Mechanism

How it works

FST 344 binds with high affinity to myostatin (GDF-8) and activin A/B, sequestering these TGF-β superfamily ligands and preventing them from engaging their cognate receptors (ActRIIA/ActRIIB). This disinhibits downstream anabolic signaling through the Smad2/3 pathway, shifting the balance toward muscle protein synthesis and satellite cell activation. Simultaneously, FST 344 can modulate FSH signaling in the pituitary axis due to its activin-antagonist properties.

Reported in research

Benefits

Significant skeletal muscle hypertrophy via myostatin inhibition
Enhanced muscle recovery and reduced exercise-induced damage
Potential improvement in bone mineral density through activin suppression
May support satellite cell proliferation, aiding long-term muscle repair and growth

Context, not a prescription

Dosing

Typical range: 100 mcg every other day or 50-100 mcg/day (Subcutaneous, Intramuscular)
Cycle length: 4-8 weeks, with extended breaks recommended
Half-life: ~24-36 hours (estimated; highly context-dependent)

Safety

Side effects & contraindications

Possible side effects

Potential tendon stress due to rapid muscle hypertrophy outpacing connective tissue adaptation
Joint discomfort if musculoskeletal imbalance develops
Possible hormonal disruption (FSH suppression) particularly relevant in women
Localized injection site reactions (redness, swelling)

Contraindications

Women trying to conceive or with reproductive hormone disorders (due to activin/FSH interference)
Active or history of hormone-sensitive cancers
Individuals with tendinopathy or connective tissue disorders
Children and adolescents (growth plate concerns)

Research information, not medical advice. Always consult a licensed clinician before considering any peptide.

In depth

Full profile

What it does

Users typically report increased muscle size, improved muscle fullness, faster recovery between workouts, and greater training capacity. Some report joint or tendon discomfort if muscle growth outpaces adaptation of supporting structures.

How it works

Imagine your muscle growth is a car with a speed limiter installed by the factory (myostatin). FST 344 is like a mechanic who removes that limiter, letting the engine — your muscles — run at full throttle.

After injection, FST 344 circulates in the bloodstream and latches onto myostatin and a related protein called activin, both of which normally tell muscles to stop growing. By grabbing onto these inhibitors, FST 344 allows muscle cells to receive more growth signals, divide, and produce more contractile protein.

What to expect

Anecdotal reports suggest noticeable changes in muscle fullness and pumps within 1-2 weeks, with meaningful hypertrophy apparent by weeks 3-4.

Week 1: Compound begins circulating; muscle cells start receiving less inhibition. Many users notice increased pumps and muscle fullness.
Weeks 2-4: Hypertrophy becomes visually noticeable. Recovery between sessions improves. Monitor for joint or tendon discomfort.
Weeks 4-8: Maximum hypertrophic gains accumulate. A cycle break is recommended to assess tendon health and allow hormonal normalization.

Good to know

Start at the lower end of the dosing range and assess tolerance before increasing
Incorporate progressive tendon conditioning (eccentric exercises) to help connective tissue keep pace with muscle growth

Staying safe

Sore or stiff joints/tendons as muscles grow faster than connective tissue
Injection site redness or mild swelling

Avoid if you have:

Women trying to become pregnant or with fertility concerns
Anyone with a history of cancer
People with existing tendon or joint injuries

Overview

Progressive skeletal muscle hypertrophy (predominantly type II fiber growth), satellite cell expansion, reduced markers of muscle proteolysis, potential increases in bone mineral density via activin suppression (activin normally inhibits osteoblastogenesis), and possibly reduced inflammatory myokine signaling.

How it works

FST 344 functions like a competitive inhibitor in enzyme kinetics — it occupies the active binding interface of myostatin with such high affinity that functional receptor engagement is thermodynamically outcompeted, analogous to a high-affinity substrate analogue blocking an enzyme's active site before the true substrate can bind.

Following subcutaneous or intramuscular administration, FST 344 distributes systemically due to reduced heparan sulfate proteoglycan binding relative to FST 288. Its interaction with activin A also modulates hypothalamic-pituitary FSH secretion, which may have downstream effects on gonadal hormone axes — particularly relevant in female subjects. Hepatic clearance and renal filtration govern elimination; glycosylation status influences half-life. At the tissue level, blunted Smad2/3 signaling upregulates MyoD, myogenin, and IGF-1 local expression, amplifying hypertrophic response.

