FGL

FGL corresponds to residues 681-695 of NCAM, the loop that physically contacts FGFR1. By reproducing that contact it triggers FGFR1 autophosphorylation and downstream PLC-gamma, PKC and MAPK signalling without needing cell-cell adhesion to occur. Rodent work shows increased presynaptic vesicle density and release probability in the dentate gyrus, restored dendritic spine architecture in aged rats, facilitated memory consolidation in water maze and social recognition tasks, and mobilisation of endogenous neural stem cells after experimental stroke. It also has documented anti-inflammatory effects on microglia. The peptide is often used as a dimer, FGL(L), because dimerisation markedly increases receptor activation.

Targets: FGF receptor 1 (FGFR1), NCAM signalling, MAPK/PKC cascade, Presynaptic vesicle release machinery

• · There is no human dose. Rodent studies typically used 2-20 mg/kg subcutaneously, which does not translate cleanly. Grey-market protocols run 0.5-2 mg daily, chosen by convention rather than evidence.
• · Nose-to-brain delivery of FG loop peptide has been studied in rodent hypoxic-ischaemic models, which is the rationale for the route.

Work out your exact syringe units →

• commonInjection-site redness or irritation
• commonNasal irritation with the intranasal route
• uncommonHeadache
• rareUnknown - the human side-effect profile is genuinely uncharacterised— FGFR1 is a growth-factor receptor expressed well beyond the brain. Nobody has measured what chronic systemic activation does in a person.

• Active malignancy - FGFR1 activation is implicated in several tumour types.
• Pregnancy and breastfeeding - no data.
• Known FGFR-driven conditions or a history of FGFR-amplified cancer.

• synergyp21 — Both target neurogenesis and synaptic structure through different receptors; theoretical synergy only.
• synergysemax — Complementary neurotrophic mechanisms - BDNF/NGF versus FGFR1. No combination data.

• · No bloodwork established.
• · Stay current on routine cancer screening if you are using an FGFR1 agonist for any length of time.

Not approved anywhere; sold as a research chemical.

• Cambon et al. 2004, a synthetic NCAM mimetic peptide promotes synaptogenesis and facilitates memory consolidation, Journal of Neuroscience (preclinical)
• Klein et al. 2016, the NCAM-derived peptide FGL mobilises endogenous neural stem cells after stroke (preclinical)

FGL is a structural mimic rather than a signalling molecule, and that distinction explains its behaviour. NCAM normally signals to FGFR1 through trans-homophilic binding between adjacent cells - two neurons touching. FGL reproduces the specific loop that makes that contact, so it triggers FGFR1 autophosphorylation without requiring the cells to be adjacent. Downstream that runs through PLC-gamma, PKC and MAPK, which is standard FGFR signalling and not specific to neurons. The rodent findings that follow are unusually concrete for this class: increased presynaptic vesicle density and release probability in the dentate gyrus, restored dendritic spine architecture in aged rats, facilitated memory consolidation in water maze and social recognition paradigms, mobilisation of endogenous neural stem cells after experimental stroke, and documented anti-inflammatory effects on microglia. Note the dimerisation point, because it matters practically: receptor tyrosine kinases are activated by ligand-induced dimerisation, and a monomeric mimetic is a much weaker activator than a dimeric one. Most published efficacy work used FGL(L), the dimer. Most grey-market material is sold without specifying which form it is, and a monomer and a dimer of the same sequence are not interchangeable. The unavoidable other half of this mechanism is that FGFR1 is a growth-factor receptor expressed throughout the body, is amplified or activated in several tumour types, and is the target of approved oncology drugs pointed in the opposite direction.

The monomer-versus-dimer problem is the specific and underappreciated failure here. FGFR1 is activated by ligand-induced dimerisation, most of the published efficacy used FGL(L), and grey-market vendors frequently do not specify which form they are selling. Someone running monomeric FGL at doses derived from dimer studies is not running the compound the papers describe. Second, the dose translation is worse than usual: rodent studies used 2-20 mg/kg subcutaneously and human protocols run 0.5-2 mg total, which is orders of magnitude below the allometric equivalent - the anecdotal doses were chosen by convention, not derivation, and may simply be too low to do anything. Third, and unlike most compounds in this class, the theoretical risk here is not vague: FGFR1 is a validated oncology target with approved inhibitors, and chronically agonising it systemically in an adult has never been studied. Fourth, there is no acute signal at all, so people have no feedback to titrate against and default to escalating on faith.

1. 500 mcgWeek 1 — Once daily subcutaneous, or 250 mcg intranasal. No human dose-response data exists, so the low end is the only rational starting point.
2. 1 mgWeeks 2-3 — Middle of the anecdotal range. Rodent studies used 2-20 mg/kg subcutaneously, which does not translate cleanly at all - the human protocols are a small fraction of the allometric equivalent.
3. 2 mgWeeks 4-6 — Top of the anecdotal range. Rodent protocols were typically short courses rather than continuous administration, and 3-6 weeks is where anecdotal use stops.

• Fibroblast growth factor receptor 1 (FGFR1) — Binding to the FGFR1 extracellular region is established from the NCAM structural work; I could not resolve a published Kd for the FGL peptide itself that I would quote

  Receptor autophosphorylation and downstream PLC-gamma, PKC and MAPK signalling. Drives neurite outgrowth, presynaptic differentiation and synapse formation. Also drives proliferation in any FGFR1-expressing tissue it reaches.

