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DPreclinical / mechanistic only
No human data

LL-37

Cathelicidin

LL-37 (the C-terminal fragment of human cathelicidin hCAP-18) is the only human cathelicidin antimicrobial peptide, a 37-amino-acid cationic host-defense peptide of the innate immune system. It is marketed in the "research peptide" / wellness space for supposed antimicrobial, anti-biofilm, wound-healing, gut-health and "immune" benefits, typically as injectable or nebulized material. There is essentially no human safety or efficacy data for systemic (subcutaneous, intramuscular, intravenous, or inhaled) LL-37 as sold to consumers. The only controlled human evidence is a single small first-in-man topical trial on leg ulcers, and that trial showed an inverted dose-response — the highest concentration worked no better than placebo, signaling that more is not better and that high local concentrations may be counterproductive. Beyond that, LL-37 is a double-edged endogenous molecule: at higher concentrations it is directly cytotoxic to host cells, and when it complexes with self-DNA/RNA it becomes an autoantigen that drives interferon production and is mechanistically implicated in psoriasis, lupus, and rosacea. The one published human report of injected (intratumoral) LL-37 documented significant dermatologic toxicity. Purity, endotoxin content, and dosing of consumer/"research" material are unverified, adding infection and immune-activation risk on top of the biology. Treat systemic use as unstudied and potentially pro-inflammatory/autoimmunity-provoking.

Clinical readoutPeptide · antimicrobial-peptide
Hepatic strainNone
CardiovascularModerate
HPTA suppressionNone
Half-life
Not established i…
Route
The only controlled hum…
Evidence
D
Active
Unknown for administere…
1·t½2·t½3·t½4·t½5·t½
Illustrative single-compartment washout · each mark = one half-life · t½ ≈ Not established in humans for exogenous administration. As an endogenous host-defense peptide LL-37 is expected to be rapidly degraded by serum and tissue proteases and bound by plasma proteins/lipoproteins; no validated human half-life, Cmax, or clearance values exist for any consumer route.
Pharmacology

Mechanism of action

LL-37 is a cationic, amphipathic alpha-helical peptide cleaved from the precursor hCAP-18. Its antimicrobial action is largely membrane-based: the cationic peptide binds anionic microbial membranes and lipopolysaccharide, inserts, and disrupts the membrane, and it also neutralizes LPS/endotoxin. Beyond direct microbicidal activity it is a potent immunomodulator: it chemoattracts neutrophils, monocytes and T cells, modulates cytokine/chemokine release, influences macrophage polarization, and promotes angiogenesis and re-epithelialization (basis for wound-healing interest). The same amphipathic membrane activity that kills microbes is also cytotoxic to host cells at higher concentrations. LL-37 binds host self-DNA and self-RNA, and these complexes are internalized by plasmacytoid dendritic cells where they trigger TLR9/TLR7 signaling and type I interferon production; LL-37 itself becomes a T-cell and B-cell autoantigen. This is the mechanistic link to psoriasis, cutaneous/systemic lupus erythematosus, and rosacea, and it is why an agent framed as "healing" can also amplify autoimmune inflammation.
Kinetics

Pharmacokinetics

Half-life

Not established in humans for exogenous administration. As an endogenous host-defense peptide LL-37 is expected to be rapidly degraded by serum and tissue proteases and bound by plasma proteins/lipoproteins; no validated human half-life, Cmax, or clearance values exist for any consumer route.

Active duration

Unknown for administered material. Native peptide is short-lived in biological fluids due to proteolysis; effect duration for systemic dosing is uncharacterized in humans.

Route

The only controlled human data are for topical application (leg ulcer gel). One human case involved intratumoral injection. Consumer-marketed subcutaneous, intramuscular, intravenous, and nebulized/inhaled routes have no published human pharmacokinetic or safety characterization.

Metabolism & clearance

Presumed proteolytic degradation into inactive fragments plus renal handling of fragments; not quantified in humans. No validated assay or washout window exists for this peptide, so washout and kinetics are simply unknown; this is a data gap, not test-evasion guidance.

For monitoring and washout planning, not drug-test evasion.

