LL-37
The only human cathelicidin antimicrobial peptide — a broad-spectrum, membrane-disrupting innate-immune peptide with antibacterial, antiviral, antifungal, and wound-healing activity.
LL-37 is the mature human cathelicidin antimicrobial peptide, a 37-amino-acid cationic, amphipathic alpha-helical peptide cleaved from the hCAP18 precursor and expressed by neutrophils, keratinocytes, and mucosal epithelial cells as a core component of innate immune defense. It kills bacteria, enveloped viruses, and fungi by directly disrupting their membranes, and separately modulates inflammation and wound repair through FPR2/ALX, P2X7, TLR, and EGFR signaling. Preclinical evidence for its antimicrobial, immunomodulatory, and wound-healing roles is extensive, though rigorous human clinical trials of native exogenous LL-37 remain limited, with topical wound-care applications the furthest developed. It has no FDA approval for any indication.
Class
Human cathelicidin antimicrobial peptide (37-amino-acid cationic amphipathic alpha-helix)
Half-life
Reported ~30 minutes (PeptideBase) to a 2-4 hour elimination half-life after subcutaneous dosing (BodyHackGuide); rapidly degraded by host proteases in vivo.
Routes
Subcutaneous, Topical, Intranasal, Intravenous
Category
Healing & Recovery
Researched benefits
What it's studied for
Broad-spectrum antimicrobial activity
LL-37 kills gram-positive and gram-negative bacteria, mycobacteria (including M. tuberculosis), and fungi by physically disrupting their membranes, with minimum inhibitory concentrations of roughly 1-20 µM in standardized assays. Because it targets bulk membrane charge rather than a single enzyme, resistance is slower to develop than with conventional antibiotics.
Activity against resistant bacteria and biofilms
Preclinical evidence shows activity against multidrug-resistant pathogens such as MRSA and against biofilm-embedded organisms that often evade standard antibiotics, via disruption of quorum sensing, reduced adhesion, and direct killing within biofilms.
Antiviral defense
LL-37 inactivates enveloped viruses — including influenza A and B, RSV, herpes simplex, vaccinia, HIV (modestly), and SARS-CoV-2 in vitro — by disrupting viral envelope integrity. Non-enveloped viruses are generally less susceptible.
Immune modulation
Through FPR2/ALX, P2X7, and TLR receptors LL-37 recruits neutrophils, monocytes, T cells, and mast cells, shapes cytokine production, and neutralizes bacterial LPS to blunt endotoxin-driven inflammation, integrating antimicrobial and immune-signaling functions.
Wound healing and re-epithelialization
LL-37 stimulates keratinocyte migration and proliferation, promotes angiogenesis via FPR2/ALX and EGFR transactivation with VEGF induction, and provides antimicrobial protection of the wound surface — the mechanism behind its most clinically advanced application, topical treatment of chronic wounds.
Mucosal and gut barrier support
As an epithelial-expressed innate defense peptide of the skin, lung, and gastrointestinal tract, LL-37 contributes to mucosal barrier integrity and frontline defense at surfaces where endogenous production is highest.
Mechanism
How it works
LL-37 is a cationic (net charge +6), amphipathic alpha-helical peptide. In aqueous solution it exists as an unstructured random coil; on contact with negatively charged target membranes it folds into an alpha-helix and inserts into the lipid bilayer. Membrane disruption is described by several models — the carpet model (peptide accumulates parallel to the surface until the bilayer collapses), the toroidal-pore model (peptide inserts perpendicularly, dragging lipid head groups to form transmembrane pores), and the less-characterized barrel-stave model. The result is rapid, minutes-scale permeabilization and killing of susceptible organisms.
Selectivity for microbial over mammalian membranes arises because bacterial membranes are rich in anionic lipids (phosphatidylglycerol, cardiolipin) and surface structures (LPS, teichoic acids) that bind cationic peptides, and lack the cholesterol that rigidifies mammalian membranes. This selectivity is not absolute: at high concentrations LL-37 can damage mammalian cells, especially those with altered membranes (cancer or apoptotic cells), giving it a relatively narrow therapeutic window. Activity is also markedly reduced by physiological salt, serum albumin, and LPS-binding protein — a major reason potent in-vitro effects translate weakly in vivo.
