One of these four has been studied more thoroughly than almost anything else sold as a peptide. That makes it the odd one out in a category better known for having no evidence at all.
Thymosin alpha-1 has been given to more than 600,000 patients. Over thirty clinical trials have enrolled more than 11,000 people between them. It is an approved prescription medicine in roughly 35 countries.[^4]
It has never been approved in the United States, its largest trial was negative, and in 2023 the FDA restricted it from compounding pharmacies.[^5]
The other three are the familiar picture. One has a little human evidence, one has none, and one comes from the same Russian research tradition our longevity guide covers.
The four compounds, sorted by evidence
| Peptide | What it is | Human evidence | Status |
|---|---|---|---|
| Thymosin alpha-1 | 28 amino acids, copies a thymus hormone | Phase 3 trials, 11,000+ participants | Approved in ~35 countries, never in the US |
| LL-37 | Your own antimicrobial peptide | Early clinical, mostly topical wound work | Not approved. Before the FDA panel by February 2027 |
| KPV | Three amino acids, from the end of alpha-MSH | Animal studies only | Not approved anywhere |
| Thymalin | A thymus extract, not a single peptide | Russian clinical studies, unblinded | Not approved outside Russia |
Why people look into immune peptides
Almost nobody arrives at immune peptides out of curiosity. They arrive because something has been happening.
Four colds a winter when everyone else gets one. A cough that never quite clears. Two years of feeling flattened since an illness that was supposed to be brief. Or a run of infections that has started to feel like a pattern rather than bad luck.
That is a reasonable thing to want solved, and the standard advice to eat well and sleep more lands badly when you have been doing both for a year.
Recurrent infection is itself a diagnostic finding. It is one of the things that prompts a doctor to look for an underlying cause. That list is long, mostly treatable, and includes several things with nothing to do with immunity at all. Iron, thyroid, sleep apnea, diabetes, and a set of genuine immune conditions that have names and treatments.
Somebody with that history buying an unapproved compound has skipped the part where a person with a stethoscope works out why. The argument is not about being careful. It is that an answer may already exist and be findable.
Is thymosin alpha-1 FDA approved?
Your thymus makes a hormone called thymosin alpha-1, which helps T cells mature and coordinates part of the immune response. The synthetic version copies it exactly, goes by thymalfasin, and sells under the brand name Zadaxin.
The scale is unusual for a compound most people have never heard of. Approved in around 35 countries for hepatitis B, hepatitis C and immunodeficiency states. Widely used in China, where most of the global sales are. Post-marketing surveillance covering more than 600,000 treated patients, with a safety record described as excellent across all of them.[^4]
So the file here is not thin. It is one of the largest in this whole subject.
The largest trial was negative
Earlier work in sepsis looked promising. A 2013 trial across six Chinese teaching hospitals reported 28 day mortality of 26.0% on thymosin alpha-1 against 35.0% on control, which is a large difference. It was single-blind, and its primary analysis reached significance only marginally.[^3]
Then came TESTS, published in the BMJ in January 2025. Twenty-two centers in China, 1,106 adults with sepsis, randomized one to one, double-blinded, placebo-controlled, injections every twelve hours for seven days.[^1]
28 day mortality was 23.4% on thymosin alpha-1 and 24.1% on placebo. The difference was not statistically significant, and the authors concluded there was no clear evidence that it reduces mortality in adults with sepsis.
No larger randomized trial of this compound has been run, and it is properly designed in every way the earlier one was not.
A possible benefit appeared in a diabetic subgroup, with a significant test for interaction. Subgroup findings from a negative trial are a hypothesis for the next study, not a result to act on. Our guide on why most peptides have no human evidence covers how findings like that get quoted afterwards.
And the meta-analyses show the same thing twice
This part is worth following, because it is the clearest example on this site of how evidence quality changes an answer.
Pooling all the randomized trials of thymosin alpha-1 in sepsis produces a benefit. One 2025 analysis of eleven trials found an odds ratio of 0.73 for 28 day mortality, which sounds convincing.[^2]
Then the same authors split the trials by quality. Among the high-quality trials alone, the benefit disappeared. Among the multicenter trials alone, it disappeared again.
An effect that vanishes when you restrict to better-conducted studies is the signature of small-study bias, and it is what the TESTS result looks like from the other direction. Small trials found something. Larger and better-conducted ones failed to confirm it.
What that evidence is actually for
The bulk of the evidence is in chronic hepatitis B and C, with a smaller body in cancer as an adjuvant and in immune deficiency states. Those three indications are what the approvals cover.
Treatment for hepatitis C has changed completely since most of that evidence was generated. Modern direct-acting antivirals cure most people in a matter of weeks. So one of the conditions where thymosin alpha-1 has its strongest evidence now has far better options. The old evidence has not become wrong, and it is worth less than it was.
None of the approvals is for general immune support, which is what the compound is sold for outside those countries.
And it is still not approved in the US
Thirty-five countries, 11,000 trial participants, 600,000 patients, and no FDA approval.
That combination confuses people and there is no single dramatic reason for it. Approval requires a sponsor to file, run trials to FDA specification, and pay for the process. Most of the evidence base was generated in Asia, much of it for hepatitis indications where US treatment moved on to antivirals that work extremely well. Nobody has yet had a commercial reason to fund the US approval program it would take.
Then in 2023 the FDA placed it in the category of peptides restricted from compounding.[^5] That decision is contested, given the safety record, and it sits in the same regulatory story our coverage of the July 2026 vote and the February 2027 panel describes.
So the summary is an awkward one. Real medicine with real evidence, genuinely useful in some settings, unavailable through any legal US route, and with its biggest trial pointing the wrong way.
What is LL-37?
