LL-37 Peptide: Dosage, Protocols and What the Evidence Actually Shows
LL-37 is the human body's only cathelicidin, a 37-amino-acid antimicrobial peptide released from neutrophils and epithelial cells. It is sold as a research peptide for immune support, chronic infection, gut problems and skin conditions. The biology is genuinely interesting. The human evidence for injecting it is thin, and the dosing figures circulating online have no trial behind them. This guide keeps those two things separate: what LL-37 does in the body, what has actually been tested in people, what the protocols in circulation look like, and where it stands with FDA as of 2026. For the wider family it belongs to, see our antimicrobial peptides guide.
What LL-37 Is
Neutrophils store a precursor protein called hCAP-18 in their granules. When the cell degranulates, the enzyme proteinase 3 cuts the C-terminal 37 residues free, and that fragment is LL-37. The name is literal: two leucines, 37 amino acids. Once released it folds into an amphipathic helix, meaning one face carries positive charge and the other is hydrophobic. That structure is the whole trick behind how it works.
Epithelial cells in skin, gut and airway produce it as well, along with several other immune cell types. It shows up in sweat, saliva, airway surface liquid and wound fluid. Unlike most compounds sold as research peptides, LL-37 is something the body already makes continuously at barrier surfaces, which is part of why the safety intuition around it runs more optimistic than the evidence justifies.
How LL-37 Works
The antimicrobial mechanism is physical. The positively charged face of the helix is drawn to the negatively charged surface of a bacterial membrane, the hydrophobic face inserts into the lipid bilayer, and the membrane loses integrity. Because the target is the membrane itself rather than an enzyme or a ribosome, resistance is harder to evolve than against conventional antibiotics, though bacteria can still defend themselves by modifying surface charge.
The immune signalling is the more consequential half. LL-37 binds the formyl peptide receptor FPR2 and recruits neutrophils, monocytes and T cells to sites of injury. It binds and neutralizes bacterial lipopolysaccharide, blunting endotoxin-driven inflammation. It promotes angiogenesis and keratinocyte migration, which is the wound-healing part of the story. And it can run the other way, amplifying inflammation in the wrong context, which is covered below under safety.
What Has Actually Been Tested in Humans
The strongest human data is topical. A controlled trial applied LL-37 directly to hard-to-heal venous leg ulcers across several concentrations and reported improved healing at the lower doses tested, with the highest concentration performing no better than control. That result is worth sitting with, because it cuts against the reflex that a bigger dose of an antimicrobial peptide is a stronger dose. At high local concentrations LL-37 is cytotoxic to host cells as well as to bacteria.
Beyond wound care the human literature thins out fast. There is a large body of observational work linking LL-37 levels to infection susceptibility, vitamin D status, and disease activity in conditions like psoriasis and cystic fibrosis. That is association, not evidence that giving exogenous LL-37 helps. Controlled trials of injected systemic LL-37 for immune support, chronic infection, or biofilm conditions do not exist in a form that would let anyone state a dose with confidence.
The honest summary: strong mechanism, a real topical wound-healing signal, no established systemic protocol. Anyone selling certainty about injected LL-37 is selling something the literature does not contain.
LL-37 Dosage: What Circulates and Why It Is Weakly Grounded
The figures below are what appear in vendor material and clinic protocols. They are not derived from dose-ranging studies, and the fact that they cluster so tightly reflects copying rather than convergence on evidence.
| Protocol | Circulating figure | Route | Duration | Evidence behind it |
|---|---|---|---|---|
| Conservative | 50–100 mcg/day | Subcutaneous | 2–4 weeks | None published |
| Most-cited | 100 mcg/day | Subcutaneous | 2–4 weeks | None published |
| Upper figure quoted | 200–300 mcg/day | Subcutaneous | Short courses | None published; more local irritation reported |
| Topical / wound | Lower concentrations outperformed higher | Applied to wound | Per study protocol | Controlled human trial in venous leg ulcers |
| Nebulized | Highly variable | Inhaled | Undefined | None; delivered dose is device-dependent |
Two things stand out. Every systemic row carries the same entry in the evidence column. And the one row with a real trial behind it points the opposite direction from the usual dose-escalation instinct. If you are going to use LL-37 anyway, the defensible reading of what exists is to stay at the low end and keep the course short, which is the opposite of what people usually do when a peptide seems to be doing nothing.