Onset & timeline

Pharmacodynamic effects on myostatin suppression likely begin within hours of administration given rapid ligand binding kinetics. Measurable changes in muscle cross-sectional area have been documented in animal studies within 2-3 weeks of sustained administration; human anecdotal timelines suggest 1-2 weeks for subjective effects and 3-4 weeks for objective hypertrophy.

Days 1-3: Systemic distribution achieved; follistatin-myostatin complexes forming rapidly. Early receptor-level disinhibition of Smad2/3 pathway begins within hours. No subjective changes typically reported.
Weeks 1-2: Upregulation of MyoD and myogenin in satellite cells; increased muscle protein synthesis rates measurable via tracer studies in rodent models. Subjective reports of improved pump, fullness, and reduced DOMS.
Weeks 2-8: Progressive muscle fiber hypertrophy accumulates, predominantly type II fibers. Bone remodeling markers may shift favorably. Connective tissue stress risk peaks around weeks 4-6; monitoring is critical. Cycle cessation recommended by week 8 to allow hormonal normalization.

Getting the most from it

Concurrent collagen peptide supplementation (10-15 g/day with vitamin C) and eccentric loading protocols to promote parallel tendon adaptation
Cycle length limitation to 6-8 weeks maximum with equivalent off-period; periodic hormone panel monitoring (LH, FSH, estradiol/testosterone) during cycle

Common side effects

Musculotendinous imbalance: rapid myofibrillar hypertrophy without proportional collagen synthesis in tendons increases enthesopathy and strain risk
Transient FSH suppression with potential temporary impairment of reproductive axis — clinically significant in females

Mechanism of action

FST 344 is the predominant circulating isoform of follistatin, differing from FST 288 (tissue-bound) by the inclusion of exon 6-encoded domain that reduces heparan sulfate binding, enabling broader systemic distribution. It acts as a high-affinity antagonist to multiple TGF-β superfamily ligands — principally myostatin (GDF-8), activin A, activin B, BMP-11, and GDF-11 — with Kd values in the low picomolar range. By sequestering these ligands extracellularly, FST 344 prevents receptor complex formation at ActRIIA/ActRIIB, blocking downstream Smad2/3 phosphorylation and its transcriptional suppression of muscle anabolic genes. This shifts the mTORC1/Akt axis toward protein synthesis and promotes satellite cell proliferation through reduced atrogene (MAFbx/MuRF-1) expression.

Following subcutaneous or intramuscular administration, FST 344 distributes systemically due to reduced heparan sulfate proteoglycan binding relative to FST 288. Its interaction with activin A also modulates hypothalamic-pituitary FSH secretion, which may have downstream effects on gonadal hormone axes — particularly relevant in female subjects. Hepatic clearance and renal filtration govern elimination; glycosylation status influences half-life. At the tissue level, blunted Smad2/3 signaling upregulates MyoD, myogenin, and IGF-1 local expression, amplifying hypertrophic response.

Pharmacodynamics

Pharmacodynamic effects on myostatin suppression likely begin within hours of administration given rapid ligand binding kinetics. Measurable changes in muscle cross-sectional area have been documented in animal studies within 2-3 weeks of sustained administration; human anecdotal timelines suggest 1-2 weeks for subjective effects and 3-4 weeks for objective hypertrophy.

Progressive skeletal muscle hypertrophy (predominantly type II fiber growth), satellite cell expansion, reduced markers of muscle proteolysis, potential increases in bone mineral density via activin suppression (activin normally inhibits osteoblastogenesis), and possibly reduced inflammatory myokine signaling.

Timeline

Days 1-3: Systemic distribution achieved; follistatin-myostatin complexes forming rapidly. Early receptor-level disinhibition of Smad2/3 pathway begins within hours. No subjective changes typically reported.
Weeks 1-2: Upregulation of MyoD and myogenin in satellite cells; increased muscle protein synthesis rates measurable via tracer studies in rodent models. Subjective reports of improved pump, fullness, and reduced DOMS.
Weeks 2-8: Progressive muscle fiber hypertrophy accumulates, predominantly type II fibers. Bone remodeling markers may shift favorably. Connective tissue stress risk peaks around weeks 4-6; monitoring is critical. Cycle cessation recommended by week 8 to allow hormonal normalization.

