What it is
LL-37 is the only human cathelicidin antimicrobial peptide. Cathelicidins are a family of antimicrobial peptides found in mammals. LL-37 is studied for broad antimicrobial activity, immune modulation and roles in wound healing.
In the PeptidesAI encyclopedia it is categorized under immune and healing. The human trials to date tested LL-37 applied to the skin, not injected.
How it's thought to work
LL-37 is described as disrupting microbial membranes for direct antimicrobial action, while also modulating immune responses, promoting angiogenesis (the growth of new blood vessels) and supporting wound healing. Reviews describe several antimicrobial mechanisms, including membrane permeabilization and disruption of viral envelopes.
What the research says
The basic-science literature on its antimicrobial and immunomodulatory roles is extensive. Human trials have tested LL-37 applied topically to hard-to-heal venous leg ulcers. In a first-in-man trial with 34 participants, healing rate constants were higher than with placebo at the two lower concentrations tested, with no safety concerns. A later phase IIb trial in 148 people found no significant improvement in healing in the full study population, and a post hoc subgroup analysis pointed to a possible benefit in people with large ulcers. There is no approved therapeutic use of the peptide itself, and it is not FDA-approved.
Reported side effects
One concern is that LL-37 can be pro-inflammatory in some conditions. People with rosacea have been reported to express abnormally high levels of cathelicidin in facial skin. The PeptidesAI encyclopedia also notes elevated LL-37 in some autoimmune contexts. Human safety data beyond these topical trials is limited.
Reconstitution
LL-37 is supplied as a lyophilized (freeze-dried) powder and is typically reconstituted with bacteriostatic (BAC) water before use. It commonly comes in 5 mg and 10 mg vials. The amount of peptide in the vial is fixed; the water added only changes how concentrated the solution is.
Worked example. A 5 mg vial is 5,000 mcg. Reconstituted with 1 mL of BAC water, that works out to 5,000 mcg ÷ 1 mL = 5,000 mcg/mL. On a U-100 insulin syringe, where each unit is 0.01 mL, each unit holds 5,000 × 0.01 = 50 mcg. Use the peptide reconstitution calculator to work out the concentration and units for any vial size and water volume.
Important notes
LL-37 is context-dependent and can be pro-inflammatory in some conditions. It is sold only as a research chemical and is not FDA-approved.
Sources
- Treatment with LL-37 is safe and effective in enhancing healing of hard-to-heal venous leg ulcers: a randomized, placebo-controlled clinical trial.. Grönberg A et al. Wound Repair and Regeneration, 2014. Human trial
- Evaluation of LL-37 in healing of hard-to-heal venous leg ulcers: A multicentric prospective randomized placebo-controlled clinical trial.. Mahlapuu M et al. Wound Repair and Regeneration, 2021. Human trial
- Decoding LL-37: Structure and antimicrobial mechanisms against microbial threats.. Neshani A et al. Infection, Genetics and Evolution, 2025. Review
- Increased serine protease activity and cathelicidin promotes skin inflammation in rosacea.. Yamasaki K et al. Nature Medicine, 2007. Preclinical
Frequently asked questions
These questions cover the same ground as the sections above, in a shorter format.
What is LL-37?
The only human cathelicidin antimicrobial peptide, studied for broad antimicrobial activity, immune modulation and roles in wound healing.
How is LL-37 thought to work?
It is described as disrupting microbial membranes for direct antimicrobial action, while also modulating immune responses, promoting angiogenesis and supporting wound healing.
What does the research on LL-37 say?
The basic-science literature is extensive. Two randomized trials of topical LL-37 in venous leg ulcers had mixed results, and there is no approved therapeutic use of the peptide itself.
What are the reported side effects of LL-37?
It can be pro-inflammatory in some conditions, such as rosacea, and human safety data is limited.
How much is one unit from a 5 mg LL-37 vial?
With 1 mL of BAC water, a 5 mg vial is 5,000 mcg/mL, so one unit on a U-100 syringe (0.01 mL) holds 50 mcg.
The full entry, plus the calculator
The PeptidesAI app has the complete encyclopedia entry for LL-37 — one of 70 compounds covered — plus the reconstitution calculator, a dose log and reminders. Free to download on iPhone.
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