LL-37
Also known as: LL-37 · Cathelicidin
The body's main cathelicidin — a broad-spectrum antimicrobial peptide. Strong in vitro, useful in topical research, almost nothing settled about systemic use.
Overview
LL-37 is the active fragment of human cathelicidin, the only cathelicidin our species makes. It's a 37-amino-acid antimicrobial peptide produced by neutrophils and epithelial cells as part of the innate immune response, and it punches holes in bacterial membranes the way detergents punch holes in oily films. The in vitro story is impressive: broad activity against gram-positive and gram-negative bacteria, several fungi, and some viruses, plus a role in wound healing and biofilm disruption. The clinical story is much smaller. Early human trials of synthetic LL-37 analogues exist for chronic wounds and venous leg ulcers; a few topical formulations have made it to small Phase 2 work. Systemic injection in humans is essentially untested at the doses people are using grey-market, and the immune-modulation effects raise as many questions as they answer.[1]
Evidence quality
Extensive in vitro and animal data dating back to the 1990s. The Gudmundsson and Agerberth groups in Sweden have driven much of the foundational work. Human trials exist for topical LL-37 analogues in chronic wounds and venous leg ulcers (early Phase 2). Systemic injectable LL-37 in healthy users has essentially no formal evidence base.
Benefits & timeline
Benefits
- Broad-spectrum antimicrobial activity in vitro — bacteria, fungi, some viruses, including biofilm-forming organisms
- Wound healing benefit in early topical trials for chronic ulcers
- Immune modulation, both anti-inflammatory and pro-inflammatory depending on context — interesting, but not necessarily what you want unsupervised
- Useful research interest as a template for next-generation antimicrobials, where antibiotic resistance is squeezing the conventional pipeline
Timeline
Week 1–2
Subjective improvement in chronic infection symptoms in users who respond; nothing for non-responders.
Week 4
Inflammatory and infection-marker trends are usually clear by now.
Week 6–8
End of typical cycle — assess and decide whether continuation is justified.
Off-cycle
At least 4 weeks off before any repeat course. Continuous dosing may dysregulate immune signalling in ways the literature hasn't characterised.
Dosage protocols
Advanced
500 mcg
three times weekly
Beginner
100 mcg
three times weekly
Standard
300 mcg
three times weekly
Titration & adjustment
Start at 100 mcg subcutaneously three times weekly. After 2 weeks escalate to 300 mcg three times weekly. Maximum 500 mcg three times weekly. Because immune signalling can be perturbed long-term, never cycle longer than 8 weeks and always pause for at least 4 weeks before repeating.
Injection timing
Three times weekly, time of day flexible. Subcutaneous abdomen or thigh, rotate sites because injection-site irritation is the most common side effect.
Side effects & contraindications
- moderateInjection-site irritation and flushing — the most consistent and dose-related side effect.
- moderateChronic dosing may perturb immune signalling, including biasing T-cell responses in directions the user can't monitor without specialist labs.
- moderateLimited long-term human safety data at injectable doses. The endogenous form exists in your body; the exogenous form at supplemental concentrations isn't a comparable exposure.
- mildHistamine release at the injection site in sensitive users — itchy weal, resolves within hours.
Contraindications
- Pregnancy
- Active autoimmune disease — the immune-modulation effects could destabilise an already-misfiring immune system
- Active cancer — LL-37 has been reported to promote tumour growth in some preclinical contexts via angiogenesis and proliferation pathways
- Concurrent biologic immunosuppression — interaction is undefined and the mechanisms overlap
Reconstitution & injection
A 5 mg vial reconstituted with 2 ml bacteriostatic water gives 2.5 mg/ml. A 300 mcg dose is 0.12 ml, which is 12 units on a U-100 insulin syringe. Subcutaneous in the abdomen or thigh, three times weekly. Rotate sites because injection-site flushing is the most common reason people stop. Topical formulations exist for wound and skin protocols at 0.5–1% in a clean carrier — for chronic ulcers, apply once daily under a clean dressing.
Open calculator pre-filledStorage after reconstitution
Refrigerate at 2–8 °C after reconstitution. Do not freeze. Light-protected. LL-37 is a longer cathelicidin peptide and less stable than smaller peptides in solution — realistic fridge window is 14–21 days. Because LL-37 is also injection-site-irritating, users typically run shorter cycles, so a single mix can match a dosing block.
Cost & sourcing red flags
Typical price range: Research-grade 5 mg vials: $60–150 per vial. LL-37 is a 37-amino-acid peptide and one of the more expensive peptides to synthesise at high purity. A 4-week cycle at 300 mcg three times weekly runs $150–400 in raw material. Compounded LL-37 from US pharmacies has effectively disappeared post-2023 FDA restrictions on bulk peptide compounding.
Red flags
- 5 mg vials priced under $40. Synthesising a 37-mer at >95% purity is non-trivial; bargain-priced LL-37 is overwhelmingly truncated sequence, oxidised peptide, or relabelled shorter cathelicidin fragments.
- No mass-spec COA tied to the batch. LL-37 has internal methionine and tryptophan residues that oxidise readily; HPLC-only purity certificates miss the oxidised forms that account for most lost activity.
- Vials marketed for 'oral LL-37' or 'sublingual LL-37'. LL-37 is a cationic amphipathic peptide and is destroyed by gastric and salivary proteases within minutes; oral routes deliver essentially zero active peptide.
- Suppliers promoting LL-37 for Lyme, biofilm disruption, or chronic Bartonella without flagging the well-documented autoimmune-flare risk. LL-37 is the autoantigen in psoriasis and is implicated in lupus pathophysiology; uncontrolled self-use in autoimmune-prone individuals is a real hazard.