Comparisons

FST 344 — effectiveness High, safety Moderate, cost $$$$, High to use
ACE-031 (ActRIIB-Fc) — effectiveness Very High, safety Caution, cost N/A (discontinued), High to use
IGF-1 LR3 — effectiveness High, safety Moderate, cost $$$, Medium to use

Adverse effects

Common:

Musculotendinous imbalance: rapid myofibrillar hypertrophy without proportional collagen synthesis in tendons increases enthesopathy and strain risk
Transient FSH suppression with potential temporary impairment of reproductive axis — clinically significant in females

Rare:

Aberrant tissue growth in non-target organs expressing ActRIIB (e.g., cardiac hypertrophy at supraphysiological doses in rodent models — incidence in humans unknown)

Contraindications & risk mitigation

Contraindicated in:

Patients with hormone-sensitive neoplasms (breast, ovarian, prostate) due to activin/FSH axis disruption
Individuals with established musculoskeletal tendinopathy or prior tendon rupture
Those on anticoagulant therapy (injection-related considerations) or with immunocompromised states

Concurrent collagen peptide supplementation (10-15 g/day with vitamin C) and eccentric loading protocols to promote parallel tendon adaptation
Cycle length limitation to 6-8 weeks maximum with equivalent off-period; periodic hormone panel monitoring (LH, FSH, estradiol/testosterone) during cycle

Qué hace

Users typically report increased muscle size, improved muscle fullness, faster recovery between workouts, and greater training capacity. Some report joint or tendon discomfort if muscle growth outpaces adaptation of supporting structures.

Cómo funciona

Imagine your muscle growth is a car with a speed limiter installed by the factory (myostatin). FST 344 is like a mechanic who removes that limiter, letting the engine — your muscles — run at full throttle.

After injection, FST 344 circulates in the bloodstream and latches onto myostatin and a related protein called activin, both of which normally tell muscles to stop growing. By grabbing onto these inhibitors, FST 344 allows muscle cells to receive more growth signals, divide, and produce more contractile protein.

Qué esperar

Anecdotal reports suggest noticeable changes in muscle fullness and pumps within 1-2 weeks, with meaningful hypertrophy apparent by weeks 3-4.

Week 1: Compound begins circulating; muscle cells start receiving less inhibition. Many users notice increased pumps and muscle fullness.
Weeks 2-4: Hypertrophy becomes visually noticeable. Recovery between sessions improves. Monitor for joint or tendon discomfort.
Weeks 4-8: Maximum hypertrophic gains accumulate. A cycle break is recommended to assess tendon health and allow hormonal normalization.

Bueno saber

Start at the lower end of the dosing range and assess tolerance before increasing
Incorporate progressive tendon conditioning (eccentric exercises) to help connective tissue keep pace with muscle growth

Manteniéndose seguro

Sore or stiff joints/tendons as muscles grow faster than connective tissue
Injection site redness or mild swelling

Evitar si tienes:

Women trying to become pregnant or with fertility concerns
Anyone with a history of cancer
People with existing tendon or joint injuries

Descripción general

Progressive skeletal muscle hypertrophy (predominantly type II fiber growth), satellite cell expansion, reduced markers of muscle proteolysis, potential increases in bone mineral density via activin suppression (activin normally inhibits osteoblastogenesis), and possibly reduced inflammatory myokine signaling.

Cómo funciona

FST 344 functions like a competitive inhibitor in enzyme kinetics — it occupies the active binding interface of myostatin with such high affinity that functional receptor engagement is thermodynamically outcompeted, analogous to a high-affinity substrate analogue blocking an enzyme's active site before the true substrate can bind.

Following subcutaneous or intramuscular administration, FST 344 distributes systemically due to reduced heparan sulfate proteoglycan binding relative to FST 288. Its interaction with activin A also modulates hypothalamic-pituitary FSH secretion, which may have downstream effects on gonadal hormone axes — particularly relevant in female subjects. Hepatic clearance and renal filtration govern elimination; glycosylation status influences half-life. At the tissue level, blunted Smad2/3 signaling upregulates MyoD, myogenin, and IGF-1 local expression, amplifying hypertrophic response.

