LL-37: the body’s own antimicrobial peptide, and why the wound trials disappointed

LL-37 is a genuinely important human molecule: your skin, gut lining and white blood cells make it to kill bacteria and to direct inflammation. That doesn't make it a good drug. The biggest trial of LL-37 as a treatment, 148 people with leg ulcers, found no benefit over placebo, and too much LL-37 is part of what drives psoriasis and rosacea.

What it is

LL-37 is the only cathelicidin humans make. The name describes it: 37 amino acids, starting with two leucines (L, L). Cells produce it as part of a larger precursor protein, hCAP18, and cut it free when needed. It carries a strong positive charge, which lets it stick to and break open the negatively charged membranes of many bacteria.

Its best-known role is in innate immunity, the fast, non-specific first line of defence. In a much-cited 2006 Science paper, researchers showed that when human macrophages detect a microbe, they switch on the machinery to activate vitamin D, which in turn induces cathelicidin and kills tuberculosis bacteria living inside the cell [1]. Serum from people with low vitamin D supported that response poorly. That was work in cells and donated serum, and it is often misquoted as evidence that taking LL-37, or vitamin D, treats infection.

Beyond killing microbes, LL-37 is reported to encourage blood-vessel growth and the migration and multiplication of skin cells [4], and it signals to immune cells [5], which is why it was developed as a wound treatment. These are lab and animal findings, and they describe a molecule with several jobs rather than one clean effect.

The human evidence

Three randomised trials have tested LL-37 as a medicine, all applied to chronic wounds. None tested it for infection, immunity or any of the general wellness uses it is sold for online.

The first-in-human leg-ulcer trial (2014)

A team in Sweden enrolled 34 people with hard-to-heal venous leg ulcers [2]. After three weeks on placebo for everyone, patients were randomised to LL-37 solution at 0.5, 1.6 or 3.2 mg/mL, or placebo, applied twice a week for four weeks. The healing rate was about six times faster than placebo at 0.5 mg/mL (p = 0.003) and three times faster at 1.6 mg/mL (p = 0.088, not significant). Mean ulcer area fell 68% and 50% in those two groups. The highest dose did no better than placebo.

With 34 people split four ways, each group averaged eight or nine patients. And an inverted dose response, where the strongest dose does nothing, is a result that needs replicating before anyone relies on it.

The larger trial that failed (2021)

HEAL LL-37 was that replication [3]. The Phase 2b trial randomised 148 patients across multiple centres to LL-37 at 0.5 or 1.6 mg/mL or placebo, on top of standard compression bandaging. Patients were on average 67.6 years old, their ulcers had lasted a median of 20.3 months, and the mean wound area was 11.6 cm².

Across the whole study population, LL-37 did not significantly improve healing on any measure compared with placebo. A post hoc analysis, one not planned in advance, found benefit in patients with large ulcers of at least 10 cm². The authors called for a new trial in that group. Post hoc subgroups from a negative trial are a reason to run another study, not evidence of an effect. The drug was well tolerated at both doses.

Diabetic foot ulcers (2023)

A single-centre trial in Jakarta randomised people with mildly infected diabetic foot ulcers to LL-37 cream or placebo cream twice weekly for four weeks [4]. The granulation index, a measure of healthy new tissue filling the wound, improved more with LL-37 at every check from day 7 to day 28 (p values 0.006 to 0.037). But LL-37 didn't reduce the inflammatory markers IL-1α and TNF-α, and it didn't significantly reduce bacterial colonisation. For a peptide whose selling point is killing bacteria, that last result is awkward. The abstract doesn't state the number of participants; the registry entry lists an estimated enrolment of 40 [13].

The three trials side by side

The pattern is familiar from other peptides on this site: a small early trial produces an eye-catching number, and the larger, better-powered follow-up doesn't reproduce it. The 2021 paper was published with copyright held by Promore Pharma, the Swedish company developing LL-37 for wounds [3]. That isn't a criticism; sponsors run most drug trials, and this one published a negative result, which is to its credit.

Cancer

A US cancer centre registered an early trial injecting LL-37 into melanoma tumours to stimulate an immune response; it enrolled four patients [9]. We found no published results.

Why more LL-37 isn't automatically better

Two lines of research point the other way, and both matter to anyone thinking about raising their LL-37 levels.

Psoriasis. In 2007, researchers found that LL-37 binds fragments of the body's own DNA and turns them into a trigger that makes plasmacytoid dendritic cells pour out interferon, a step they proposed drives psoriasis [5]. In 2014 the same lead author reported that two-thirds of patients with moderate-to-severe plaque psoriasis carry T cells that specifically target LL-37, treating it as a foreign invader; those cells infiltrate psoriatic skin, and their levels in blood track disease activity [6]. In psoriasis, LL-37 is an autoimmune target.

Rosacea. People with rosacea have abnormally high levels of cathelicidin in their facial skin, processed into unusual fragments by an overactive skin enzyme. Injecting those fragments into mouse skin caused inflammation, and mice without the cathelicidin gene were protected [7].

Neither finding proves that applying or injecting LL-37 would cause these conditions. They do show the peptide is an active inflammatory signal, not a neutral booster.

Engineered versions

Much current research has moved on from native LL-37 to shorter, modified fragments designed to kill bacteria with fewer side effects. One example, LL-37mini, killed MRSA in mouse wound biofilms and didn't provoke resistance in repeated lab passages [8]. That work is preclinical and tests a different molecule, so it tells you nothing about products sold as LL-37.

Safety and side effects reported

Topical LL-37 was well tolerated in the leg-ulcer trials [2,3], while the diabetic foot trial abstract doesn't report on adverse events [4]. There is no published human safety data for injected LL-37 outside the tiny melanoma study, which has no published results [9]. Given its links to psoriasis and rosacea [6,7], people with those conditions have particular reason for caution, though no study has tested this directly.

Legal and regulatory status

United States. LL-37 isn't an FDA-approved drug. It was nominated for pharmacy compounding and the nomination was later withdrawn, as the FDA's list of substances with significant safety risks records [10]. The FDA's compounding advisory committee is due to consider cathelicidin LL-37 at a meeting before the end of February 2027, according to a law-firm summary [11].

United Kingdom. Not a licensed medicine.

Sport. WADA doesn't name LL-37, but its S0 class prohibits any pharmacological substance with no current approval by a government health regulator for human therapeutic use [12].

What we still don’t know

• Whether LL-37 helps large venous leg ulcers, the one subgroup hinted at in HEAL LL-37, in a trial designed to test it.
• Why the lowest dose worked best in the first trial, and whether that was chance.
• Whether any benefit in diabetic foot ulcers holds in a larger, multicentre trial.
• Anything about injected LL-37 in people: dose, blood levels, safety or effect. Every controlled human trial so far put the peptide on the surface of a wound, which is a very different exposure from an injection that reaches the bloodstream.
• Whether engineered fragments such as LL-37mini will ever reach human trials.
• Whether raising LL-37 could worsen psoriasis, rosacea or other inflammatory conditions in practice.

Educational content only — not medical advice. Many peptides discussed on HPR are not approved for human use. Talk to a qualified clinician before making any decision about your health.

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