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FGL (FG Loop Peptide)15 residuesEVYVVAENQQGKSKAEach bubble = one amino acid. Size = residue mass. Color = chemical class.Also known as: FGL, FGLL, FG Loop peptide
Twenty-four volunteers. One Phase I trial. Then the entire clinical program shut down in 2014. FGL is a 15-amino-acid synthetic peptide that mimics NCAM's binding region on fibroblast growth factor receptor 1 (FGFR1). In rodent studies, it protects hippocampal neurons from ischemic death and boosts long-term potentiation in the dentate gyrus for up to 24 hours. Community subcutaneous doses of 1 to 2 mg sit 10 to 100 times below the Phase I intranasal doses that actually reached the brain. Early adopters chasing cognitive gains should know the dose-response gap is real.
FGL (FG Loop peptide) is a synthetic pentadecapeptide corresponding to residues E681 through A695 of NCAM's second fibronectin type III module. Whether it actually reaches the brain at the doses people inject remains the open question. The active form is FGL(L), a dimerized version of the monomer with better blood-brain barrier penetration. It was designed to mimic the NCAM-FGFR1 binding interaction, making it a targeted FGFR1 agonist. The preclinical track record is genuinely strong. Klementiev and colleagues [1] showed that a single injection after training produced memory improvements lasting a full month in rats. Bhatt's group confirmed hippocampal CA1 neuron protection against ischemic damage [2]. The Phase I intranasal trial (n=24)[3] used doses of 25, 100, and 200 mg and confirmed tolerability with detectable brain penetration. The disconnect: community subcutaneous dosing runs 1 to 2 mg daily. Phase I intranasal doses ran 25 to 200 mg. That is a 12x to 200x gap. No human subcutaneous pharmacokinetic study exists. Whether 1 to 2 mg SC even reaches the CNS in meaningful amounts is genuinely unknown. Fewer than 30 people have shared their experiences online. Longecity thread #109111 contains about 15 to 20 individual reports with mixed outcomes. Clinical development ended when the NeuroFGL consortium (CORDIS #278006) failed to advance past Phase I by 2014.
FGL binds directly to fibroblast growth factor receptor 1 (FGFR1), mimicking the natural contact between NCAM's second fibronectin type III domain and the receptor. That binding triggers FGFR1 phosphorylation and splits signaling down two paths. The first is the Ras-MAPK cascade. This arm drives neurite outgrowth, neuronal differentiation, and synaptic remodeling. Rodent studies confirmed increased synaptophysin expression within 24 to 72 hours of dosing [4]. The second is the PI3K-Akt pathway. This promotes neuronal survival by suppressing pro-apoptotic signals. In hippocampal slice cultures exposed to oxygen-glucose deprivation, FGL preserved metabolic function and presynaptic activity whether applied before or after the insult [2]. At the synapse, FGL activates PKC signaling. PKC triggers activity-dependent delivery of AMPA receptors to postsynaptic membranes. More AMPA receptors at the synapse means stronger signal transmission; this is the molecular basis for its memory-consolidating effects. FGL also mobilizes endogenous neural stem cells from the subventricular zone and hippocampus after stroke, supporting remyelination and modulating neuroinflammation through microglial regulation [5]. The active clinical form is FGL(L), a dimerized version of two monomers linked together. The dimer provides better blood-brain barrier penetration.
Phase I-only human data (intranasal, single-dose, n=24, 2007). Clinical development abandoned 2014 after NeuroFGL EU consortium failed to advance to Phase 2a. No Phase 2 conducted. Strong preclinical evidence for neuroprotection, LTP, and neurogenesis, but zero human efficacy data exists. Pro-epileptogenic signal in mouse kindling model is a real safety concern.
Bhatt DL et al. 2007 (PMID 17375985): Phase I intranasal FGL(L) in 24 healthy volunteers at 25, 100, 200 mg. All doses tolerated; mild nasal burning at 200 mg (2 subjects, <3 min). Dose-proportional systemic exposure; brain penetration detected. 8-day observation only: no chronic safety data.
No human efficacy data. Single Phase I trial (n=24, single dose, intranasal, 8-day observation only). Clinical development discontinued 2014: no Phase 2 attempted or planned. Community SC protocol (1–2 mg/day) is 10–100× below Phase I intranasal doses with no human PK basis. Pro-epileptogenic signal: FGL at 2 and 10 mg/kg SC significantly lowered generalized seizure threshold in mouse kindling model (PMC3963124). FGFR1 agonism is a theoretical oncogenesis risk (FGFR1 amplified/mutated in ~5–10% of human cancers). Must use dimerized FGL(L) form: monomer has inferior CNS penetration.
