LL-37

Assumed/theoretical. LL-37 is the active C-terminal fragment of human hCAP-18, cleaved (e.g., by proteinase-3) into a 37-residue cationic amphipathic peptide. Its assumed direct action is membrane disruption, it binds anionic microbial membranes, oligomerizes, and forms pores/extracts lipids, killing bacteria, fungi, and some viruses and disrupting biofilms in vitro. Its assumed immunomodulatory action is via receptors like FPR2/FPRL1 and scavenger/Toll-like receptors: it chemoattracts immune cells, neutralizes LPS, and, when complexed with self nucleic acids, activates plasmacytoid dendritic cells to produce type-I interferon (the same pathway that makes it a double-edged autoimmunity driver). It is not an approved drug, so all of this is mechanism, not proven clinical effect.

The two leg-ulcer trials are graded A together because the letter counts every randomized trial for that use; the tags show their results separately: the first mixed, the second did not work.

Adverse events

In topical wound trials LL-37 was well tolerated (mild-to-moderate local reactions; serious adverse events rare and judged unrelated). The standout injected-route signal is from the melanoma program: intratumoral injection produced eruptive keratoacanthoma-like atypical squamous lesions and a vesiculo-bullous reaction that mimicked malignancy, resolving ~2 months after stopping. Common reported local effects of subcutaneous use are injection-site redness/swelling and flu-like symptoms.

Contraindications

Theoretical caution flags (not formal label contraindications, since there is no approved systemic product): active or high-risk cardiovascular/atherosclerotic disease and clotting/platelet disorders; autoimmune disease, especially lupus, psoriasis, or rosacea, where LL-37 is implicated as a driver; and personal history of cancers where LL-37 is pro-tumor. Pregnancy/breastfeeding untested. Anyone on antiplatelet/anticoagulant therapy should treat the thrombosis-priming data as a real interaction concern.

Theoretical concerns

This is the central concern and why safety is graded low despite "endogenous" framing. Raising LL-37 systemically engages the same biology that, when overexpressed, is implicated in harm (FDA's own review also lists immunogenicity risk, nonclinical findings suggesting harm to male reproduction, and possible tumor promotion in some tissues): it primes platelets via GPVI and augments arterial thrombus formation; LL-37/mitochondrial-DNA complexes aggravate atherosclerotic lesions; it is a NETosis component tied to thrombosis on coronary plaques; and bound to self-DNA/RNA it acts as an autoantigen and interferon trigger central to lupus, psoriasis, and rosacea pathophysiology. Its role in cancer is dual and tissue-specific, pro-tumor in ovarian, lung, and breast models, so "anti-cancer peptide" is an oversimplification.

What nobody knows yet

No human data on long-term systemic subcutaneous dosing, the route and indication most people buy it for are essentially untested in controlled trials. Unknown whether exogenous dosing meaningfully raises tissue LL-37 long enough to matter (it is degraded within minutes in serum), and unknown whether chronic elevation tips the cardiovascular/autoimmune/oncologic risk biology in real users. Gray-market product purity, endotoxin load, and actual peptide content are unverified.

What else could explain the result

In the communities using it, LL-37 is rarely run alone. Lyme/chronic-infection users typically stack antibiotics, herbal antimicrobials, binders, and other peptides (BPC-157, thymosin alpha-1, KPV); gut users add probiotics and diet changes; recovery users stack BPC-157/TB-500 and improved sleep. Any perceived antimicrobial or immune benefit is heavily confounded by concurrent antibiotics, the natural waxing/waning of chronic illness, and regression to the mean.

Is the vial what it says?

Grades reflect: (a) real but route-mismatched human trials, topical venous-leg-ulcer RCTs (one positive first-in-man n=34, one negative phase IIb n~148) and a small intratumoral melanoma program; (b) no completed controlled human trial for systemic subcutaneous use; (c) a documented injection-route toxicity case; and (d) a substantial mechanistic harm literature (thrombosis, atherosclerosis, lupus/psoriasis autoimmunity, dual cancer role). Citations are specific PubMed records, a few PubMed search URLs where the point is an absence or a broad literature, and a ClinicalTrials.gov study ID (NCT02225366). Where evidence is thin it is labeled thin.

Half-life and storage

Half-life: Very short, minutes in unprotected biological fluids/serum, due to rapid cleavage by host serine proteases (kallikreins, elastase, trypsin-like enzymes) and serum-binding factors. It is more stable topically in wound exudate, which is part of why the credible human data are topical. This short systemic half-life undercuts the rationale for infrequent subcutaneous dosing.

Once mixed: Lyophilized powder; vendors advise storage frozen (around -20C or colder) before reconstitution. Reconstitute with bacteriostatic water and add the liquid gently down the vial wall, swirl/roll, never shake, as agitation/foaming can degrade the peptide. As a highly cationic membrane-active peptide it can adsorb to surfaces and is degradation-prone. Once reconstituted, refrigerate (2-8C) and use within ~2 weeks; aliquot and freeze for longer storage and avoid repeated freeze-thaw. Protect from heat and light.

What people report

Volume of reports: faint. Little real-world record, or mostly noise.

