LL-37 Side Effects: What Studies Report

What the published literature actually contains

LL-37 is the mature, active fragment of human cathelicidin, a host-defence peptide released mainly by neutrophils and epithelial cells. Because it is an endogenous molecule, much of the research on it describes normal immune biology rather than the tolerability of an administered product. That distinction matters when reading anything framed as "LL-37 side effects": the majority of papers in this area were not designed as safety studies at all, and several used LL-37 deliberately as an inflammatory stimulus.

Across the verified literature reviewed here, there is a single randomized human trial that stated safety as an endpoint — researchers evaluated the efficacy and safety of an oral LL-37 preparation against the Omicron BA.5.1.3 variant of SARS-CoV-2 (PMID 37605995). Everything else summarised below comes from animal models, isolated cells, or disease-model systems. PeptideU's LL-37 course covers the peptide's mechanism and pharmacology; this page is limited to what studies reported about unwanted or harmful effects.

Human Data: What Studies Report

The oral LL-37 trial is the only human interventional report in this set that explicitly framed safety alongside efficacy, and the study was conducted as a randomized trial in the context of an Omicron BA.5.1.3 infection (PMID 37605995). Its scope was a single route (oral), a single indication, and a short antiviral treatment window; it was not designed to characterise long-term exposure, injectable routes, or rare events. No verified paper in this set reported a systematic adverse-event table for injected or infused LL-37 in humans.

Two further human-relevant papers describe endogenous LL-37 rather than an administered peptide. Researchers examining neutrophil functions in patients with neutropenia due to glycogen storage disease type 1b treated with empagliflozin assessed how impaired neutrophil biology affects antimicrobial defence (PMID 38531056), which is a reminder that cathelicidin output is normally tied to neutrophil number and function. Separately, a review of vitamin D and hepatitis C described vitamin D–driven induction of cathelicidin as part of the antiviral response (PMID 34420582) — an endogenous upregulation pathway, not an exogenous dosing model.

Animal Studies: What Studies Report

Repeated mucosal administration in mice

The most directly safety-oriented animal work in this set examined repeated exposure of the female reproductive tract: the study assessed the safety of multiple administrations of spermicidal LL-37 antimicrobial peptide into the mouse female reproductive tract (PMID 37326833). That design — a defined mucosal surface, repeated dosing, and local tissue endpoints — is the kind of protocol that generates tolerability information, but its conclusions apply to that species, that route, and that contraceptive context only.

Livestock delivery via engineered bacteria

Researchers also reported that a recombinant Lactococcus lactis strain expressing human LL-37 prevented deaths from viral infections in piglets and chickens (PMID 37743432). Survival-focused veterinary studies of this kind indicate that the delivery system was compatible with the animals used, but they are efficacy experiments; they do not substitute for dedicated toxicology, and findings in poultry or swine do not transfer to humans.

Cell and Tissue Toxicity: What Studies Report

Endothelial cells and autophagy status

One of the clearest mechanistic signals of harm is conditional. The study reported that human cathelicidin peptide LL-37 induced cell death in autophagy-dysfunctional endothelial cells (PMID 35387840). The important word there is dysfunctional: cells with intact autophagy handled the peptide differently from cells in which that pathway was impaired. Findings like this suggest that the cytotoxic potential of LL-37 in the literature is context-dependent — a function of cell type, concentration, and the state of intracellular stress-handling machinery — rather than a fixed property.

Sperm function

LL-37 is one of the components released when neutrophils form extracellular traps, and researchers reported adverse effects of single neutrophil extracellular trap-derived components on bovine sperm function (PMID 35625154). This aligns with the separate use of LL-37 as a spermicidal agent in the mouse reproductive-tract work (PMID 37326833): membrane-active peptides that damage microbes can also damage host cells with exposed, lipid-rich membranes. In these reports, the anti-sperm activity was the studied effect, not an incidental observation.

Skin and Inflammatory Effects: What Studies Report

Rosacea models

In dermatology research, LL-37 is frequently the trigger rather than the treatment. A 2025 paper investigated vascular and inflammatory crosstalk in rosacea and tested cannabigerol as a dual-pathway modulator of that cascade (PMID 40725084), and a separate report described dissolving microneedles co-delivering a mitochondria-targeted nanozyme and tofacitinib for rosacea (PMID 41386376). Both sit inside a research tradition in which elevated cathelicidin activity in skin is treated as part of the disease process to be suppressed. For anyone reading about LL-37 tolerability, the relevant takeaway from that literature is directional: excess local cathelicidin activity has been studied as pro-inflammatory in skin, and researchers in these papers were attempting to interrupt it.