Extremely limited community experience. Fewer than 5 Reddit threads across r/Nootropics and r/Peptides. Primary data from Longecity forum thread #109111. Theoretical enthusiasm (FGFR1 mechanism, LTP, neuroprotection) exceeds actual user reports. Mixed outcomes: occasional cognitive clarity reports alongside brain fog and mood dysregulation.
Science establishes neuroprotective and LTP-enhancing effects in well-controlled rodent studies. Community adoption is too minimal and dose-uncertain to validate or refute the science. The community SC protocol (1–2 mg/day) may be producing no meaningful CNS effect: making both positive and negative community outcomes uninformative about the true mechanism.
| Level | Dose / Injection | Frequency |
|---|---|---|
| Beginner | 1mg | 5x/week |
| Moderate | 1.5mg | 5x/week |
| Aggressive | 2mg | 5x/week |
The single most important thing: confirm your vendor is selling dimerized FGL(L), not the monomer. The monomer has inferior CNS penetration and is the wrong form entirely. Ask for a CoA specifying dimeric form and molecular weight around 3,476 Da. Reconstitution math for a 5 mg vial: add 2 mL bacteriostatic water to get 2.5 mg/mL. A 1 mg dose is 0.4 mL, which is 40 units on a U-100 insulin syringe. A 2 mg dose is 0.8 mL (80 units). For a 10 mg vial with 2 mL BAC water, you get 5 mg/mL. A 2 mg dose becomes 0.4 mL (40 units). Swirl gently for 15 or more minutes after adding water. FGL(L) is stubborn to dissolve. If you see particles that won't clear, discard the vial. HA-FGL from WholisticResearch (about $122 per vial) is a hyaluronic acid conjugate. Different compound entirely with zero published human data. Not interchangeable with FGL(L).
Standard protocol is 5 days on, 2 days off for 6 weeks. Evaluate cognitive response before continuing. Some practitioners recommend 4 weeks off between cycles to maintain receptor sensitivity.
Continuous FGFR1 agonism risks receptor downregulation: a well-documented response to sustained growth factor receptor activation. The 6-weeks-on / 4-weeks-off protocol aims to restore FGFR1 sensitivity. The pro-epileptogenic preclinical signal also argues for periodic off-cycles to detect any emerging neurological changes before continuing.
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Expected: Anecdotal: possible improvements in focus, verbal recall, and memory consolidation by weeks 3–6. No human efficacy data exists to validate these outcomes. Dose adequacy for CNS effects in humans is unknown.
Monitor: Weekly mood/cognition self-check. Any seizure-like symptoms (head sensations, visual disturbances, muscle twitching): stop immediately. Confirm vendor is supplying dimerized FGL(L) before use.
Pull 2 mL of bacteriostatic water into a syringe and inject it slowly into your 5 mg FGL(L) vial. Aim the stream against the glass wall, not directly onto the powder.
Swirl the vial gently for 15 minutes or until fully dissolved. Don't shake or vortex. If cloudiness or particles persist after 15 minutes, discard the vial.
From a 5 mg/2 mL vial: 1 mg equals 40 units (0.4 mL). From a 10 mg/2 mL vial: 2 mg equals 40 units (0.4 mL).
Pinch abdominal fat and inject subcutaneously at a 45-degree angle. Use a 29 to 31 gauge needle. Rotate injection sites daily.
Follow the 5-days-on, 2-days-off schedule (Monday through Friday).
Move to 1.5 to 2 mg only if you tolerate the starting dose without brain fog, mood changes, or neurological symptoms.
Use within 21 days. Do not freeze after reconstitution. Lyophilized vials keep at minus 20 degrees C for up to 24 months.
Phase I intranasal doses: 25–200 mg. Community SC doses: 1–2 mg. Routes are not interchangeable: Phase I doses are 12–200× higher than community SC doses.
The only human data (PMID 17375985) used intranasal FGL(L) at 25, 100, and 200 mg. Some community members are experimenting with intranasal FGL at 1–5 mg as a compromise for direct CNS delivery, but this has no clinical validation. Mild nasal burning reported in 2/8 subjects at 200 mg in Phase I is not expected at lower doses.
Complementary cognitive mechanism: Semax activates BDNF/TrkB and dopaminergic pathways while FGL targets FGFR1/AMPA trafficking. Community describes as "dopaminergic clarity + synaptic plasticity" combination. Semax has far more community evidence and is an approved drug in Russia.
Semax 300–600 mcg intranasal 2x/day + FGL 1–2 mg SC daily, same 5-on-2-off schedule.
