# LL-37

A natural defense peptide made by skin and white blood cells. It kills microbes in a dish and steers inflammation. Its one sizable human trial, a gel for leg ulcers, failed.

Also called: Cathelicidin, CAP-18 fragment
Source page: https://peplogue.com/peptides/ll-37/
Reviewed: 2026-10-12. Not medical advice.

## How it works

- Where it acts: It punches holes in microbe membranes and signals to immune cells.
- What it changes: It kills microbes in a dish, and can calm or fuel inflammation depending on the setting.
- What people notice: A gel for leg ulcers failed in a trial. High levels are linked to inflammation in psoriasis and rosacea.

## What the evidence shows

- **Fighting infections**: Lab only. Lab studies only. Injections for infections, mold illness or Lyme disease have not been tested in people.
- **Hard-to-heal leg ulcers (topical)**: Failed. In a 148-patient trial, ulcers closed in about 25% of people on either dose and 25% on placebo.

## Side effects and risks

- unknown: FDA cites lab and animal findings of harm to male fertility and of tumor promotion in some tissues.
- unknown: Risk of immune reactions to injected peptide and its impurities.

Who should avoid it: Injected use. Men trying to conceive should be especially cautious.

## How it has been given in studies

Applied to the wound bed twice a week for 13 weeks. There is no established dose for injection.

## Legal status

- United States: undefined
- European Union: undefined
- Spain: undefined. Research peptide. No authorised medicine in Spain that we found (not individually checked in CIMA). Not lawful to sell to the public.
- Germany: undefined. Not approved in Germany. Selling it as a medicine without approval is a crime (AMG § 21, § 96); sold online only as 'research' products.
- France: undefined. No French authorisation found. Not named by ANSM, but its Jul 2026 alert says peptides bought outside the official circuit are illegal.
- Italy: undefined. No Italian authorisation found. Not lawful to sell to the public as a medicine. Research-grade vials online are unverified products.
- Netherlands: undefined. No Dutch marketing authorisation found. Selling it as a medicine without one is illegal per IGJ, even if labelled 'research use'.
- United Kingdom: undefined
- Canada: undefined
- Australia: undefined

## Questions people ask

**Does LL-37 work for Lyme disease or mold illness?**

Injections for infections, mold illness or Lyme disease have not been tested in people. The evidence comes from lab studies only.

**Has LL-37 been tested in humans?**

Its one sizable human trial, a gel for hard-to-heal leg ulcers in 148 patients, failed: ulcers closed in about 25% of people on either dose and 25% on placebo.

**Is LL-37 safe?**

FDA cites lab and animal findings of harm to male fertility and of tumor promotion in some tissues, plus the risk of immune reactions to injected peptide. Men trying to conceive should be especially cautious.

**Can LL-37 be compounded in the US?**

FDA advisers are due to review it for compounding by February 2027.

## Sources

- EU Clinical Trials Register: LL-37 in venous leg ulcers, results: https://www.clinicaltrialsregister.eu/ctr-search/trial/2018-000536-10/results
- FDA: bulk substances that may present significant safety risks: https://www.fda.gov/drugs/human-drug-compounding/certain-bulk-drug-substances-use-compounding-may-present-significant-safety-risks