• Presynaptic vesicle release machinery — Downstream consequence, not a binding target

  Increased vesicle density and release probability at dentate gyrus synapses. This is the electrophysiological substrate for the memory-consolidation findings.

• Microglia — Not quantified

  Anti-inflammatory modulation of activated microglia, contributing to the post-stroke regenerative findings.

• Endogenous neural stem cell pool — Not applicable

  Mobilisation of endogenous neural stem cells after focal ischaemia, with enhanced remyelination and immune modulation.

Rodent memory-consolidation effects appeared within hours to days of dosing - notably faster than most structural neurotrophic compounds, because facilitating consolidation is a functional effect rather than an architectural one. The structural synaptic changes, spine density and presynaptic vesicle density, took weeks. Anecdotal human use runs 3-6 weeks on that basis. There is no acute subjective effect and no human timeline of any kind. If you use it, treat 4-6 weeks with an objective cognitive measure at both ends as the minimum informative course, and accept that a null result is the most likely outcome.

The theoretically coherent stacks are with compounds hitting different neurotrophic receptors - Semax on BDNF/NGF, P021 on the LIF-CNTF axis - and there is no combination data for any of them. What is worth flagging is the aggregate growth-factor load: FGL activates FGFR1, Dihexa is proposed to potentiate c-Met, BPC-157 pushes angiogenic signalling, and GHK-Cu drives tissue growth. Each individually carries a theoretical proliferation concern; running several together aggregates a risk nobody has quantified. If your reason for caution with any one of them is oncological, that reasoning applies to the stack rather than to the individual compound. The practical note is the dimer question: if you are combining FGL with anything, know whether your material is the monomer or FGL(L), because the efficacy literature largely used the dimer and the two are not equivalent at a receptor activated by dimerisation.

Against Semax: Semax raises BDNF expression, FGL activates a receptor directly. Semax has Russian human data and an acute subjective effect you can titrate against; FGL has rodent data of respectable quality and no human anything. If you want feedback from your own body, Semax gives it and FGL does not. Against P021: both are rodent-only synthetic fragments aimed at neurogenesis and synaptic architecture, both slow, both without human trials. P021 has a longer and more coherent single-lineage literature; FGL has some independent replication, including the 2016 stroke work from a German group, which is worth something. Against Dihexa: FGL's literature is intact, which is more than can be said for Dihexa's, and both carry a growth-factor-receptor oncological question. Against Cerebrolysin, which also claims neurotrophic and neurogenic effects: Cerebrolysin has thousands of randomised patients and ambiguous outcomes, FGL has none. The general point across this comparison set is that the compounds with cleaner mechanisms have less human evidence, and the one with human evidence has no identified mechanism.

$50–$200/month. A 30 mg vial typically runs 80-150 USD. At 1 mg daily that is a month per vial, so 80-150 USD monthly is the realistic figure at mid-range dosing. At 2 mg daily it doubles. Availability is limited and pricing is less standardised than for Semax or Selank, and the dimeric FGL(L) form, where it is offered at all, costs more.

• ?No human trial of FGL has ever been published. I searched specifically for a phase 1 and found nothing.
• ?No pharmacokinetic data exists in any species - no tmax, half-life, bioavailability, volume of distribution, clearance or protein binding.
• ?I could not resolve a published binding affinity for FGL at FGFR1 that I would quote with confidence.
• ?The monomer versus FGL(L) dimer distinction is central to potency and is rarely specified by suppliers. Which form the anecdotal dose ranges refer to is unknown.
• ?The anecdotal 0.5-2 mg daily human range is far below the allometric equivalent of the 2-20 mg/kg rodent doses and has no traceable derivation.
• ?The serum phosphate concern is a mechanistic prediction from FGFR biology, not an observed effect of FGL. Nobody has measured it.
• ?Whether the intranasal route achieves better CNS exposure than subcutaneous for this peptide has never been compared directly.
• ?No chronic safety data exists in any species. The consequences of sustained systemic FGFR1 agonism in an adult human are entirely uncharacterised.
• ?The molecular weight of 1649.8 Da in the Core record is consistent with the 15-residue sequence but I did not verify it against a primary characterisation.

• A synthetic neural cell adhesion molecule mimetic peptide promotes synaptogenesis, enhances presynaptic function, and facilitates memory consolidation Cambon K, Hansen SM, Venero C, Herrero AI, Skibo G, Berezin V, Bock E, Sandi C, Journal of Neuroscience, 2004 · PMID 15115815

  The foundational FGL paper. Synaptogenesis, enhanced presynaptic function and facilitated memory consolidation in rats - the study every claim about this peptide traces back to.

• The Neural Cell Adhesion Molecule-Derived (NCAM)-Peptide FG Loop (FGL) Mobilizes Endogenous Neural Stem Cells and Promotes Endogenous Regenerative Capacity after Stroke Klein R, Mahlberg N, Ohren M, Ladwig A, Neumaier B, Graf R, Hoehn M, Albrechtsen M, Rees S, Fink GR, Rueger MA, Schroeter M, Journal of Neuroimmune Pharmacology, 2016 · PMID 27352075

  The post-stroke regeneration study - stem cell mobilisation, remyelination and immune modulation after focal ischaemia. Independent group, which matters given how much of this class comes from single lineages.