N-acetyl Semax preferred by some users for better intranasal bioavailability. Same rationale as Semax: dopaminergic complement to FGL's FGFR1/LTP mechanism.
Anxiolytic complement: used to offset potential mood disruption from FGL. Selank modulates GABAergic and serotonergic systems. Anecdotally reported to smooth the cognitive enhancement experience.
Broader neurotrophic support: Cerebrolysin provides BDNF, NGF, CNTF; FGL specifically drives FGFR1 activation. Described as "restorative + plasticity" combination. Cerebrolysin has significantly more human clinical data (stroke/TBI/Alzheimer's studies).
Cerebrolysin 5–10 mL IV or IM on alternate days; FGL 1–2 mg SC daily. Not recommended for beginners.
Both are FGFR1 agonists. Concurrent use risks additive FGFR1 signaling: oncogenesis risk theoretically compounded and seizure threshold potentially further lowered. No combination data exists in any species. Dihexa also has two foundational mechanism papers retracted (April 2025, data fabrication).
Do not combineDirect pharmacodynamic antagonism: FGFR inhibitors block FGL's receptor-mediated effects entirely. No combination data, but mechanism predicts complete antagonism.
Do not combineCaution: FGL may reduce effective anticonvulsant coverage by lowering seizure threshold (PMC3963124). Do not initiate FGL in patients actively managed for seizure disorders.
FGFR1 is an oncogenic driver in ~5–10% of all human cancers (breast, lung, bladder, endometrial). FGL agonism could theoretically worsen FGFR1-amplified malignancies. Absolute contraindication in any active malignancy.
Do not combinePricing updated 2026-04-09
The seizure risk is the lead concern. In a mouse kindling model of epilepsy, FGL at 2 and 10 mg/kg subcutaneous lowered the stimulation threshold for generalized seizures (PMC3963124). This is not a theoretical flag. It is a published preclinical finding that directly contradicts casual use in anyone with a seizure history. The Phase I human data covers 24 volunteers receiving single intranasal doses of 25, 100, or 200 mg of FGL(L) [3]. Two of eight subjects at the 200 mg dose reported brief nasal burning lasting less than 3 minutes. No ECG abnormalities, no vital sign changes, no laboratory parameter shifts across any dose group. That is reassuring for single-dose safety, but 8 days of observation in 24 people tells you almost nothing about chronic subcutaneous use. FGFR1 carries an oncogenesis concern. FGFR1 is amplified or mutated in roughly 5 to 10 percent of human cancers, including breast, lung, bladder, and endometrial malignancies. Sustained FGFR1 agonism could theoretically worsen any FGFR1-driven tumor. No cancer events appeared in the Phase I trial, but single-dose exposure is a poor test of this risk. Active malignancies are an absolute contraindication. Community side effects from the small Longecity user base include brain fog, mood dysregulation, and cognitive dulling, particularly in the first two weeks. These come from fewer than 30 total reports, so frequency estimates are not possible. An important caveat about community side effects: the subcutaneous doses people use (1 to 2 mg) may be far below CNS-effective concentrations. Allometric scaling of the animal doses (2 to 10 mg/kg in rats) translates to 140 to 700 mg for a 70 kg human. Both positive and negative community outcomes may be noise rather than real pharmacological signal. Injection site discomfort is mild and transient. Reconstitution can be tricky; FGL(L) dissolves slowly, and cloudy solutions with persistent particulate should be discarded. Contraindications: pregnancy and breastfeeding (no reproductive safety data). Seizure disorders or epilepsy history. Active brain tumors or CNS malignancies. Known hypersensitivity to NCAM-derived peptides. Children under 18. When to stop: any seizure-like symptoms including unusual head sensations, visual disturbances, or muscle twitching. Mood disruption persisting beyond one week. Any new cancer diagnosis.
Verify FGL (FG Loop Peptide) dosing and safety with a second opinion
Extremely niche compound with minimal vendor competition and no standardized manufacturing accountability. The critical distinction between monomer and dimerized FGL(L) is rarely verified by independent testing. HA-FGL (WholisticResearch) is a different compound (hyaluronic acid conjugate) commonly confused with FGL(L). No community analytical testing data available for purity verification.
| Test | When | Target |
|---|---|---|
| Neurological self-assessment | Weekly throughout protocol | No new neurological symptoms; stable or improving mood baseline |
| Liver function tests (ALT, AST, bilirubin) | Baseline and at cycle end (week 6) | Within normal laboratory reference ranges |
| Cognitive self-assessment (memory, verbal fluency) | Weekly using consistent test (digit span, verbal recall) | — |
Early detection of mood changes, cognitive disruption, or prodromal seizure symptoms. FGL's preclinical pro-epileptogenic signal (PMC3963124) warrants routine self-monitoring for unusual head sensations, visual disturbances, or muscle twitching.