Reconstitution and Dose Math
LL-37 arrives lyophilized, most often as 5 mg. Two workable dilutions. Five millilitres of bacteriostatic water gives 1 mg/mL, where 0.1 mL (10 units on a U-100 insulin syringe) is 100 mcg and 0.05 mL is 50 mcg. Or 2.5 mL gives 2 mg/mL, where 0.05 mL is 100 mcg. The 1 mg/mL version uses more diluent but makes the units-to-mcg conversion trivial, which matters more than it sounds when you are drawing volumes this small.
Run the diluent slowly down the inside wall of the vial rather than squirting it onto the powder cake, swirl instead of shaking, and refrigerate once reconstituted. LL-37 is a relatively long peptide and is not especially stable in solution, so a short in-use window and consistent cold storage matter more here than with smaller, hardier compounds. The rules in our peptide storage guide and the diluent choices in our bacteriostatic water guide both apply directly. Technique is unchanged from any other small-volume subcutaneous peptide, covered in how to inject peptides.
Side Effects and the Case for Caution
Reported effects are mostly local: stinging on injection, a red raised weal, itching, occasional flushing and transient malaise. The local reaction has a plausible mechanism, since LL-37 triggers mast cell degranulation, so histamine release at the site is expected rather than aberrant.
The more serious concern is conceptual. LL-37 is not a one-way protective molecule. In psoriasis it is markedly elevated in lesional skin, and complexes of LL-37 with self-DNA activate plasmacytoid dendritic cells through TLR9, driving the interferon response that sustains the plaques. In rosacea, abnormal cathelicidin processing is part of the disease mechanism rather than incidental to it. That makes psoriasis, rosacea and lupus reasonable contraindications on mechanistic grounds, and it means the framing of LL-37 as a broadly safe immune booster is not supported by its own biology. Our peptides and autoimmune conditions guide covers the general version of this problem.
The Vitamin D Route to Raising LL-37
The gene that encodes LL-37 carries a vitamin D response element, so the active hormone form of vitamin D directly drives its transcription. This is one of the cleaner findings in the field, and it explains a chunk of the observed relationship between vitamin D deficiency and infection susceptibility.
The practical implication is unglamorous but real. Correcting a documented deficiency raises endogenous LL-37 through the pathway the body already uses, at a known dose, with a known safety profile, and no injection. For anyone whose interest in LL-37 is general immune resilience rather than a specific refractory problem, that is the better-evidenced starting point. Our peptides for immune support guide covers the rest of that category.
LL-37 Compared With KPV and Thymosin Alpha-1
These three get grouped as immune peptides and they are not close substitutes. KPV is a tripeptide anti-inflammatory derived from the C-terminus of alpha-MSH, not from LL-37, despite the two frequently being described as related in vendor copy. It calms inflammation and does not meaningfully kill bacteria. Thymosin alpha-1 modulates T-cell function and carries the strongest clinical record of the group, having been used as an approved drug in a number of countries for decades.
LL-37 sits apart as the only one with direct antimicrobial action, which is precisely why the case for it rests on mechanism rather than outcomes. If the objective is documented immune modulation, thymosin alpha-1 is better supported. If it is inflammation, KPV has the cleaner profile. LL-37 is the choice you make when the mechanism is what you are after and you accept that the human dosing evidence is not there. See also peptides for inflammation.
Legal and FDA Compounding Status in 2026
LL-37 is not an FDA-approved drug in the United States. On the compounding side, FDA identified cathelicidin LL-37 for removal from the Category 2 list, the bucket for substances presenting significant safety risk, and for referral to the Pharmacy Compounding Advisory Committee. It was not among the seven peptides taken up at the July 23–24, 2026 PCAC meeting, where the committee recommended six of seven for the 503A bulks list. FDA has indicated LL-37 will be considered at a meeting before the end of February 2027. Our PCAC vote breakdown covers how that process actually works, and the 14-peptide Category 1 reclassification covers the parallel track.
What that means in practice: a PCAC recommendation is not authorization. FDA still has to propose adding a substance to the 503A bulks list through notice-and-comment rulemaking before a compounding pharmacy can legally dispense it from bulk. Until then, LL-37 sold online is research-grade material labeled not for human use, and the buyer carries the identity and purity risk. See are peptides legal and buying peptides online safely for the practical version of that problem.
Who Has the Clearest Reason to Skip It
Anyone with psoriasis, rosacea or lupus, on the interferon and cathelicidin mechanisms described above. Anyone with an active autoimmune condition, where adding a molecule that amplifies innate immune signalling is a coin flip in the wrong direction. Pregnancy and breastfeeding, where there is nothing resembling safety data. And anyone treating a diagnosable infection, where the relevant comparison is not LL-37 versus nothing but LL-37 versus an antibiotic with a known spectrum and a known dose.