- Vials reconstituted in plain bacteriostatic water with no carrier protein or excipient. LL-37 is famously prone to adsorbing to glass and plastic surfaces in dilute solution; without proper formulation, a stated 100 mcg dose may deliver substantially less.
Pricing rots fast and varies by region and supplier. We list no vendors.
Common mistakes
Using it as a general antibiotic substitute.
Better approach: It isn't an antibiotic in the way you'd substitute for amoxicillin. The in vitro activity is real; the in vivo dose-response, tissue penetration, and clinical reliability of injected LL-37 are not characterised. Treat real infections with real antibiotics; use LL-37 as an adjunct for chronic, biofilm-driven contexts where conventional treatment has stalled.
Running it continuously.
Better approach: The immune-modulation profile is biphasic and dose-dependent. Continuous high exposure can shift things in directions you can't see without specialist labs. Cycle 6–8 weeks maximum, with at least 4 weeks off between courses.
Ignoring the cancer caveat.
Better approach: Preclinical data shows LL-37 can promote tumour growth in some contexts via angiogenesis. If there's any active or recent cancer history, the risk-benefit math doesn't work; pick a different tool.
Buying generic 'antimicrobial peptide' powder.
Better approach: Purity matters enormously with a peptide this long. Mis-folded or truncated material may have very different activity than full-length LL-37. Buy from a source with a published certificate of analysis, or don't bother.
Real-world tips
- Rotate injection sites systematically — left abdomen, right abdomen, left thigh, right thigh. Injection-site reactions are the most common cause of dropouts.
- Inject slowly. Faster injection produces more histamine release and a bigger local weal.
- For chronic wound use, the topical formulation has the strongest evidence. Don't reach for the injection if a topical application can reach the target.
- Don't combine with active immunosuppressive courses (steroids, biologics) without medical guidance. The mechanism interplay is undefined.
- Track one objective marker — wound size for a chronic ulcer, infection-marker labs for a chronic infection. Subjective improvement on a peptide like this is too easy to over-interpret.
What users report
Aggregated from r/Peptides and chronic-Lyme forum threads. Thin data well — among the smallest user-report bases of any commonly sold peptide. Not clinical data.
Onset: Subcutaneous 100–300 mcg three times weekly: most users report no acute effect from any single injection. Subjective shifts in chronic-infection symptoms typically land at 4–6 weeks; users who notice nothing by week 6 generally stop.
Common reports
- Localised injection-site burning and redness lasting 30–90 minutes is reported by a clear majority of users, attributed to LL-37's strong cationic charge interacting with local mast cells.
- Mild flu-like symptoms (low-grade fever, body aches) in the first 24 hours after dosing, particularly at higher doses (200+ mcg) or in users with chronic infection load.
- Slow improvements in chronic-infection symptom clusters (post-Lyme fatigue, recurrent UTIs, treatment-resistant sinus infections) at week 4–8, reported alongside concurrent antibiotic protocols rather than as monotherapy.
- Autoimmune-flare reports in users with prior psoriasis, eczema, or unspecified inflammatory skin conditions. This is the most clinically significant adverse-event signal in the user base and frequently misattributed to other variables.
- Effect is poorly differentiated from concurrent antibiotic or anti-inflammatory therapies in most self-reports; users rarely run clean monotherapy and attribution is messy.
Where reports diverge from theory: The mechanistic literature on LL-37 is dominated by in-vitro antimicrobial activity and biofilm-disruption assays. Forum users dosing 100–300 mcg subcutaneously three times weekly are unlikely to reach the systemic plasma concentrations required for direct antimicrobial activity at infection sites; the doses are 2–3 orders of magnitude below the in-vitro effective ranges. Whatever benefit users describe is more plausibly immunomodulatory (dendritic cell signalling, chemokine induction) than direct antibacterial. The persistent marketing of LL-37 as a 'biofilm buster' at these doses outruns the pharmacokinetics.
When something else is the better tool
Conventional antibiotics
Use instead when: Always the first answer for acute or characterised bacterial infections. Susceptibility-guided antibiotic therapy is the gold standard; LL-37 isn't in the same evidence class.
Topical silver or honey dressings
Use instead when: Chronic wound contexts where antimicrobial coverage plus moist wound healing is the goal. Both are cheaper, better-studied, and don't require a peptide pipeline.
Bacteriophage therapy (specialist centres only)
Use instead when: Antibiotic-resistant chronic infection with no remaining conventional option. Phage therapy has both more clinical track record and better targeting than systemic antimicrobial peptides in 2026.
Based on 1 peer-reviewed study
- Is this like an antibiotic?
- Antimicrobial, but not in the conventional small-molecule sense. The activity profile is broad, penetration into infected tissue is variable, and dose-response in humans is poorly characterised. Don't substitute it for an antibiotic course.
- Can I use it for biofilm infections?
- Biofilm disruption is one of the more interesting LL-37 properties in vitro. Some chronic-infection clinics are exploring it as an adjunct. As a self-administered protocol without supervision, the math doesn't add up — get the diagnosis confirmed and the protocol overseen.
- What's the cancer concern?
- Preclinical studies show LL-37 can promote angiogenesis and proliferation in some tumour models. Whether systemic exogenous dosing recapitulates that in humans is unknown. Don't use it if there's active or recent cancer.
- Why do I get a weal at the injection site?
- Histamine release. LL-37 is a known mast-cell activator. Inject slowly and rotate sites. If weals are large and persistent, stop and reassess.
- Topical or injectable?
- Topical has the better evidence base for skin and wound use and a much lower side-effect ceiling. Injectable is what people use for systemic infection contexts, where evidence is much thinner.
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