Inicio y cronología

Pharmacodynamic effects on myostatin suppression likely begin within hours of administration given rapid ligand binding kinetics. Measurable changes in muscle cross-sectional area have been documented in animal studies within 2-3 weeks of sustained administration; human anecdotal timelines suggest 1-2 weeks for subjective effects and 3-4 weeks for objective hypertrophy.

Days 1-3: Systemic distribution achieved; follistatin-myostatin complexes forming rapidly. Early receptor-level disinhibition of Smad2/3 pathway begins within hours. No subjective changes typically reported.
Weeks 1-2: Upregulation of MyoD and myogenin in satellite cells; increased muscle protein synthesis rates measurable via tracer studies in rodent models. Subjective reports of improved pump, fullness, and reduced DOMS.
Weeks 2-8: Progressive muscle fiber hypertrophy accumulates, predominantly type II fibers. Bone remodeling markers may shift favorably. Connective tissue stress risk peaks around weeks 4-6; monitoring is critical. Cycle cessation recommended by week 8 to allow hormonal normalization.

Cómo aprovecharlo al máximo

Concurrent collagen peptide supplementation (10-15 g/day with vitamin C) and eccentric loading protocols to promote parallel tendon adaptation
Cycle length limitation to 6-8 weeks maximum with equivalent off-period; periodic hormone panel monitoring (LH, FSH, estradiol/testosterone) during cycle

Efectos secundarios comunes

Musculotendinous imbalance: rapid myofibrillar hypertrophy without proportional collagen synthesis in tendons increases enthesopathy and strain risk
Transient FSH suppression with potential temporary impairment of reproductive axis — clinically significant in females

Mecanismo de acción

FST 344 is the predominant circulating isoform of follistatin, differing from FST 288 (tissue-bound) by the inclusion of exon 6-encoded domain that reduces heparan sulfate binding, enabling broader systemic distribution. It acts as a high-affinity antagonist to multiple TGF-β superfamily ligands — principally myostatin (GDF-8), activin A, activin B, BMP-11, and GDF-11 — with Kd values in the low picomolar range. By sequestering these ligands extracellularly, FST 344 prevents receptor complex formation at ActRIIA/ActRIIB, blocking downstream Smad2/3 phosphorylation and its transcriptional suppression of muscle anabolic genes. This shifts the mTORC1/Akt axis toward protein synthesis and promotes satellite cell proliferation through reduced atrogene (MAFbx/MuRF-1) expression.

Following subcutaneous or intramuscular administration, FST 344 distributes systemically due to reduced heparan sulfate proteoglycan binding relative to FST 288. Its interaction with activin A also modulates hypothalamic-pituitary FSH secretion, which may have downstream effects on gonadal hormone axes — particularly relevant in female subjects. Hepatic clearance and renal filtration govern elimination; glycosylation status influences half-life. At the tissue level, blunted Smad2/3 signaling upregulates MyoD, myogenin, and IGF-1 local expression, amplifying hypertrophic response.

Farmacodinamia

Pharmacodynamic effects on myostatin suppression likely begin within hours of administration given rapid ligand binding kinetics. Measurable changes in muscle cross-sectional area have been documented in animal studies within 2-3 weeks of sustained administration; human anecdotal timelines suggest 1-2 weeks for subjective effects and 3-4 weeks for objective hypertrophy.

Progressive skeletal muscle hypertrophy (predominantly type II fiber growth), satellite cell expansion, reduced markers of muscle proteolysis, potential increases in bone mineral density via activin suppression (activin normally inhibits osteoblastogenesis), and possibly reduced inflammatory myokine signaling.

Cronología

Days 1-3: Systemic distribution achieved; follistatin-myostatin complexes forming rapidly. Early receptor-level disinhibition of Smad2/3 pathway begins within hours. No subjective changes typically reported.
Weeks 1-2: Upregulation of MyoD and myogenin in satellite cells; increased muscle protein synthesis rates measurable via tracer studies in rodent models. Subjective reports of improved pump, fullness, and reduced DOMS.
Weeks 2-8: Progressive muscle fiber hypertrophy accumulates, predominantly type II fibers. Bone remodeling markers may shift favorably. Connective tissue stress risk peaks around weeks 4-6; monitoring is critical. Cycle cessation recommended by week 8 to allow hormonal normalization.

