LL-37

What is LL-37?

LL-37 is the only cathelicidin your body makes, a 37-amino-acid antimicrobial peptide that starts with two leucines (hence the name). Your skin, immune cells, and mucosal surfaces release it as a front-line part of innate immunity. It kills microbes, but it also signals to the immune system and plays a role in wound repair, which makes it a genuinely double-edged molecule. It is a naturally occurring peptide, but there is no approved LL-37 drug; the synthetic version is sold as a research compound.

How LL-37 works

LL-37 does two very different jobs. As an antibiotic it punches holes in bacterial membranes directly, which is hard for microbes to develop resistance against. As an immune signal it binds receptors like FPR2 on neutrophils, macrophages, and other cells, pulling them toward a site of infection or injury and nudging processes like angiogenesis and tissue repair. The catch is that this signaling cuts both ways: in the right context it resolves inflammation, but too much LL-37, or LL-37 in the wrong tissue, can drive it instead.

What the research shows

LL-37 is one of the most heavily studied human peptides, but most of that work is basic science in cells and animals rather than outcome trials in people. The clearest human data is topical: an early-phase trial applying LL-37 to chronic venous leg ulcers reported improved healing. On the other side of the ledger, elevated or dysregulated LL-37 is repeatedly tied to inflammatory disease, including rosacea, psoriasis, and systemic lupus, where it can act as a self-antigen. So the picture is nuanced. Real antimicrobial and wound-healing promise, alongside evidence that more is not automatically better.

Dosage and how it is used

There is no standardized human protocol. The clinical work that exists has been topical, formulated into wound dressings or applied to the wound bed, not self-injected. Figures circulating for injectable use come from anecdote rather than dose-finding trials, and given the pro-inflammatory risk they should be treated with real caution.

→ How to reconstitute LL-37: calculator, units to draw & storage

Side effects and safety

The main safety concern is not toxicity in the usual sense but the peptide's own biology. Because high or misplaced LL-37 is linked to inflammatory and autoimmune skin conditions, adding more is not obviously benign. Lab studies also show it can be cytotoxic to human cells at higher concentrations, and it breaks down quickly, which complicates any consistent dosing.

• Research depth: moderate. Extensive basic science; limited human outcome trials, mostly topical wound care.
• Dual-edged biology: elevated LL-37 is associated with rosacea, psoriasis, and lupus.
• Not FDA-approved. Legal status varies by jurisdiction; verify it independently.

Frequently asked questions

References

• Kahlenberg JM, Kaplan MJ. LL-37: cathelicidin-related antimicrobial peptide with pleiotropic activity. Curr Rheumatol Rep / PubMed 27117377, 2016.
• Dombrowski Y, Schauber J. Cathelicidin LL-37: an antimicrobial peptide with a role in inflammatory skin disease. Ann Dermatol (PMC3346901), 2012.