01 What is LL-37?
In plain English.
LL-37 is a small antimicrobial peptide that your own body makes. It is the only cathelicidin humans produce, a family of innate-immune peptides shared across mammals. Neutrophils, skin keratinocytes, and the cells lining your airways and gut all secrete LL-37 (or its precursor, hCAP-18) as part of the first-line defence against bacteria, viruses and fungi. The name simply comes from its sequence, it starts with two leucines (LL) and is 37 amino acids long.
Synthetic LL-37, sold by research-peptide vendors and increasingly visible in biohacker circles, is a lab-made copy of the same molecule. It is not an approved medicine in any jurisdiction. The fact that the body makes LL-37 naturally does not mean injecting more of it is safe, endogenous antimicrobial peptides also play roles in inflammation and cancer biology that get messier the more of them you add.
02 How it works
The simple version, then the science.
LL-37 punches holes in microbial membranes, its positive charge sticks to the negatively charged outside of bacterial cells, then the peptide inserts itself into the membrane and disrupts it. That makes LL-37 broadly active against bacteria (including some antibiotic-resistant strains), some viruses and fungi. Separately, LL-37 acts as a signalling molecule to the immune system: it recruits white blood cells, modulates inflammation, and influences how skin and other tissues heal after injury.
Go deeper · the proposed mechanism
LL-37 is the active C-terminal fragment of hCAP-18, the protein product of the CAMP gene, cleaved out by proteases such as proteinase 3. As an amphipathic α-helical cationic peptide, it disrupts bacterial membranes via the carpet/toroidal-pore model. Beyond direct antimicrobial action it engages receptors including FPR2, P2X7 and EGFR, and modulates TLR signalling, driving chemotaxis of neutrophils, monocytes and T cells, promoting angiogenesis and re-epithelialisation, and altering cytokine output. Vitamin D upregulates CAMP transcription, which is part of why low vitamin D status has been linked to impaired innate immunity. LL-37 is also a known autoantigen in psoriasis and is overexpressed in some chronic inflammatory skin conditions, where its immunomodulatory effects appear to drive, not just respond to, pathology.
03 What it's used for
Each use graded by how strong the evidence actually is.
- PreclinicalBroad-spectrum antimicrobial activityRobust in vitro and animal data against gram-positive and gram-negative bacteria, biofilms, some viruses and fungi. No approved antimicrobial drug based on LL-37 exists.
- LimitedChronic wound healing (venous leg ulcers, diabetic foot ulcers)A small Swedish RCT in hard-to-heal venous leg ulcers (Grönberg et al., 2014) reported faster healing with topical LL-37; a 2021 follow-up multicentric RCT was less clear-cut. A 2023 RCT of an LL-37 cream in diabetic foot ulcers reported enhanced healing. All small, none with regulatory approval.
- PreclinicalImmune modulation in inflammatory skin diseaseStudied in atopic dermatitis, psoriasis and rosacea biology, where LL-37 expression is altered. Therapeutic role is mechanistic, not established as a treatment.
- AnecdotalLongevity / biohacker antimicrobial protocolsPromoted online as an "immune booster" or chronic-infection adjunct. No human evidence supports general supplementation in healthy adults.
04 What the evidence says
The case for LL-37 as biology is strong, thousands of papers describe its structure, antimicrobial mechanism and immunomodulatory roles, and several review articles map the field comprehensively. The case for LL-37 as a medicine is much thinner. A handful of small randomised trials in chronic wounds (hardest-to-heal venous leg ulcers and diabetic foot ulcers) have reported benefit, with mixed consistency between studies and no approved product to date. The clinical-development history of LL-37 and related cathelicidins (and of derived "innate defence regulator" peptides such as IDR-1018) is one of recurrent promise and slow translation: antimicrobial peptide drugs have repeatedly failed late-stage trials elsewhere in the field. The cancer literature is genuinely conflicted, LL-37 has been reported as pro-tumour in breast and ovarian cancers and anti-tumour in colon and gastric cancers, depending on receptor context and tissue. Honest read: real biology, modest early human signal in chronic wounds, unresolved cancer signal, no approved therapeutic.
05 Dosing & administration
Reported in the literature, information not advice.
Because there is no approved human protocol outside of investigational trials, no safe or effective dose has been established for general use. Published clinical work in venous leg ulcers used topical LL-37 in low microgram-per-millilitre concentrations applied to the wound under occlusion in a controlled trial setting. Online research-chemical communities describe subcutaneous injection of milligram-range doses, but these regimens are not supported by clinical evidence and the purity of vendor-supplied LL-37 is unregulated. A qualified clinician should be consulted before considering any peptide.
06 Side effects & safety
Long-term safety in humans is unknown, published trials of LL-37 are small, short and mostly topical. The mechanistic concerns are real and specific. LL-37 has dual signals in cancer biology: it has been reported to promote tumour growth, invasion and metastasis in breast, ovarian, lung and skin cancers (often via FPR2 / EGFR signalling), and to inhibit tumour growth in some gastric and colon cancer models. The direction depends on the cancer, the receptor context and the dose, and there is no clinical safety data to settle which dominates in any given person. LL-37 also drives inflammation in psoriasis (where it is a known autoantigen), rosacea, and possibly atopic dermatitis flares. Injection-site reactions, immune-stimulation effects and unknown interactions are all plausible. Products sold as "LL-37" are unregulated research chemicals, purity, identity and dose-by-dose consistency are not guaranteed. People with a personal or family history of cancer, inflammatory skin disease, autoimmunity, or who are pregnant, breastfeeding or immunocompromised should be especially cautious.
07 Where to buy
Sold "research use only", so verification matters more than price.
08 Legal & regulatory status
- UKNot licensed as a medicine. Sold only as a "research chemical", not for human use.
- USNot FDA-approved. Not on the FDA's list of bulk substances permitted for compounding.
- EU / AUS / CANNo approved human medicine containing LL-37. Sale for human use is unlawful in most jurisdictions; "research use only" framing is standard.
- Sport (WADA)
09 Clinical studies & research
Primary sources. Read the science yourself.