LL-37 is a naturally occurring human peptide studied intensively in immunology and microbiology. It is a research-stage molecule, not an approved therapy. This page describes its studied biological roles, not established treatments.
The short answer
LL-37's benefits split into two categories that are routinely merged: what the peptide does as part of your own innate immune system, which is extensively documented, and what happens when it is administered as a drug, which has been tested in humans exactly once at scale — topically, on venous leg ulcers — and did not meet its primary endpoint. There is no published evidence for injected LL-37 in people.
Holding those apart is the entire skill required to read this literature honestly.
What the molecule does in your body
LL-37 is the only cathelicidin-derived antimicrobial peptide humans make: a 37-residue amphipathic helical peptide cleaved from the hCAP18 precursor. A widely cited Biochimica et Biophysica Acta review (PMID 16716248) consolidates the functional picture: it is expressed in epithelial cells of the skin, testis, gastrointestinal tract, and respiratory tract, and in monocytes, neutrophils, T cells, NK cells, and B cells. Beyond broad-spectrum antimicrobial activity, it regulates inflammatory responses, chemoattracts adaptive immune cells to wounds and infection sites, binds and neutralizes bacterial LPS, and promotes re-epithelialization.
Its antibacterial activity is also conformation- and environment-dependent. Structural work in the Journal of Biological Chemistry (PMID 9452503) showed LL-37 is disordered in plain water, becomes helical in the presence of specific anions, and that the degree of helicity tracks its antibacterial potency. Human serum inhibited that activity. A molecule whose function switches with local ionic conditions is not a molecule whose effect you can assume from a milligram figure.
The human trials nobody mentions
LL-37 has been in registered human trials, which is more than most catalog peptides can say — and the results are more instructive than the marketing.
- First-in-man, 2014. Thirty-four participants with hard-to-heal venous leg ulcers, a 3-week placebo run-in, then 4 weeks of twice-weekly topical LL-37 at 0.5, 1.6, or 3.2 mg/mL. Healing rate constants were roughly sixfold higher than placebo at 0.5 mg/mL (p=0.003) and threefold at 1.6 mg/mL (p=0.088), with mean ulcer area falling 68% and 50% respectively. The highest dose, 3.2 mg/mL, was no different from placebo. No local or systemic safety concerns were reported (PMID 25041740).
- Phase IIb, 2021. HEAL LL-37 enrolled 148 patients with hard-to-heal venous leg ulcers, randomized to topical LL-37 at 0.5 or 1.6 mg/mL or placebo alongside compression therapy. In the full study population it found no significant improvement in healing versus placebo. A post hoc subgroup with wounds of at least 10 cm² showed improvement on several healing parameters, which the authors framed as an observation warranting an adequately powered study. The drug was well tolerated at both strengths (PMID 34687253).
Two features of that record deserve emphasis. The route was topical, applied to an open wound bed, in low milligram-per-millilitre concentrations. And the dose-response was inverted at the top of the range — more LL-37 produced less healing.
The findings that cut the other way
LL-37's own literature is candid that it can drive disease.
In rosacea, a Nature Medicine study (PMID 17676051) found abnormally high cathelicidin levels in facial skin, processed into different forms than in healthy skin. Injecting those rosacea-associated cathelicidin peptides into mouse skin produced inflammation, and mice lacking the cathelicidin gene were protected. In psoriasis, a Nature paper (PMID 17873860) showed LL-37 binds self-DNA and delivers it into plasmacytoid dendritic cells, triggering type I interferon production and breaking tolerance to the body's own DNA.
The biophysics explains why. LL-37 is described as non-cell-selective, acting on membranes by a detergent-like "carpet" mechanism (PMID 10417311), and it is cytotoxic to eukaryotic cells at 13–25 µM while killing E. coli at around 5 µM (PMID 9452503). That is a narrow window, and it is why the leg-ulcer trial's highest dose lost its effect.
What this adds up to
LL-37 is important biology and a real drug-development target, mostly framed around antibiotic resistance. But the tested application is a low-concentration topical gel on a chronic wound, its largest controlled trial was negative on its primary endpoint, and the dose curve is non-monotonic. Nothing published supports claims about immune enhancement, infection clearance, or systemic healing from injected LL-37. The molecule is real; the product category has run ahead of the trials.