Beyond direct killing, LL-37 is a signaling peptide. It acts through formyl peptide receptor 2 (FPR2/ALX) to drive leukocyte chemotaxis, through P2X7 to influence inflammasome activation, through EGFR transactivation to promote wound repair and angiogenesis, and through TLR9 when complexed with self-DNA. It neutralizes LPS, modulates cytokine output in a context-dependent manner, and can promote Th17 differentiation and plasmacytoid dendritic cell activation — protective in acute infection but pathogenic when dysregulated in psoriasis and rosacea.
Endogenous LL-37 is regulated by vitamin D: the CAMP gene carries a vitamin D response element, and 1,25-dihydroxyvitamin D substantially upregulates LL-37 production. Healthy plasma concentrations run about 1-5 µg/mL and vary with infection status, vitamin D level, and circadian rhythm. Exogenous subcutaneous LL-37 is rapidly absorbed but heavily protein-bound and quickly degraded by tissue proteases, which limits systemic bioavailability and has driven interest in protease-resistant analogs and encapsulated or hydrogel delivery.
Dosing protocols
Dosing & administration
Dosing reflects protocols reported in research and community literature for educational purposes. It is not medical advice or a recommendation. Most peptides here are not approved for human use.
Reconstitution
A 5 mg vial reconstituted with 2 mL bacteriostatic water yields 2.5 mg/mL: on a U-100 insulin syringe 1 unit (0.01 mL) = 25 µg, 4 units = 100 µg, 8 units = 200 µg, 20 units = 500 µg. Alternatives: 5 mg + 1 mL = 5 mg/mL (1 unit = 50 µg) for higher doses in low volume, or 5 mg + 5 mL = 1 mg/mL (1 unit = 10 µg) for fine titration. Add BAC water slowly down the vial wall and swirl gently — do not shake or vortex, which can denature the alpha-helix. Solution must be perfectly clear; refrigerate at 2-8 C (stable 3-4 weeks, do not freeze reconstituted), protect from light, and use glass vials rather than plastic.
Beginner (tolerance test)
- Dose
- 100 µg
- Frequency
- Single test dose
- Timing
- Morning
- Duration
- Day 1, then wait 48-72 h
- Route
- Subcutaneous
Small test dose to assess tolerance — watch for excessive injection-site inflammation, systemic symptoms, or any psoriasis/rosacea flare before proceeding. Optimize vitamin D (25-OH-D >40 ng/mL) and rule out contraindications first.
Standard (chronic infection support)
- Dose
- 200 µg escalating to 500 µg
- Frequency
- 3x weekly (M/W/F)
- Timing
- Morning
- Duration
- Week 1 at 200 µg, weeks 2-6 at 500 µg, weeks 7-8 taper to 250 µg 2x weekly, then 4 weeks off
- Route
- Subcutaneous
Cyclical course; reassess clinical status and repeat only if benefit is documented. Injection-site reactions and mild flu-like symptoms are more common than with benign peptides and usually diminish with continued dosing.
Systemic antimicrobial support
- Dose
- 250-500 µg
- Frequency
- Daily
- Timing
- Morning
- Duration
- 2-4 weeks
- Route
- Subcutaneous
For suspected systemic infection unresponsive to conventional therapy alone; combine with relevant conventional antimicrobials and monitor response with clinical and laboratory markers.
Intermediate (chronic infection syndromes)
- Dose
- 500 µg
- Frequency
- Daily induction, then 3x weekly maintenance
- Timing
- Morning
- Duration
- 500 µg daily for 4-6 weeks (induction), then 500 µg 3x weekly for 8-16 weeks, then 4-8 week break
- Route
- Subcutaneous
For chronic Lyme, viral reactivation, or mold-related syndromes after a completed beginner cycle. Longer courses raise both potential benefit and side-effect risk; escalate gradually (100-250 µg every 4-7 days) and monitor labs regularly.