Low. Independent, credible community/forum experience reports for the injectable form are sparse; most of the visible online content is vendor, compounding-clinic, and wellness-blog marketing rather than peer user reports. Lyme and chronic-illness forums mention it but with little detailed first-hand dosing-and-outcome reporting.

Mixed and hard to read. Where users do report, accounts split between "felt immune/gut improvement" and "noticed nothing," with effects that are subjective and easily attributable to co-administered antibiotics, herbals, or the natural course of chronic infection. No consistent, distinctive, objective effect emerges from anecdote.

Low. The bulk of positive sentiment originates from sources with a commercial interest in selling the peptide (vendors, peptide clinics, affiliate blogs), which are discounted. Genuinely independent, verifiable user reports are thin, so the real-world signal cannot be weighted as meaningful support.

• Anecdote, not proof: Lyme and chronic-infection users describe trying it for biofilm/'stealth pathogen' support, with some reporting a sense of improved immune resilience or fewer flares and others reporting no discernible change.
• Some users of the subcutaneous form report injection-site redness, swelling, or transient flu-like feelings in the first days.
• Gut-focused users anecdotally pair it with diet/probiotic changes and credit improved digestion, though attribution is muddy.
• A recurring theme even among enthusiasts is uncertainty about whether anything is happening, given the absence of an objective marker and the peptide's short half-life.

Reports are anecdote, not proof.

Can you tell if it is working?

First documented human use: First documented controlled human use was a 2014 first-in-man, placebo-controlled trial of TOPICAL synthetic LL-37 on hard-to-heal venous leg ulcers (Gronberg et al., Wound Repair Regen, 34 patients). Intratumoral injection into melanoma metastases followed in a small phase 1/2 trial (ClinicalTrials.gov NCT02225366, 4 patients, completed November 2020; a published case describes an 8-week weekly intratumoral course). No completed controlled human trial exists for the systemic subcutaneous "wellness/immune/anti-aging" use that gray-market vendors promote, for that route the honest answer is: no controlled human trial has been completed.

1. 1995Human cathelicidin (hCAP-18) and its C-terminal LL-37 fragment first identified/characterized.
2. 2014First-in-man placebo-controlled trial of topical LL-37 on venous leg ulcers (n=34), positive dose-response signal at lower doses.
3. 2015-2020Phase 1/2 intratumoral LL-37 trial in melanoma (NCT02225366, 4 patients, completed November 2020, results posted); a 2018 case report documented skin toxicity from weekly injections.
4. 2021Larger phase IIb HEAL LL-37 venous-leg-ulcer trial (~148 patients) reports negative primary endpoint, no benefit over placebo overall.
5. 2020sMarketed by compounding pharmacies and longevity/wellness clinics as an injectable antimicrobial/immune peptide despite absence of supportive systemic human trials.

• Gronberg 2014, first-in-man topical LL-37 venous leg ulcer RCT (PMID 25041740)(Small positive first-in-human trial; topical route; dose-response at lower concentrations.)
• Mahlapuu et al. 2021, HEAL LL-37 phase IIb venous leg ulcer trial (PMID 34687253)(Larger ~148-patient RCT; NEGATIVE primary endpoint, no benefit vs placebo overall.)
• Dermatologic toxicity of intratumoral LL-37 in melanoma (PMID 29665030)(Case report: keratoacanthoma-like lesions/bullae from weekly injections, resolved on stopping.)
• ClinicalTrials.gov NCT02225366, intratumoral LL-37 for melanoma(Early-phase intratumoral injection study; the injectable human-use anchor.)
• Zhang et al. 2023, cathelicidin in thrombosis and thromboinflammation (PMID 37090695)(Mechanistic evidence LL-37 drives clotting, plaque, and autoimmunity, basis for the low safety grade.)
• LL-37 antimicrobial mechanism & membrane action (PubMed search)(In-vitro/mechanistic basis for the antimicrobial and immunomodulatory claims.)
• FDA: bulk drug substances that may present significant safety risks (withdrawn table, cathelicidin LL-37 row)(FDA lists immunogenicity, male-reproduction and possible tumor-promotion concerns; no human safety data.)
• LL-37 stability / proteolytic degradation (PMID 21547341)(Documents rapid degradation/short half-life and relative stability in wound exudate.)
• LL-37 in cancer, dual pro/anti-tumor role (PubMed search)(Tissue-specific; pro-tumor in ovarian/lung/breast, tempers 'anti-cancer' marketing.)
• Gronberg A, et al. Treatment with LL-37 is safe and effective in enhancing healing of hard-to-heal venous leg ulcers: a randomized, placebo-controlled clinical trial. Wound Repair Regen. 2014. PMID 25041740. doi:10.1111/wrr.12211(dose table)
• Mahlapuu M, et al. Evaluation of LL-37 in healing of hard-to-heal venous leg ulcers: a multicentric prospective randomized placebo-controlled clinical trial. Wound Repair Regen. 2021;29(6):938-950. PMID 34687253. doi:10.1111/wrr.12977(dose table)
• ClinicalTrials.gov record NCT02225366, MD Anderson Cancer Center; started 2015-07-08, completed 2020-11-24; results posted.(dose table)