Reported effects

Physiological & performance effects

  • Direct antimicrobial and anti-biofilm activity against bacteria (including some Gram-negative and Gram-positive organisms), fungi, and enveloped viruses in vitro — the basis of interest, but demonstrated in laboratory/preclinical systems, not in treating systemic infection in people
  • LPS/endotoxin neutralization in vitro
  • Immunomodulation: chemotaxis of neutrophils/monocytes/T cells, cytokine modulation (context-dependent, can be pro- or anti-inflammatory)
  • Pro-angiogenic and pro-re-epithelialization activity relevant to wound closure
  • In a single small topical RCT on hard-to-heal venous leg ulcers, low concentrations (0.5 and 1.6 mg/mL) improved wound-healing predictors versus placebo, while the highest concentration (3.2 mg/mL) was no better than placebo — an inverted dose-response indicating a narrow useful range and no benefit from higher exposure
  • Endogenous roles in barrier immunity of skin, gut, and airway
Safety

Adverse effects by system

Cardiovascular

No human safety data for administered LL-37. Preclinical/mechanistic literature suggests concentration-dependent effects on endothelium and platelets and a role in vascular inflammation/atherosclerotic plaques, but no clinical cardiovascular outcome data exist. Treat cardiovascular risk as uncharacterized.

Hepatic

No human hepatotoxicity data. LL-37 is a peptide not known to undergo hepatic first-pass in the manner of oral 17-alpha-alkylated agents; however, no liver-safety monitoring data exist for any administered route.

Endocrine / HPTA

Not a hormonal or steroidal agent; no known direct effect on the hypothalamic-pituitary-gonadal axis and no human endocrine data. No evidence of testosterone/gonadotropin suppression, but also no formal study — absence of data, not proof of safety.

Reproductive

No human reproductive or pregnancy safety data. Use in pregnancy/lactation is unstudied and should be avoided.

Neuropsychiatric

No human psychiatric adverse-effect data. No established neuropsychiatric signal.

Renal

No human renal safety data. Peptide fragments would be renally handled; injected impure/endotoxin-containing material could theoretically stress the kidney via systemic inflammation, but this is unstudied.

Hematologic

No human hematologic safety data. LL-37 interacts with platelets and neutrophils in vitro (potential effects on platelet activation and NET formation), but clinical hematologic effects of administered LL-37 are uncharacterized.

Dermatologic

The best-documented human harm. Topical use in the leg-ulcer RCT was reported as well tolerated, but injected (intratumoral) LL-37 in a melanoma patient produced multiple verrucous papules, a vesiculo-bullous lesion, and a lichenoid inflammatory infiltrate with atypical squamous proliferations mimicking squamous cell carcinoma/keratoacanthoma; lesions resolved within ~2 months of stopping. Mechanistically LL-37 is implicated in psoriasis, cutaneous lupus, and rosacea flares. Injection-site inflammation is a plausible risk.

Recovery

HPTA suppression & recovery

Suppression: None expected on mechanistic grounds, but not formally studied

LL-37 is an antimicrobial peptide with no known steroidal or gonadotropic activity, so HPTA suppression is not an anticipated mechanism and no post-cycle intervention is indicated on that basis. There is, however, no human endocrine study to confirm this. Any concern about hormonal status, fertility, or recovery should be evaluated with bloodwork and directed by an endocrinologist rather than self-managed; single-SERM approaches are outside the scope of a non-hormonal peptide and are not applicable here.

Bloodwork & vitals

Monitoring

Recommended labs & checks
Baseline and follow-up CBC with differentialComprehensive metabolic panel (liver and kidney function) as general safety baselines given absent organ-safety dataInflammatory markers (CRP, ESR) if any inflammatory or autoimmune symptoms emergeANA and autoimmune serologies if new skin, joint, or systemic inflammatory symptoms develop, given the psoriasis/lupus autoantigen linkInjection-site and skin examination by a clinician

Cadence: Because systemic use is unstudied, there is no validated monitoring schedule; if a person uses it despite the lack of data, obtain baseline labs before starting, re-check within a few weeks, and evaluate promptly at any new symptom rather than on a fixed interval. All monitoring should be clinician-supervised.

