At a glance

What is LL-37?

LL-37 is the active 37-amino acid fragment of the only human cathelicidin, a natural host-defense (antimicrobial) peptide produced by immune and skin cells. It is a well-studied part of the body's innate immune system. It is sold through wellness and "research peptide" channels for antimicrobial and immune support, and it is not an approved medicine.

How does it work?

LL-37 has two broad actions. Its positively charged, membrane-active structure can directly disrupt the membranes of microbes, and it also modulates the immune system, influencing immune-cell recruitment, inflammation, and wound healing, and binding some microbial factors. These roles are documented in a large laboratory and animal literature. Importantly, its effects are context-dependent: it can be either pro- or anti-inflammatory and has been implicated in some inflammatory skin diseases. This describes its biology, separate from demonstrated benefit from giving it as a therapy.

Is it FDA-approved?

No. There is no FDA-approved exogenous LL-37 therapeutic, and gray-market versions are unapproved. It was not among the peptides reviewed favorably at the 2026 FDA compounding advisory meetings.

What does the research show?

The evidence is preclinical for any injected wellness use. LL-37's biology in innate immunity, wound healing, and skin disease is documented in humans and in extensive laboratory work, including studies of how it disrupts microbes and binds microbial factors (example¹).

However, there are no controlled human trials establishing efficacy or safety of self-administered LL-37 for antimicrobial, immune, or healing wellness uses, and its context-dependent, sometimes pro-inflammatory, behavior argues against simple benefit claims.

Frequently asked questions

Is LL-37 FDA-approved?

No. LL-37 is not FDA-approved for any indication, and as of September 2026 it is not eligible for 503A compounding: FDA removed it from Category 2² in April 2026 after the nomination was withdrawn, but it is not on the 503A bulks list or in FDA's Category 1³. FDA has announced an advisory committee meeting⁴, to be held before the end of February 2027, that will discuss LL-37.

What does LL-37 do in the body naturally?

LL-37 is an endogenous antimicrobial peptide produced by neutrophils and various epithelial cells⁵, including skin cells. In neutrophils, its precursor protein hCAP-18 is cleaved by proteinase 3 after release⁶ from the cell, freeing the active peptide. It kills bacteria, viruses, and fungi directly through membrane disruption; recruits neutrophils, monocytes, and T cells to infection sites via FPRL1 receptor signaling; and supports wound healing through skin re-epithelialization⁷ and new blood-vessel formation⁸.

How does vitamin D affect LL-37 levels?

Vitamin D receptor activation by 1,25-dihydroxyvitamin D drives transcription of the CAMP gene⁹, which encodes hCAP-18 (the LL-37 precursor). In laboratory studies of human immune cells, serum with low 25-hydroxyvitamin D was less able to support cathelicidin induction¹⁰.

Can LL-37 make autoimmune conditions worse?

Potentially, yes. LL-37 has been identified as a psoriasis T-cell autoantigen¹¹, and it can bind self-DNA to activate plasmacytoid dendritic cells¹² via TLR9, triggering an interferon response that may drive autoimmunity in psoriasis. It is also implicated in rosacea¹³ as an inflammatory mediator when abnormally processed by elevated serine proteases.

What is the difference between LL-37 and defensins?

LL-37 is a cathelicidin, while defensins are a structurally distinct family¹⁴ of antimicrobial peptides. LL-37 takes on a helical shape in membrane-like environments and kills through amphipathic membrane disruption; defensins contain six cysteine residues linked by three disulfide bonds into a beta-sheet structure. Both are components of the human innate immune system.

What has the human clinical research examined?

The most advanced human clinical evidence for LL-37 comes from topical wound-healing research. A multicenter phase IIb randomized trial of topical LL-37 in 148 patients with hard-to-heal venous leg ulcers found no significant benefit across the full cohort¹⁵, with a signal in larger ulcers on post-hoc analysis. A 2023 randomized trial in diabetic foot ulcers reported consistently greater granulation¹⁶ in the LL-37 group. This is the research application with the most human trial data, but the evidence does not meet the threshold for an FDA-approved indication.

Who should not use LL-37?

Based on LL-37's dual-edged biology, the populations of greatest concern include individuals with active or history of psoriasis (LL-37 is a psoriasis autoantigen and, bound to self-DNA, activates plasmacytoid dendritic cells through TLR9), rosacea or rosacea-prone skin (abnormal cathelicidin processing is implicated in rosacea), active malignancy (LL-37 is upregulated in ovarian, breast, and lung tumors⁸, and FDA notes nonclinical findings that it can be protumorigenic in some tissues²), systemic autoimmune conditions involving innate immune dysregulation, pregnancy or breastfeeding (no safety data), and known hypersensitivity to LL-37. These are illustrative mechanistic concerns, not a formal contraindication checklist.

More for immune & defense