The general signals to watch for with any research peptide are in our peptide side effects guide, and the background on what research-grade actually means is in research peptides.
Bottom Line
LL-37 has better mechanistic credentials than most peptides in this category and worse dosing evidence than almost any of them. The one solid human trial is topical and points toward lower concentrations rather than higher. Every systemic figure in circulation, including the near-universal 100 mcg per day, traces back to practice rather than to a study. The vitamin D pathway is a real and cheap way to raise your own cathelicidin. And the regulatory picture gets clearer at the PCAC meeting scheduled before the end of February 2027, which for anyone not in a hurry is a reasonable thing to wait for.
Frequently Asked Questions About LL-37
LL-37 is the only cathelicidin antimicrobial peptide humans make. It is a 37-amino-acid chain, named for the two leucines at its start, released when the enzyme proteinase 3 cleaves the precursor protein hCAP-18 stored in neutrophil granules. It kills bacteria by disrupting their membranes rather than by hitting a single molecular target, and it also signals to immune cells, neutralizes bacterial endotoxin, and participates in wound repair.
There is no validated systemic dose. No regulator has approved LL-37, and no published human trial has established a safe and effective injected dose for the uses people buy it for. The figures circulating in research and clinic protocols are roughly 100 mcg per day subcutaneously, sometimes quoted up to 300 mcg, for short runs of two to four weeks. Those numbers come from practice rather than from dose-finding studies, and this guide treats them as what people do, not as what the evidence supports.
Some, and it is narrower than the marketing suggests. The most substantive human work is topical, not injected: a controlled trial in hard-to-heal venous leg ulcers tested LL-37 applied to the wound and reported better healing at the lower concentrations tested, with no benefit at the highest. That is a useful reminder that more peptide is not automatically better. Almost everything else is cell-culture and animal work. Systemic injected LL-37 for immune support or chronic infection has not been established in controlled human trials.
It is not FDA approved. On the compounding side, FDA has identified cathelicidin LL-37 for removal from the Category 2 list and for consultation with the Pharmacy Compounding Advisory Committee. It was not one of the seven peptides taken up at the July 2026 PCAC meeting; FDA has indicated the review will happen at a meeting before the end of February 2027. Until a substance is actually added to the 503A bulks list through rulemaking, pharmacy compounding from bulk is not permitted, and material sold today is research-grade, labeled not for human use.
It ships as a lyophilized powder, usually in 5 mg vials. Adding 5 mL of bacteriostatic water gives 1 mg/mL, at which 0.1 mL on a U-100 insulin syringe (10 units) is 100 mcg and 0.05 mL is 50 mcg. Adding 2.5 mL instead gives 2 mg/mL, where 0.05 mL is 100 mcg. Add the diluent slowly against the glass, swirl rather than shake, and refrigerate afterward.
The commonly reported ones are local: stinging, redness, and swelling at the injection site, consistent with the peptide degranulating mast cells. Flushing and transient flu-like malaise are also described. The theoretical concerns matter more than the anecdotal ones. LL-37 is not purely protective. It is elevated in psoriasis and rosacea, and in psoriasis it forms complexes with self-DNA that drive an interferon response. Anyone with an inflammatory skin condition or an autoimmune disease has a mechanistic reason for caution rather than reassurance.
KPV is a three-amino-acid fragment, lysine-proline-valine, that comes from the C-terminus of alpha-MSH rather than from LL-37, despite the two being grouped together in a lot of peptide marketing. KPV is an anti-inflammatory signal with essentially no direct antimicrobial action; LL-37 is a full-length membrane-disrupting antimicrobial that also modulates immunity. For calming inflammation they are not interchangeable, and KPV has the cleaner safety story.
People do it, aiming at sinus and lower-airway targets, and delivering an antimicrobial peptide directly to the mucosa is not an unreasonable idea on its face. But nebulized LL-37 has no established human protocol, the delivered dose depends heavily on the device, and inhaled peptides can provoke bronchospasm. This is a route with more enthusiasm behind it than data.
Yes, and this is one of the better-supported pieces of LL-37 biology. The gene encoding LL-37 carries a vitamin D response element, so the active hormone form of vitamin D drives its expression. That is part of why vitamin D status tracks with innate immune competence. Correcting a documented deficiency is a plausible way to support endogenous LL-37 and carries none of the unknowns of injecting the peptide.
Thymosin alpha-1 has by far the strongest human record of the peptides marketed for immune support, having been used as an approved drug in a number of countries for decades. LL-37 has more mechanistic appeal and less clinical evidence behind exogenous administration. Choosing on the strength of the data rather than on novelty points to thymosin alpha-1.