Thymosin alpha-1 vs LL-37 compares two immune peptides that are often grouped together online but work in very different ways. The short answer: thymosin alpha-1 (thymalfasin) is an immune-modulating thymus peptide approved in more than 30 countries according to its maker, with large human trials, while LL-37 is an antimicrobial peptide tested only in small topical wound trials.[3][11] Neither is FDA-approved, and the largest recent thymosin alpha-1 trial, in 1,106 sepsis patients, found no survival benefit.[8]
Thymosin alpha-1 vs LL-37 at a glance
| Thymosin alpha-1 | LL-37 | |
|---|---|---|
| Size | 28 amino acids[4] | 37 amino acids[10] |
| Where it comes from | Thymus; first isolated in 1977[4] | Skin, gut, airways and white blood cells[10] |
| Main job | Activates dendritic cells and T cells[5] | Kills microbes, recruits immune cells, helps wounds close[10] |
| Approvals | Zadaxin abroad, mainly hepatitis B; not in the US[1] | None anywhere |
| Largest trial | 1,106-patient sepsis trial, no mortality benefit[8] | 148-patient leg ulcer trial, missed main goal[12] |
| Half-life | Under 3 hours after injection under the skin[21] | Not established for injections |
| US compounding | Advisers voted 17 to 4 against (2024)[2] | Advisory review planned by February 2027[18] |
How each one works
Thymosin alpha-1 was isolated from calf thymus tissue by Allan Goldstein’s team in 1977.[4] It acts as a conductor of the adaptive immune system, switching on dendritic cells through toll-like receptors and supporting T-cell responses.[5] Despite the name, it is unrelated to thymosin beta-4 or TB-500.
LL-37 is the only cathelicidin humans make, cut from a larger protein called hCAP18. It is part of innate immunity, the fast first line of defense: it punches holes in bacteria, neutralizes bacterial toxins and calls immune cells to wounds.[10] Active vitamin D directly switches on its gene.[22]
Put simply, thymosin alpha-1 tunes the immune response, while LL-37 is a frontline weapon.
Evidence quality
Thymosin alpha-1
Thymosin alpha-1 has decades of randomized trials, with mixed results:
- Hepatitis B: a Taiwan trial reported a 40.6% complete response at 18 months versus 9.4% in controls,[6] but a US phase 3 trial (14% vs 4%) did not reach significance.[7]
- Sepsis: the 2025 TESTS trial in China, 1,106 patients, found 28-day mortality of 23.4% with thymosin alpha-1 and 24.1% with placebo, no meaningful difference.[8]
- COVID-19: a 2023 meta-analysis of more than 5,000 patients found no effect on mortality.[9]
LL-37
LL-37’s human evidence is limited to topical wound treatment:
- Venous leg ulcers, 2014: in 34 patients, the lower doses sped healing compared with placebo, but the highest dose did no better than placebo.[11]
- Venous leg ulcers, 2021: a larger 148-patient phase 2b trial found no significant overall benefit; the report is copyrighted by the company developing it.[12]
- Diabetic foot ulcers, 2023: LL-37 cream increased healing tissue but did not reduce bacteria or inflammation markers more than placebo.[13]
We found no published human trial of injected LL-37 for immunity, infection or wellness.
Side effects and safety
Thymosin alpha-1 is generally well tolerated. FDA’s review lists injection-site discomfort as the most common effect, with rare rash, redness, joint pain and temporary liver enzyme rises. It advises against use in pregnancy, in children and in transplant recipients on deliberate immune suppression, and urges caution in autoimmune disease.[1]
LL-37 has a double edge:
- Inflammation: abnormally high levels of cathelicidin fragments drive skin inflammation in rosacea,[14] and about two-thirds of people with moderate-to-severe psoriasis had T cells that target LL-37.[15]
- Cancer: lab and animal studies suggest LL-37 may promote ovarian, lung and breast cancers while suppressing colon and gastric cancers.[16]
- FDA concerns: compounded LL-37 may trigger immune reactions and have impurity problems, and nonclinical data suggest harm to male reproduction and tumor promotion in some tissues.[17]
Legal and regulatory status in 2026
- Thymosin alpha-1: sold as Zadaxin in China, India, Italy and other countries, though FDA could not independently verify every approval.[1][3] In the US, FDA reviewed it for 13 possible uses and advised against compounding; advisers agreed 17 to 4 in December 2024.[1][2] It is not on FDA’s May 2026 list of nominated substances.[19]
- LL-37: FDA lists it among former category 2 substances whose nominations were withdrawn and still describes its safety concerns.[17] FDA says its compounding advisory committee will discuss LL-37 before the end of February 2027.[18]
- Sport: neither is named on WADA’s 2026 list. Because thymosin alpha-1 is approved abroad, the non-approved category likely does not apply; LL-37, with no approvals, likely falls under S0.[20]
Why they get marketed together
Online, both peptides are often sold as “immune” or “longevity” peptides and sometimes bundled in protocols for chronic fatigue, mold illness or frequent infections. That pairing is a marketing idea, not a research finding. We found no study testing them together, and their mechanisms point in different directions: thymosin alpha-1 was developed to strengthen adaptive immune responses,[5] while LL-37 is a frontline antimicrobial that can also fuel inflammation in some skin diseases.[14] Before combining immune-active products, it matters what problem is actually being treated.
Who might consider which? Questions for a clinician
- What specific condition am I trying to treat, and is there an approved therapy with better evidence?
- Do I have an autoimmune disease, rosacea, psoriasis or a history of cancer?
- Am I taking immune-suppressing medicines or planning a transplant or pregnancy?
- Would checking and correcting my vitamin D level be a reasonable first step?
The verdict
Thymosin alpha-1 is the better-studied peptide by a wide margin, with approvals outside the US and a long tolerability record, but its large modern trials have mostly failed to show hard benefits. LL-37 is a fascinating part of natural immunity with only small topical trials, and raising it carries known risks for inflammatory skin disease and possibly some cancers. Neither is approved in the US. Related reading: LL-37 vs defensins vs magainin, epitalon vs TA-65, peptides vs exosomes and the longevity peptides hub.
This is general information, not medical advice.
