with high affinity in the bloodstream, preventing them from reaching the activin type IIB receptor on muscle cell membranes and blocking the resulting Smad2/3-mediated suppression of muscle protein synthesis. Lacking a strong heparin-binding domain, it is thought to circulate more freely rather than concentrating at cell surfaces, potentially favoring broader systemic distribution over localized muscle-tissue action.
Dosing ranges
| Tier | Dose | Frequency |
|---|---|---|
| Low | 100 mcg | once daily |
| Moderate | 200 mcg | once daily |
| High | 300 mcg | once daily |
Estimate your dose
Dose estimator
Educational estimate for Follistatin 315, scaled to body size. Not medical advice.
Read before using
This tool produces a rough, educational estimate by scaling commonly-published research ranges to lean body mass. It is not a prescription, medical advice, or a substitute for a licensed clinician. Most peptides use fixed dosing, not weight-based dosing. Individual needs vary. You are solely responsible for how you use this information.
Mixing & syringe units
Target dose follows the estimate above. Change it there, or edit it directly below.
Reconstitution calculator
How much water to add and how many insulin-syringe units to draw for Follistatin 315. Educational only.
Draw to this mark
4units
= 0.04 mL for a 200 mcg dose
Concentration
5,000 mcg/mL
Doses per vial
50
A U-100 syringe reads 100 units per mL. This is a reconstitution math helper only, not a prescription or dosing advice. Verify every calculation and consult a qualified healthcare provider.
Possible side effects
Stacking notes
Sometimes discussed as an alternative to follistatin 344 in research contexts; no direct human comparative data exist to guide combined or sequential use.
Common protocols
Common questions
What is Follistatin 315 used for?
Follistatin 315 is the more abundant circulating isoform of follistatin, lacking the heparin-binding domain present in the 344 variant, which makes it behave more as a freely circulating systemic inhibitor of activin and myostatin. It is studied in research settings for the same underlying rationale as follistatin 344: reducing myostatin-driven suppression of muscle growth. Because it does not bind cell surfaces as readily, its distribution and activity profile in tissue is understood to differ somewhat from the 344 isoform, though comparative human data are minimal. It remains an emerging research compound rather than an established therapeutic. Reported benefits include proposed systemic reduction in myostatin/activin signaling, suggested support for muscle mass preservation and growth in research models, may offer a different distribution profile than follistatin 344 for those studying systemic effects.
What is a typical Follistatin 315 dose?
Reported ranges run 100-300 mcg. A low protocol is 100 mcg once daily, and a high protocol is 300 mcg once daily. This is reference information, not a dosing recommendation.
What is the half life of Follistatin 315?
Estimated at several hours; not well characterized in humans. That is what sets the dosing frequency, and it is why the timing guidance is morning.
How is Follistatin 315 administered?
Subcutaneous: into fat, under the skin.
How long is a Follistatin 315 cycle?
4-8 weeks on, followed by 8-12 weeks off. As with other follistatin isoforms, conservative cycling is generally discussed given the absence of long-term safety data.
What are the side effects of Follistatin 315?
Reported side effects include unestablished long-term human safety profile, theoretical disruption of activin-dependent reproductive and tissue-repair processes, injection site irritation, unpredictable systemic effects from broad tgf-beta superfamily inhibition. Evidence level for this compound is emerging research.
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Educational use only, not medical advice. This profile is provided for research and educational purposes. Follistatin 315 may be a research compound or a prescription medication; nothing here is intended to diagnose, treat, cure, or prevent any disease. Dosing figures reflect commonly-published research ranges, not a prescription. Consult a qualified healthcare provider before making any health decision. Read the full disclaimer →