Precautionary monitoring for any novel peptide with CNS growth factor signaling. No hepatotoxicity data exists for chronic SC FGL dosing.
Objective tracking of any cognitive signal: positive or negative. Identifies non-responders before completing full 6-week cycle. Also catches early cognitive worsening.
Minimal noticeable effects. The peptide begins engaging FGFR1 signaling and initiating synaptic remodeling processes. Some users report subtle improvements in mental clarity.
Gradual improvements in focus, verbal recall, and working memory. Synaptic plasticity changes are building as AMPA receptor trafficking increases.
Peak cognitive benefits typically emerge. Noticeable improvements in memory consolidation, learning speed, and mental endurance.
Benefits often persist beyond the active dosing period due to structural synaptic changes and ongoing neurotrophic factor upregulation.
Week 1: Most people won't notice anything. FGFR1 phosphorylation and downstream MAPK/PI3K signaling begin at the cellular level, and synaptophysin upregulation appears within 24 to 72 hours in preclinical models. Structural synaptic changes take weeks to build. Some users report subtle mental clarity; distinguishing that from placebo with single-user anecdotes is basically impossible. Common side effects: mild injection site discomfort, occasional headache, and brain fog reported by a minority at 2 mg doses. Weeks 2 to 3: This is when a signal starts building, if one appears at all. Rodent studies showed detectable LTP improvements in the dentate gyrus by week 2 [6]. AMPA receptor trafficking increases with continued FGFR1 activation. A subset of Longecity users report improved verbal recall and mental clarity in this window. Many report nothing. Mood changes (positive or negative) surface for some users here. Weeks 4 to 6: Peak claimed benefit window. Positive responders (a minority) describe improved memory consolidation, learning speed, and mental endurance. The animal data comes from ICV administration, and community SC doses are unlikely to reach ICV-equivalent CNS concentrations. Negative responders still report ongoing brain fog or mood disruption. Fatigue shows up for some users toward cycle end. Weeks 7 to 10 (off-cycle): Structural synaptic changes from FGFR1-mediated remodeling may outlast active dosing. Animal ICV studies showed benefit persistence four weeks post-administration. A few Longecity users claim sustained cognitive benefits two to four weeks after stopping. Based on very few reports; reliability is low.
FGFR1 phosphorylation and MAPK/PI3K cascade activation begin with first dose. Preclinical studies show synaptophysin upregulation within 24–72h. Structural synaptic changes require weeks.
Most users report nothing notable in week 1. Occasional "mental clarity" reports are difficult to distinguish from placebo given single-user anecdote quality.
LTP improvements in dentate gyrus detectable by week 2 in rodents (PMID 21508096). AMPA receptor trafficking increases with continued FGFR1 activation. Memory consolidation effects emerged in weeks 2–4 in rodent fear-conditioning and water-maze models.
A subset of Longecity users report improved verbal recall and mental clarity starting week 2–3. Effect onset is inconsistent; many users report no effect through week 3.
Cumulative synaptic remodeling: animal memory improvements persisted up to 1 month post-injection after ICV administration. Community SC dosing is unlikely to achieve ICV-equivalent CNS exposure; extrapolation uncertain.
Positive responders (a minority) report best cognitive results: improved memory consolidation, learning speed, mental endurance. Negative responders report ongoing brain fog or mood disruption.
Structural synaptic changes from FGFR1-mediated remodeling may outlast active dosing. Animal ICV studies showed benefit persistence 4 weeks post-administration. Whether SC human dosing replicates this is unknown.
Some Longecity users report sustained subjective benefits 2–4 weeks post-cycle. Based on very few reports; not reliable.
Source: Estimated 4-6 hours following subcutaneous administration; CSF concentrations stable up to 5 hours post-injection in rats (Anand et al., 2007)
Loading the interactive decay curve.
FGL completed a single Phase I safety trial in 2007 using intranasal delivery [3]. It is not approved by the FDA or any regulatory agency for human therapeutic use. The NeuroFGL consortium's EU-funded clinical program (CORDIS #278006) was discontinued in 2014 without advancing to Phase 2. FGL is available through research chemical suppliers and select peptide vendors. It is sold for research purposes only. No compounding pharmacy availability exists. Athletes should note that peptides with growth factor receptor signaling activity may fall under WADA prohibited substance categories. Check current guidance before use in any tested sport. This content is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment recommendations. Consult a qualified healthcare provider before using any research peptide.
Peptide Schedule Research TeamReviewed Apr 20268 Citations