Severe cutaneous reactions

At the most severe end of neutrophil-driven skin pathology, the study reported that neutrophils initiate and exacerbate Stevens-Johnson syndrome and toxic epidermal necrolysis (PMID 34193610). Because LL-37 is a neutrophil-derived mediator, this work is often cited when discussing the destructive potential of neutrophil effector molecules in epidermis. It is a disease-mechanism paper — it did not administer LL-37 to participants and did not report adverse events from a peptide product.

Other Reported Effects

LL-37 binds nucleic acids, and that property has consequences beyond antimicrobial killing. Researchers examined the role of LL-37 in oral bacterial DNA accumulation in dental plaque (PMID 38093556), describing how the peptide interacts with bacterial DNA in a real human microenvironment. Peptide–DNA complexes are the same general phenomenon invoked in autoimmune-adjacent cathelicidin research, which is why this line of work appears in discussions of unintended immune consequences rather than pure antimicrobial benefit.

Finally, some frequently circulated claims about LL-37 rest on computational work only. An in silico analysis proposed a scientific basis for LL-37 as a therapeutic for COVID-19 (PMID 34333809). Modelling papers of that type generate hypotheses about binding and mechanism; they contain no tolerability information of any kind.

Summary Table of Reported Findings

What Is Not Documented

The absence of data is itself a finding, and it should be stated plainly. In the verified literature summarised here, there is no systematic human adverse-event profile for injected, subcutaneous, intravenous, nebulised, or topical LL-37; no dose-ranging toxicity study in humans; no long-term exposure data; no pharmacokinetic safety characterisation across routes; and no reported interaction profile with medications. The one randomized human trial addressed an oral preparation in a specific viral indication (PMID 37605995), and its scope cannot be extended to other routes or populations.

Anything presented elsewhere as a definitive list of LL-37 side effects in people — injection-site reactions, flushing, immune activation, dose thresholds — is not supported by the papers above. Where such claims appear, the underlying source is usually a cell-culture experiment, an animal model, or a disease-mechanism paper of the kind described here, generalised beyond its design.

How to Read This Evidence Base

• Trigger versus treatment. In rosacea research, LL-37 activity was something investigators sought to suppress (PMID 40725084), while in antiviral research it was the candidate agent (PMID 37605995). The same molecule occupies opposite roles depending on tissue and concentration.
• Context changes toxicity. Cell death was reported specifically in autophagy-dysfunctional endothelial cells (PMID 35387840), not as a universal endothelial effect.
• Species and route limits. Mucosal mouse safety work (PMID 37326833) and livestock survival work (PMID 37743432) do not predict human tolerability.
• In silico is not evidence of safety. Computational modelling reported structural rationale only (PMID 34333809).

LL-37 is not an approved therapeutic product in the United States, and peptide material sold for laboratory purposes carries research-use-only status. This page is for educational purposes only and is not medical advice; consult a licensed physician about any health question, symptom, or treatment decision.

References

• Efficacy and safety of Oral LL-37 against the Omicron BA.5.1.3 variant of SARS-COV-2: A randomized trial (Journal of Medical Virology, 2023)
• Safety of multiple administrations of spermicidal LL-37 antimicrobial peptide into the mouse female reproductive tract (Molecular Human Reproduction, 2023)
• Human Cathelicidin Peptide LL-37 Induces Cell Death in Autophagy-Dysfunctional Endothelial Cells (Journal of Immunology, 2022)
• Adverse Effects of Single Neutrophil Extracellular Trap-Derived Components on Bovine Sperm Function (Animals, 2022)
• Neutrophils initiate and exacerbate Stevens-Johnson syndrome and toxic epidermal necrolysis (Science Translational Medicine, 2021)
• Targeting Vascular and Inflammatory Crosstalk: Cannabigerol as a Dual-Pathway Modulator in Rosacea (International Journal of Molecular Sciences, 2025)
• Dissolving microneedle co-deliver mitochondria-targeted Nanozyme and Tofacitinib for synergistic treatment of Rosacea (Journal of Controlled Release, 2026)
• Recombinant Lactococcus lactis Expressing Human LL-37 Prevents Deaths from Viral Infections in Piglets and Chicken (Probiotics and Antimicrobial Proteins, 2024)
• Role of LL-37 in Oral Bacterial DNA Accumulation in Dental Plaque (Journal of Dental Research, 2024)
• Neutrophil functions in patients with neutropenia due to glycogen storage disease type 1b treated with empagliflozin (Blood Advances, 2024)
• An in silico scientific basis for LL-37 as a therapeutic for Covid-19 (Proteins, 2022)
• Inhibition of hepatitis C virus by vitamin D (Vitamins and Hormones, 2021)