Comparaciones

FST 344 — efectividad High, seguridad Moderate, costo $$$$, High de usar
ACE-031 (ActRIIB-Fc) — efectividad Very High, seguridad Caution, costo N/A (discontinued), High de usar
IGF-1 LR3 — efectividad High, seguridad Moderate, costo $$$, Medium de usar

Efectos adversos

Comunes:

Musculotendinous imbalance: rapid myofibrillar hypertrophy without proportional collagen synthesis in tendons increases enthesopathy and strain risk
Transient FSH suppression with potential temporary impairment of reproductive axis — clinically significant in females

Raros:

Aberrant tissue growth in non-target organs expressing ActRIIB (e.g., cardiac hypertrophy at supraphysiological doses in rodent models — incidence in humans unknown)

Contraindicaciones y mitigación de riesgos

Contraindicado en:

Patients with hormone-sensitive neoplasms (breast, ovarian, prostate) due to activin/FSH axis disruption
Individuals with established musculoskeletal tendinopathy or prior tendon rupture
Those on anticoagulant therapy (injection-related considerations) or with immunocompromised states

Concurrent collagen peptide supplementation (10-15 g/day with vitamin C) and eccentric loading protocols to promote parallel tendon adaptation
Cycle length limitation to 6-8 weeks maximum with equivalent off-period; periodic hormone panel monitoring (LH, FSH, estradiol/testosterone) during cycle

Reference data

Specifications

Molecular formula: C₁₅₁H₂₁₈N₄₂O₅₇S₇ (approximate for recombinant core; glycosylation adds variability)
Molecular weight: ~31,500 Da (core protein; ~35,000-40,000 Da with glycosylation)
Half-life: ~24-36 hours (estimated; highly context-dependent)
Route: Subcutaneous, Intramuscular
Cycle length: 4-8 weeks, with extended breaks recommended
Storage: Store lyophilized powder at -20°C long term; once reconstituted, store at 4°C and use within 4-7 days. Protect from light and repeated freeze-thaw cycles.
Legal status: Research compound; not approved by the FDA or any major regulatory body for human therapeutic use. Classified as a research chemical in most jurisdictions.

FAQ

Common questions

How does FST 344 differ from FST 288 in terms of pharmacology?

FST 344 includes the domain encoded by exon 6, which reduces its affinity for heparan sulfate proteoglycans (HSPGs). This prevents membrane tethering, resulting in greater systemic bioavailability and broader biodistribution compared to FST 288, which is preferentially localized to tissues expressing HSPGs such as the gonads and pituitary.

Is there any human clinical trial data supporting its use?

Gene therapy trials (e.g., Mendell et al., 2009 in STXBP1-related disorders and Becker MD context) have demonstrated safety of follistatin gene delivery, but recombinant FST 344 protein administration in healthy humans for performance enhancement has no published controlled trial data. Evidence remains primarily preclinical.

What is the relationship between FST 344 and IGF-1?

Myostatin suppression indirectly enhances local IGF-1 expression in muscle via Akt/mTOR pathway derepression. Stacking exogenous IGF-1 LR3 with FST 344 may produce synergistic anabolic effects but also amplifies risks of hypoglycemia and unregulated tissue proliferation.

What is the evidence level?

This compound is classified as Animal data. Most data comes from preclinical animal studies. Human clinical trial evidence is limited or absent.

Research

Research & sources

Animal data

Current evidence for FST 344 is rated as Animal data. Research is based primarily on animal models.

1. Follistatin gene therapy for Becker muscular dystrophy: a non-randomized controlled trial (2015) — Molecular Therapy
2. Inactivation of myostatin does not result in hyper-muscularity in dystrophin-deficient mdx mice (2006) — Journal of Clinical Investigation
3. Systemic delivery of follistatin 288 promotes muscle hypertrophy and reduces fat accumulation in mice (2008) — Endocrinology
4. Follistatin isoforms and their roles in muscle and reproductive biology (2011) — Molecular and Cellular Endocrinology

Explore

Often discussed alongside

IGF-1 LR3 · growth BPC-157 · healing TB-500 · healing GHRP-6 · growth