Page last checked
Human cathelicidin antimicrobial peptide (research compound) · safety
LL-37 side effects
The ll-37 side effects reported most often are anemia, actinic keratosis and skin hypopigmentation. The full list is below, with the cited trial report cited at the bottom of the page.
Rarer, but worth being able to spot: squamous cell carcinoma and crops of wart-like and blistering skin lesions away from the injected sites.
The sources describe these effects in relation to the dose and to the early weeks.
LL-37 is a peptide 37 amino acids long that your own white blood cells make. It kills bacteria by breaking open their outer membrane. It is sold for stubborn infections, for biofilms, and for gut complaints.
It has reached people twice in registered work: injected into melanoma deposits, and applied to venous leg ulcers. Neither study tested an infection. On the ulcers the two lower strengths improved healing while the highest did no better than placebo. The FDA cited animal findings of harm to male reproduction, and of tumor-promoting activity in some tissues.
At a glance
Common reactions listed
7
Serious reactions listed
2
Boxed warning
No label exists
Strongest source
limited human data
Common reactions
The full list, as reported across the available trial evidence. There is no approved label to read frequency bands off, so these are the trials’ own figures.
| Reaction | Frequency |
|---|---|
| Anemia | 2 of 3 treated participants (phase 1/2, n=3) |
| Actinic keratosis | 1 of 3 treated participants (phase 1/2, n=3) |
| Skin hypopigmentation | 1 of 3 treated participants (phase 1/2, n=3) |
| Hypothyroidism | 1 of 3 treated participants (phase 1/2, n=3) |
| Decreased lymphocyte count | 1 of 3 treated participants (phase 1/2, n=3) |
| Decreased white blood cell count | 1 of 3 treated participants (phase 1/2, n=3) |
| Increased blood bilirubin | 1 of 3 treated participants (phase 1/2, n=3) |
Serious reactions to watch for
These are rarer, and they are the ones clinicians monitor for. Seek medical care if you have symptoms.
| Reaction | Frequency |
|---|---|
| Squamous cell carcinoma | 1 of 3 treated participants (phase 1/2, n=3) |
| Crops of wart-like and blistering skin lesions away from the injected sites | One patient, about 45 days after starting weekly injections — 12 lesions biopsied, 11 sharing one pattern, all resolved within 2 months of stopping |
When to seek medical care
The trial report does not give a separate list of serious reactions. There is also no approved label. That gap does not mean serious reactions cannot happen. This compound has been studied in fewer people and for less time than an approved drug.
1Read deeper
When these effects tend to occur
The cited evidence describes these effects in relation to the dose and to the early period of taking it, rather than as a constant background throughout.
When a study reports how side effects changed over time, it describes the group as a whole. It cannot predict one person’s experience.
Common questions
How long do ll-37 side effects last, and do they go away?
The sources do not commit to a set number of days. What they describe is a pattern — reactions cluster after you start and after each increase, then ease while the dose is held steady. Any page quoting an exact number of days is quoting something the sources do not say.
What are the most common ll-37 side effects?
Anemia, actinic keratosis, skin hypopigmentation and hypothyroidism — the table above has the rest, each with its frequency band and source.
Does ll-37 have a boxed warning?
No, because a boxed warning is part of an FDA-approved label and ll-37 has no approved label. That reflects what regulatory review has and has not covered, not a finding that serious risks were looked for and ruled out.
Can the ll-37 dose be lowered if side effects are hard to tolerate?
That decision belongs to the prescribing clinician. No approved schedule exists for this compound, so there is no labeled framework for adjusting a dose. That leaves the prescriber as the only place the decision can sit.
Does ll-37 cause hair loss?
Hair loss is not among the reactions listed on this page for ll-37, because the cited trial report cited here does not carry it. Anything missing from the tables is outside what can be sourced here rather than ruled out, and a symptom that worries you belongs with a clinician.
Is fatigue a listed ll-37 side effect?
No — the cited trial report cited here does not list it. That describes what was reported, not proof that the symptom cannot happen.
Is constipation a listed ll-37 side effect?
Not separately. If a source groups stomach and gut events together, this page keeps that group intact. It does not invent named reactions the source never listed.
What should someone do about a serious ll-37 reaction?
Contact a clinician promptly, and emergency services if symptoms are severe or getting worse quickly. The section above describes what those reactions actually look like, so they can be spotted early rather than explained away.
Do the frequencies on this page mean the chance it happens to one person?
No. These figures describe groups in a study. A band shows how often a reaction appeared under that trial’s conditions. It cannot predict what will happen to one person.
Related
The schedule these reactions were recorded against, the comparisons, and the sourcing rules behind every grade on this page.
Questions readers ask
What are the benefits of the LL-37 peptide?
How long does it take for LL-37 to work?
- Induction of Antitumor Response in Melanoma Patients Using the Antimicrobial Peptide LL37 (NCT02225366) — MD Anderson Cancer Center, phase 1/2, completed, results postedretrieved 2026-08-12Human trial
- Dermatologic toxicity from novel therapy using antimicrobial peptide LL-37 in melanoma: a detailed examination of the clinicopathologic features (Journal of Cutaneous Pathology, 2018)retrieved 2026-08-12limited human data