LL-37

What it is

LL-37 is the mature, active fragment of hCAP18, the only human member of the cathelicidin family of antimicrobial peptides S2. It is encoded by the CAMP gene, whose 18 kDa precursor (hCAP18) is cleaved extracellularly — for example by proteinase 3 — to release the 37-residue peptide that begins with two leucines, giving it the name "LL-37" S2. The peptide is stored in the granules of neutrophils and other cells and deployed as part of the innate immune response S2. Its biology is unusually broad for a single small peptide: it is at once a direct antimicrobial, an immune modulator, and a signal in tissue repair S1.

Marketed as

On the research and gray market, LL-37 is typically sold as a "research use only" peptide and is discussed in wellness circles around immune support, antimicrobial defense, gut and skin health, and wound healing S6. These are marketing and enthusiast framings rather than approved uses. There is no approved LL-37 drug product, and the vendor-side positioning does not reflect a settled clinical evidence base S3S6.

Regulatory status (US)

LL-37 has no FDA approval for any use S6. It is not an approved drug and is not an established dietary ingredient; industry summaries place it in an informal "gray" category — neither on the FDA's nominal compounding review lists nor given a formal classification for compounding use S6. In practice that means material sold as LL-37 sits outside the approved-drug framework and is not quality-assured as a medicine.

Around the world

LL-37 itself is an endogenous human molecule, so as a natural peptide it is universally present in people; the regulatory question is about synthetic LL-37 sold as a product, which lacks marketing authorization as a medicine in major jurisdictions S3S6. Clinical development that has occurred — for LL-37 and for engineered relatives — has largely been in early-phase academic and small-company trials rather than approved products S3.

Evidence

The mechanistic and preclinical literature on LL-37 is substantial. It permeabilizes the cell walls of gram-positive and gram-negative bacteria, disrupts viral envelopes and fungal membranes, and breaks up bacterial biofilms at relatively low concentrations S1. Beyond killing microbes, it neutralizes bacterial endotoxins, promotes chemotaxis of immune cells, and can preferentially trigger death of infected host cells to contain an infection locally S1. In tissue repair, it is described as promoting cell proliferation and migration and contributing to wound closure and angiogenesis S2. Human therapeutic translation remains early: a review of skin therapeutics reports that locally applied LL-37 (developed as ropocamptide) was safe and well tolerated on nonhealing lower-leg ulcers, and that most LL-37 trials remain in Phase II S3. LL-37-derived peptides such as OP-145 (a 24-residue derivative) have been studied in randomized Phase II work for chronic ear infection S3.

Anti-doping

LL-37 is not a growth-hormone secretagogue and is not specifically named on the WADA Prohibited List S6. It is a host-defense/antimicrobial peptide rather than a classic performance agent. However, its status should be verified against the current WADA list before assuming it is permitted, because WADA applies catch-all provisions to substances without regulatory approval for human therapeutic use S6. Athletes subject to testing should treat any non-approved peptide cautiously.

Safety

LL-37's biology is genuinely double-edged. The same peptide central to host defense is implicated in the pathogenesis of inflammatory skin diseases: in psoriasis, LL-37 can complex with self-nucleic acids to drive interferon responses, and LL-37-specific T cells appear in roughly two-thirds of moderate-to-severe psoriasis patients S2S4. Elevated cathelicidin/LL-37 is likewise implicated across the subtypes of rosacea S2S4. At physiologically relevant concentrations LL-37 can be cytotoxic to host cells S1, and reviews of the engineered peptides note challenges including protease degradation, toxicity, cost, and emerging microbial resistance S3S5. Because material sold as LL-37 is unapproved and unregulated, its identity, purity, and safety in humans are not established S6.

What's changing

Interest is shifting from native LL-37 toward engineered fragments and analogues designed to keep the antimicrobial and immunomodulatory activity while reducing toxicity and improving stability S5. A 2025 clinical-translation review frames cathelicidins as promising but still gated by the same four hurdles — protease susceptibility, production economics, resistance, and formulation S3. The near-term action is concentrated in local and topical applications and in derivative peptides (e.g., ropocamptide, OP-145) rather than approved systemic LL-37 S3. The scientific picture continues to sharpen even as the consumer/gray-market supply runs well ahead of the evidence S3S6.