History

LL-37 was discovered in 1995 by Agerberth and colleagues at the Karolinska Institute, identified from a human bone-marrow cDNA library and originally named FALL-39. It was soon renamed LL-37 for its two leading leucine residues and 37-residue length. In 2000, De Yang and colleagues showed it recruits immune cells through the FPR2/FPRL1 receptor. In the 2010s the Swedish firm Promore Pharma advanced a topical form (ropocamptide) for venous leg ulcers; an early trial was positive but a larger Phase IIb study failed in 2021. In 2026, FDA removed it from a compounding watch list after the nomination was withdrawn.

LL-37 is unusual among the peptides people ask about because it is not a synthetic invention. It is a molecule your own body produces as a first line of immune defense. That fact gets used in marketing to suggest it is both safe and proven, but the truth is more complicated. As a natural immune signal, LL-37 is one of the best-studied antimicrobial peptides in biology. As a drug you would buy and use, the human evidence is small, early, conflicted, and in the one well-powered trial, negative. This page separates the well-characterized biology from the much weaker case for using LL-37 as a therapeutic.

What it is

LL-37 is the only human member of the cathelicidin family of antimicrobial peptides. It is a short, 37-amino-acid chain that is cationic (positively charged) and amphipathic, meaning one face attracts water and the other repels it. That structure lets it insert into and disrupt microbial membranes.

The body makes it indirectly. The CAMP gene encodes a precursor protein called hCAP18, and an enzyme (proteinase 3) clips off the active C-terminal fragment, LL-37, inside neutrophil granules and at skin and mucosal surfaces. The peptide folds into a helix-break-helix shape with a kink around its twelfth residue.

Its biology is genuinely dual-edged. Beyond directly killing bacteria, LL-37 acts as an “alarmin”: it chemoattracts neutrophils, monocytes, and T cells through the FPR2/FPRL1 receptor, nudges cytokine release, and can promote new blood-vessel growth and wound re-epithelialization. But the same membrane activity that damages bacteria can damage human cells, and LL-37 can bind a person’s own DNA to form complexes that trigger inflammation through TLR9, a mechanism tied to the skin disease psoriasis.

The claims

Two claims rest on actual clinical development. The first is topical wound healing, specifically venous leg ulcers. The second is intratumoral cancer treatment, studied in melanoma.

Beyond those, the broader literature explores LL-37 for antibacterial, antibiofilm, antifungal, and antiviral activity, and for immune modulation generally. Some of this work proposes repurposing it for cancer, though the cancer findings are contradictory: LL-37 looks anti-tumor in some tissues and pro-tumor in others, including melanoma.

Separately, a wave of gray-market vendor and “peptide reference” sites promote LL-37 for immune support, gut health, chronic infection or biofilm, and anti-aging. These products are typically sold “for research use only,” which is not a legal basis for human use. None of those marketed uses is backed by an approval or by adequate human trials, and those claims should be read as marketing rather than evidence.

The evidence

There are three relevant human studies, and reading them in order tells the real story.

The first, a Phase I/IIa trial in venous leg ulcers (Grönberg, 2014), was positive but tiny. It enrolled 34 patients and used a placebo run-in followed by four weeks of twice-weekly topical LL-37 at three doses. The two lower doses sped healing, while the highest dose did not beat placebo, an odd non-monotonic pattern. The study was run by the developer.

The second study is the one that matters most. The Phase IIb trial (Mahlapuu, 2021), randomized about 149 patients across multiple centers in a double-blind, placebo-controlled design with compression therapy. It failed its primary endpoint. Confirmed complete wound closure occurred in 26.5%, 24.7%, and 25.3% of patients in the low-dose, high-dose, and placebo groups respectively, essentially identical. A post-hoc look at large ulcers hinted at benefit, but post-hoc subgroups generate hypotheses, they do not prove anything. The trial was funded by Promore Pharma, and several authors were company employees.

The third human study is in melanoma (NCT02225366) at MD Anderson. It enrolled just four patients, and its primary endpoint was finding an optimal biological dose based on toxicity. In other words, it was a safety and feasibility study, not a test of whether the peptide shrinks tumors.

The animal and lab data are extensive: antimicrobial and antibiofilm activity, wound-healing and angiogenesis effects, and both pro- and anti-tumor effects depending on the tissue. None of that predicts human benefit.

The honest summary: no adequately powered, confirmatory randomized trial supports any primary use of LL-37 as a drug. The single well-powered trial was negative. The positive data are small, developer-funded, and inconsistent in their dose response. There is no human efficacy data for oral or systemic use, and poor stability, host-cell toxicity, and production cost have all slowed development.

Safety and side effects

In the controlled topical wound trials, LL-37 was generally well tolerated. The Phase IIb study reported local reactions such as redness, swelling, and raised skin temperature in a substantial fraction of patients, mostly mild to moderate, without a major systemic safety signal.

The melanoma program produced one documented dermatologic toxicity case: a 63-year-old woman developed multiple warty papules and a blistering eruption about 45 days after starting weekly intratumoral injections. A biopsy showed an inflammatory infiltrate with eosinophils, and the lesions resolved within roughly two months of stopping (Dolkar, 2018).

Two mechanism-based concerns deserve attention. First, LL-37 damages host cell membranes (red cells, lymphocytes, fibroblasts) in the lab at concentrations near its antimicrobial range, which is a real constraint on giving it systemically. Second, LL-37 bound to a person’s own DNA can activate immune cells through TLR9, a process implicated in psoriasis. Associations with other inflammatory conditions are discussed in the wider literature. There is also the oncologic ambivalence noted above: in several tissues LL-37 can promote tumor behavior, which is part of why regulators have flagged it.

There is no long-term human safety data for systemic or chronic use.

Legal and regulatory status

LL-37 is not FDA-approved for any indication. It was historically listed in the interim 503A “Category 2” of bulk drug substances that may present significant safety risks for pharmacy compounding. In April 2026, FDA announced the removal of twelve peptides, including cathelicidin LL-37, from Category 2 because those nominations were voluntarily withdrawn. Importantly, removal from that list does not mean approval. It reflects withdrawn nominations and a pending advisory review. LL-37 was not part of the July 2026 advisory committee meeting and is slated for a separate review by the end of February 2027. The “peptides are legal again” framing seen from some vendors is promotional and overstated.

There is no record of LL-37 or ropocamptide being approved as a marketed drug in the EU, UK, or elsewhere; the Swedish developer did not reach approval after the negative Phase IIb.

For athletes, LL-37 does not appear by name on the WADA 2026 Prohibited List, and it is not a peptide hormone or growth factor. However, as an exogenous, non-approved pharmacological substance, it could plausibly be captured by the S0 “Non-Approved Substances” catch-all. No explicit ruling was found either way, so it should not be assumed permitted.

In practice, LL-37 is sold online as a “research use only” lyophilized powder, with no approved human use, no USP monograph, and no manufacturing-quality guarantee. Purity, identity, and endotoxin levels are unverified at the consumer level.

Bottom line

LL-37 is a genuine, well-studied human immune peptide, and that biology is real. But as a therapeutic you would actually use, it sits at the weak end of “preliminary human.” The supporting trials are small and developer-run, the one adequately powered trial failed its primary endpoint, and the cancer work is a four-patient dose-finding study. Layer on real concerns about host-cell toxicity, pro-inflammatory autoimmune mechanisms, and context-dependent pro-tumor effects, and the picture is of a molecule that is fascinating in the lab but unproven and not approved as a drug. Marketing that presents it as a safe, established immune booster is running well ahead of the evidence.

LL-37: A real human peptide with thin, mostly negative clinical data