LL-37 is sold as an immune peptide, usually to be injected. The research behind the name is real and it is about something else entirely: a cream, rubbed on leg ulcers that will not close. Following that research is worth doing, because it ends somewhere more interesting than either the sellers or the skeptics suggest. It sits under the recovery goal, where healing claims live.
What LL-37 is
LL-37 is not a designed drug. It is a fragment of a protein the human body already makes, cathelicidin, released at skin and mucosal surfaces as part of innate immune defense. It kills bacteria directly, and it also signals: it draws immune cells in, encourages new blood vessels, and pushes skin cells to migrate and divide. Those last properties are why anyone tried it on a wound.
The first trial in people, which looked striking
A first-in-man study published in 2014 enrolled 34 participants with hard-to-heal venous leg ulcers[1]. It opened with a three-week single-blind run-in on placebo, which is a careful design: it weeds out ulcers that were going to improve anyway. Then four weeks of twice-weekly topical LL-37 at 0.5, 1.6 or 3.2 mg/mL, or placebo, and four weeks of follow-up.
The healing rate constants were approximately sixfold and threefold higher than placebo at 0.5 and 1.6 mg/mL (p = 0.003 and p = 0.088). Mean ulcer area fell 68% in the lowest dose group and 50% in the middle one. And at the highest dose, 3.2 mg/mL, there was no difference from placebo at all.
The larger trial, which did not confirm it
HEAL LL-37 was the phase IIb [2]. It was double-blind, randomized and placebo-controlled, across multiple centers, in 148 patients with hard-to-heal venous leg ulcers treated alongside compression therapy. Mean age was 67.6 years, median ulcer duration 20.3 months, mean wound size 11.6 cm² at randomization. It carried forward the two doses that had worked, 0.5 and 1.6 mg/mL, against placebo.
Efficacy analysis on the full study population did not identify any significant improvement in healing with LL-37 compared with placebo. A post hoc analysis found statistically significant improvement in several related healing measures within a subgroup of patients. That sentence is the whole result, and the order of its two halves matters: the trial missed, and the positive finding was located afterwards inside a slice of the data.
A post hoc subgroup is a hypothesis for the next trial, not a result from this one. It is the most common way a negative study gets reported as a positive one, and the paper itself is straightforward about which is which.
A separate randomized double-blind trial published in 2023 tested an LL-37 cream on diabetic foot ulcers with mild infection, applied twice weekly for four weeks, measuring wound area alongside inflammatory markers and bacterial counts [3].
What none of this supports
Every controlled trial above put LL-37 on the surface of a wound. None of them injected it, and none of them studied immunity, infection resistance or general recovery in a healthy person. A registry search compounds the confusion: most records mentioning LL-37 are studies that measure cathelicidin as an outcome of something else, usually vitamin D status, rather than studies of giving it as a drug.
So the position is unusual. LL-37 has better clinical evidence than most of this shelf and none of it is evidence for the product being sold.
Where it stands with the FDA
Neither LL-37 nor cathelicidin appears in the 503A bulk drug substances categories, updated May 14, 2026, or in the 503B categories, updated March 21, 2025. No pharmacy has a lawful route to compound it. For how that framework works, see what makes a peptide legal to prescribe. For the other peptides sold for repair, see BPC-157 and TB-500 and KPV.