Advanced (pulse dosing)
- Dose
- 500-750 µg
- Frequency
- Daily during pulse
- Timing
- Morning
- Duration
- 2-4 week high-intensity pulses separated by longer recovery periods
- Route
- Subcutaneous
Short high-intensity courses aim for maximum antimicrobial effect while limiting cumulative exposure. Doses above 1 mg per injection are not supported by evidence; cumulative daily doses above 1.5 mg should not be exceeded without specialist guidance.
Topical (wound healing)
- Dose
- Compounded formulation
- Frequency
- Typically once daily under dressing
- Timing
- Per wound-care schedule
- Duration
- Per clinician direction
- Route
- Topical
The most clinically evidenced route — Phase II trials of topical LL-37 for chronic venous leg ulcers have shown benefit. Requires compounding-pharmacy preparation; reconstituted lyophilized peptide in water cannot simply be applied to a wound.
- Optimize vitamin D first: the CAMP gene is directly upregulated by vitamin D, so achieving 25-OH-D >40 ng/mL boosts endogenous LL-37 and may make exogenous peptide unnecessary. This is the single most important preparatory step.
- Trial dose before any standard dosing — never start at 500+ µg without a 100-200 µg tolerance test.
- Inject slowly over 3-5 seconds using a 29-31G insulin needle and rotate sites aggressively (abdomen, flanks, outer thighs); faster injection amplifies local irritation.
- Use only high-quality peptide with a published HPLC certificate showing >95% purity and low endotoxin (<1 EU/mg); the 37-residue length and high cationic charge make LL-37 harder to synthesize cleanly, and truncated fragments or endotoxin raise both inefficacy and inflammatory risk.
- Do not use LL-37 for vague wellness goals — it is indication-specific (chronic/biofilm infection, chronic wounds, immune dysregulation), not a general-purpose peptide, and should not be relied on as monotherapy for serious infection.
- Use intermittent cyclical courses (typically 8-week cycles with 4-8 week breaks); continuous daily dosing beyond 16 weeks is uncharacterized. Monitor CBC, CRP/ESR, and renal function every 4-6 weeks on longer protocols.
Evidence
Research & clinical studies (11)
A protease-sensing circuit links neutrophil inflammation to virulence regulation in Streptococcus pyogenes
LL-37 normally represses S. pyogenes virulence genes, but neutrophil proteases released during NETosis degrade a bacterial repressor and reactivate virulence, revealing a feedback loop where host inflammation paradoxically amplifies bacterial virulence.
PMID 42182189Evaluation of the antibacterial activity of stem cells from apical papilla and their conditioned medium against Escherichia coli and Staphylococcus aureus
Stem cells from apical papilla and their secreted factors showed potent antibacterial activity against E. coli and S. aureus, with LL-37 identified as a key antimicrobial compound responsible for the effect.
PMID 42176067Overcoming LPS-mediated resistance in gram-negative pathogens: a review of LL-37 analogs and computational design strategies
LL-37 analogs designed through structural modification and computational methods show promise in overcoming resistance in gram-negative bacteria by improving membrane penetration while reducing human-cell toxicity.
PMID 42183878Serine protease HtrA promotes Campylobacter jejuni intestinal colonization through degrading antimicrobial peptide LL-37
C. jejuni evades LL-37 by using the serine protease HtrA to cleave and inactivate the peptide, with HtrA induced on LL-37 exposure to allow resistant strains to survive intestinal colonization.
PMID 42160414Optimized LL-37-Derived Peptides Exhibit Antitubercular Activity, Induce Membrane Disruption, and P-Type ATPase Transcriptional Responses in Mycobacterium tuberculosis
The optimized D-LL37 variant showed antimycobacterial activity against M. tuberculosis via partial membrane disruption combined with upregulation of P-type ATPase stress-response genes.