Warning signs — seek care
  • New or worsening skin lesions: plaques, scaling, blistering, warty/verrucous papules, or a rosacea-like or psoriasis-like rash
  • Non-healing or atypical skin growths (reported to mimic squamous cell carcinoma) — needs dermatologic/biopsy evaluation
  • Signs of injection-site or systemic infection: spreading redness, swelling, pus, fever, chills (also flags contaminated/endotoxin-laden product)
  • New joint pain, photosensitivity, malar rash, or other autoimmune features
  • Any allergic/anaphylactic signs: hives, swelling, wheeze, throat tightness — seek emergency care
Do not use if

Contraindications

  • Personal or family history of autoimmune/autoinflammatory disease (especially psoriasis, systemic or cutaneous lupus erythematosus, and rosacea), given LL-37's mechanistic role as a self-DNA-complexing autoantigen driving type I interferon
  • Pregnancy and breastfeeding (no safety data)
  • Active or recurrent inflammatory skin disease at or near intended injection sites
  • Use of unverified, non-pharmaceutical-grade / 'research use only' material of unknown purity and endotoxin content (universal caution for this product class)
  • Any systemic (SC/IM/IV) or inhaled use as self-treatment — unstudied in humans and cannot be assumed safe
  • Known hypersensitivity to the peptide or excipients
Combinations

Interaction profile

  • ContraindicatedWith DNP: Additive cardiovascular strain

Check a specific combination in the interaction checker.

Harm reduction

Reducing harm & when to stop

  • Understand the evidence gap: no human study supports systemic or inhaled LL-37 for antimicrobial, healing, gut, or 'immune' benefit; the only controlled human data are topical and show that higher concentrations are not better.
  • Do not assume 'natural/endogenous' means safe — at higher concentrations LL-37 is cytotoxic to human cells and can act as an autoantigen that drives autoimmune-type inflammation.
  • Avoid entirely if you or close family have psoriasis, lupus, rosacea, or other autoimmune/autoinflammatory disease, and during pregnancy or breastfeeding.
  • 'Research use only' peptides carry real risks of impurity, incorrect content, and bacterial endotoxin contamination; injecting such material adds infection and systemic-inflammation risk independent of the peptide itself.
  • Stop and seek medical care promptly for: new or spreading skin lesions or non-healing growths, blistering or psoriasis/rosacea-like rashes, injection-site or systemic infection (fever, spreading redness, pus), new joint pain or photosensitivity/malar rash, or any allergic reaction (hives, swelling, breathing difficulty).
  • If considering use despite the lack of data, do so only under a clinician who obtains baseline bloodwork and monitors you — not as self-treatment.
  • This monograph is harm-reduction information, not an endorsement or a dosing recommendation; there is no established safe or effective consumer dose.
Evidence

Citations (6)

Every clinical claim above is tied to a primary source. Overall evidence grade D — this compound lacks adequate human data; claims rest on preclinical or mechanistic evidence.

  1. 01

    LL-37 is the single human cathelicidin, a 37-amino-acid cationic antimicrobial/host-defense peptide of the innate immune system with immunomodulatory activity.

    ReviewDOI 10.3390/vaccines8030517 ↗

  2. 02

    LL-37 complexed to self-DNA acts as an autoantigen in psoriasis and lupus erythematosus and induces type I interferon production by plasmacytoid dendritic cells, initiating autoimmune inflammatory cascades.

    ReviewPMID 32927756 ↗

  3. 03

    In a first-in-man double-blind RCT (n=34) topical LL-37 for hard-to-heal venous leg ulcers was safe and well tolerated with no local or systemic safety concerns; lower concentrations (0.5 and 1.6 mg/mL) improved healing predictors while the highest concentration (3.2 mg/mL) was no better than placebo (inverted dose-response).

    RCTPMID 25041740 ↗

  4. 04

    Intratumoral LL-37 injection in a melanoma patient (phase 1 context) caused dermatologic toxicity: multiple verrucous papules, a vesiculo-bullous lesion, and lichenoid infiltrates with atypical squamous proliferations resembling squamous cell carcinoma/keratoacanthoma, resolving within ~2 months of stopping therapy.

    Case reportDermatologic toxicity from novel therapy using antimicrobial peptide LL-37 in melanoma: A detailed examination of the clinicopathologic features.PMID 29665030 ↗

  5. 05

    LL-37 is mechanistically implicated in rosacea pathogenesis: intradermal LL-37 injection induces a rosacea-like inflammatory dermatitis in mice (erythema, mast-cell activation, epidermal thickening), a model used to study rosacea pathogenesis and candidate therapies.

    PreclinicalPMID 41296102 ↗

  6. 06

    There is no adequate human pharmacokinetic, systemic-safety, or organ-specific (hepatic, renal, cardiovascular, endocrine, hematologic, psychiatric) safety data for subcutaneous, intramuscular, intravenous, or inhaled LL-37 as marketed to consumers.

    ReviewPMID 32927756 ↗

Last reviewed 2026-07-06 · Verified against PubMed · Educational, not medical advice