PMID 42194015Antimicrobial peptoids pass rapidly through bacterial membranes and flocculate ribosomes and DNA: A single-cell fluorescence study
Synthetic peptoids mimicking LL-37 rapidly permeabilized bacterial membranes and aggregated intracellular ribosomes and DNA faster than native LL-37, while offering resistance to enzymatic degradation.
PMID 42372144Immunomodulatory effects of synthetic antimicrobial peptides on LPS-induced inflammatory responses in THP-1 macrophages
Synthetic antimicrobial peptides modulated LPS-induced inflammatory responses in THP-1 macrophages, supporting the immunomodulatory, endotoxin-neutralizing role of cathelicidin-class peptides.
PMID 42358966The Role of Vitamin D in Modulating the Innate Immune Response in Children with Vesicoureteral Reflux
Vitamin D status was linked to innate immune modulation in children with vesicoureteral reflux, consistent with vitamin D's role in driving CAMP/LL-37 expression.
PMID 42353981Beta-Amyloid (Abeta) and Human Cathelicidin LL-37: Two Sides of the Same Coin?
Reviews parallels between amyloid-beta and LL-37, examining shared aggregation and immune-signaling behaviors relevant to neuroinflammation and host defense.
PMID 42353177Cathelicidin LL-37 in severe Streptococcus pyogenes soft tissue infections in humans
Human biopsy specimens from severe streptococcal soft-tissue infections showed large amounts of neutrophil-derived LL-37 colocalized with bacteria and the streptococcal protease SpeB, providing in vivo evidence for LL-37 as a frontline innate effector subject to bacterial resistance.
PMID 18490458Antimicrobial and protease inhibitory functions of the human cathelicidin (hCAP18/LL-37) prosequence
The cathelin-like prosequence of hCAP18/LL-37 inhibited cysteine protease activity and killed E. coli and MRSA at 16-32 µM, identifying dual antimicrobial and protease-inhibitory functions complementary to the C-terminal LL-37 peptide.
PMID 12713586Combinations
Stacking & blends
Thymosin Alpha-1 + LL-37: Immune Support
Broad-spectrum immune support targeting both innate and adaptive arms
Thymosin Alpha-1 restores adaptive immune competence by promoting T-cell maturation and dendritic/NK cell activity, while LL-37 provides direct antimicrobial action and innate immune modulation — complementary mechanisms for chronic-infection contexts.
Chronic Lyme Integrative Protocol
Multi-modal support for chronic tick-borne / co-infection syndromes
LL-37 (500 µg SC 3x weekly) supplies antimicrobial and anti-biofilm coverage, Thymosin Alpha-1 (1.6 mg SC 2x weekly) restores adaptive immunity, and mitochondrial support (CoQ10, alpha-lipoic acid, MOTS-c) plus conventional antibiotics, herbal antimicrobials, and biofilm disruptors address the multifactorial illness. LL-37 is adjunctive, not curative.
Chronic Infected Wound Adjunct
Accelerate healing of chronic or infected wounds
Topical plus systemic LL-37 (250-500 µg daily) provides antimicrobial and re-epithelialization support, BPC-157 (250 µg BID) and TB-500 (2 mg 2x weekly) drive tissue repair and angiogenesis, alongside conventional wound care and debridement.
Post-Viral Syndrome Protocol
Immune support for long COVID / post-EBV syndromes
Cautious low-dose LL-37 (250-500 µg SC 3x weekly) for lingering viral activity paired with Thymosin Alpha-1 (1.6 mg 2x weekly) for adaptive immune restoration; LL-37's pro-inflammatory potential means dosing starts low with monitoring for worsening.
Safety
Side effects & considerations
Commonly reported effects
Contraindications & cautions
- Active psoriasis or psoriatic arthritis (LL-37 complexed with self-DNA drives the Th17 psoriasis cascade)
- Active rosacea (dysregulated cathelicidin processing drives rosacea inflammation)
- Active systemic lupus erythematosus or severe autoimmune dermatosis
- Known hypersensitivity to LL-37 or cathelicidin preparations
- Pregnancy and breastfeeding
- Severe mast cell activation syndrome
- Active malignancy without oncology supervision
- Personal or strong family history of psoriasis or rosacea (strong relative contraindication)
- Moderate-to-severe renal impairment
- Sepsis or uncontrolled systemic infection (may be pro-inflammatory at high doses)
LL-37 has a narrower therapeutic window than most peptides because the same membrane-disrupting activity that kills microbes can damage mammalian membranes at high concentrations, and it can be pro-inflammatory in certain doses and disease contexts. Its established role in psoriasis and rosacea pathogenesis makes it genuinely risky for susceptible individuals — any new psoriasis- or rosacea-like skin findings warrant immediate discontinuation and dermatology evaluation. Long-term safety is not well characterized (trials run weeks to months, not years); monitor renal function on extended courses and avoid continuous dosing beyond about 16 weeks.
FAQ
LL-37 — common questions
What makes LL-37 different from regular antibiotics?
LL-37 kills microbes by physically disrupting their membranes rather than targeting a specific enzyme, giving it a broad spectrum (gram-positive, gram-negative, fungi, some enveloped viruses, biofilm-embedded organisms) and making resistance harder to develop. The trade-off is a narrower therapeutic window, sensitivity to salt and protein binding in vivo, and immunomodulatory effects that can help or harm depending on context — so it is a complement to, not a replacement for, conventional antibiotics.
Can LL-37 cause or worsen psoriasis?
Yes — this is a genuine, established concern. When complexed with self-DNA, LL-37 activates plasmacytoid dendritic cells via TLR9 and drives the Th17 cascade characteristic of psoriasis. People with active psoriasis should not use LL-37, those with a personal or family history should be very cautious, and any new psoriasis-like plaques warrant immediate discontinuation. A lesser version of the same concern applies to rosacea.
Should I optimize vitamin D before using LL-37?
Yes — it is the single most important preparatory step. The CAMP gene that encodes LL-37 is directly upregulated by vitamin D through a response element in its promoter, so achieving 25-OH-D above roughly 40 ng/mL substantially boosts endogenous LL-37, sometimes enough that exogenous peptide is unnecessary. Optimizing vitamin D first (4-8 weeks) is cheaper, better supported, and may resolve the underlying issue.
Is LL-37 effective against antibiotic-resistant bacteria?
Preclinical evidence shows LL-37 activity against multidrug-resistant pathogens including MRSA, with the membrane-disruption mechanism being less prone to traditional resistance pathways. Human clinical evidence for antimicrobial-resistance applications, however, is still limited.
Can I use LL-37 topically for wounds?
Topical use has the most clinical evidence — Phase II trials of topical LL-37 for chronic venous leg ulcers have shown benefit, and this is the most likely first FDA-approved indication. It requires a compounded formulation from a pharmacy; you cannot effectively apply lyophilized peptide reconstituted in water to a wound. For simple acute infections, standard antibiotics are usually sufficient.
What are the most common side effects of LL-37 injection?
Injection-site reactions are more prominent than with many peptides — expect red, sore, sometimes itchy sites for 24-48 hours early on. Some users note mild flu-like symptoms (low-grade fever, fatigue, aches) in the first few doses, which usually diminish with continued dosing. The uncommon but serious effects are worsening or new-onset psoriasis/rosacea and, rarely, severe allergic reactions.
Is LL-37 legal and FDA-approved?
LL-37 is not approved for any medical indication anywhere in the world and is sold for laboratory research use only. It was removed from the FDA's Category 2 bulk substances list on April 22, 2026 and is scheduled for Pharmacy Compounding Advisory Committee review in early 2027, but that removal does not by itself authorize compounding-pharmacy production.
Is LL-37 safe for long-term use?
Long-term safety is not well characterized — published trials run weeks to months, not years. Theoretical concerns with chronic use include cumulative renal effects, induction of inflammatory skin disease in susceptible people, and effects on the skin and mucosal microbiome. Most users adopt intermittent cyclical protocols (about 8-week courses with breaks) rather than continuous daily use; continuous dosing beyond several months is uncharacterized.