LL-37 is a peptide your own body makes. It is a cathelicidin, part of innate immunity, and it does two things at once: kills bacteria directly and signals to other immune cells.
The human evidence is early and mostly topical, in wound infection and biofilm work. Nothing like the thymosin alpha-1 file.[^6]
It is one of five compounds going before the FDA compounding panel by the end of February 2027. Our preview of that meeting covers the evidence and the safety record in detail, including the animal findings that make it a harder case than the July slate.
Does KPV have any human evidence?
KPV is three amino acids, lysine-proline-valine, taken from the tail end of alpha-MSH. That parent hormone is the one melanotan II mimics, and KPV takes none of the pigmentation effect with it.
The interest is that it appears to damp inflammation without broadly suppressing the immune system, which would be a useful combination if it held up in people.
No completed human trial exists. The work is cell and animal studies, mostly in colitis and skin inflammation models.[^6]
It also gets sold in oral form on the reasoning that the animal gut work used oral dosing. Whether an oral tripeptide survives human digestion intact enough to matter has not been established, and our guide on what peptides are covers why swallowing them is usually the problem.
What is thymalin?
Thymalin is a thymus extract and not a single peptide, developed by the same Russian research tradition that produced epitalon.[^6]
If you have read our longevity guide, the issue will be familiar. The human studies are Russian, largely unblinded, and concentrated in one research lineage. The substance is an extract, not a defined molecule, which makes reproducibility harder even for people trying in good faith.
That makes three families where the evidence belongs to a preparation somewhat different from the thing on sale. Three instances is where it stops looking like coincidence.
Is thymosin beta-4 the same as thymosin alpha-1?
It comes up here because of the name, and it belongs on a different page.
Thymosin beta-4 and thymosin alpha-1 are both named for the thymus and they do different things. Alpha-1 works on immune signalling. Beta-4 acts on cell migration and tissue repair, which is why it sits in the healing category rather than this one.
TB-500, the thing actually sold, is a seven amino acid fragment of thymosin beta-4 and not the protein itself. Our guide on the healing pair covers that distinction, which is one of the clearest examples of the pattern described above.
How each one is taken
Route matters more in this family than in most, and the four differ completely.
Thymosin alpha-1 is injected subcutaneously, twice weekly in the approved schedule, with a half-life of roughly two hours and effects that outlast it.
LL-37's human work is largely topical, applied to wounds rather than injected.
Thymalin is given intramuscularly in the Russian studies.
KPV is the interesting one. It is sold orally, and the reasoning is that the animal colitis work used oral dosing. Whether a three amino acid peptide survives human digestion in a useful state is the whole question, and nobody has answered it. Our guide on what peptides are covers why swallowing them is normally the problem, and a very short peptide is the case where the usual answer is least certain.
What all four have in common
They act on the immune system, which is a different proposition from acting on a muscle or a receptor in the skin.
That is worth pausing on. An immune system that is too quiet leaves you vulnerable to infection. One that is too active attacks you. Anything that shifts the balance can shift it the wrong way, and the compounds here that have been studied properly are studied in people who have a specific problem, not in healthy people looking for an edge.
Nobody has tested any of them in a healthy person for general immune support, which is what they are mostly sold for.
And if you have an autoimmune condition, are on immunosuppressants, or have had a transplant, this is the compound family where the interaction question is least theoretical. That is a conversation with your specialist before anything else, and our guide on telling your doctor covers how to have it.
Where this stops being useful
Whether thymosin alpha-1 would help you, which depends on having a condition it has been tested for.
What the FDA compounding panel will decide about LL-37, which is scheduled and not yet held.
Whether any of these does anything for general immunity in a healthy person. No trial has asked.
Common questions
Is thymosin alpha-1 FDA approved?
No. It is approved in roughly 35 countries for hepatitis B, hepatitis C and immunodeficiency, and the FDA has never approved it. In 2023 it was also restricted from US compounding pharmacies.
Does thymosin alpha-1 work?
For some conditions there is real evidence, particularly in hepatitis. In sepsis, the largest trial ever run, with 1,106 patients, found no mortality benefit.[^1] For general immune support in healthy people, nobody has tested it.
Why is it approved in other countries but not the US?
Nobody has run the US approval program. The evidence was mostly generated in Asia for hepatitis indications where American practice moved to antivirals, and approval requires a sponsor willing to fund it.
Is KPV safe?
No completed human trial exists, so nobody can answer that from evidence. The work is animal and cell studies.
Does oral KPV work?
Unknown. The animal gut studies used oral dosing, and whether a tripeptide survives human digestion intact enough to matter has not been established.
What is LL-37 used for?
Its human work is mostly topical, in wound infection and biofilm. It goes before the FDA compounding panel by the end of February 2027.
Can I take these for general immune support?
That is what they are mostly sold for and it is the thing nobody has tested. Every properly conducted trial in this group enrolled people with a specific diagnosed problem.
I keep getting sick. Should I try these?
That history is a reason to get investigated, not to self-treat. Recurrent infection is a diagnostic finding, and the causes include iron deficiency, thyroid problems, sleep apnea and diabetes, most of them treatable.
Is thymosin beta-4 the same thing?
No. Both are named for the thymus and they do different things. Beta-4 acts on tissue repair and belongs with the healing compounds. TB-500, the thing sold, is a fragment of it rather than the protein.
What is thymosin alpha-1 actually approved for?
Chronic hepatitis B and C, and immunodeficiency states, in the countries that approve it. Not general immune support, which is what it is sold for elsewhere.
I have an autoimmune condition. Can I take these?
Of everything covered here, that question matters most with these, because shifting immune activity is the whole point of them. Ask your specialist first.
Sources
[^1]: Wu J, Pei F, Zhou L, et al. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ 2025;388:e082583. doi:10.1136/bmj-2024-082583. PMID 39814420. Peer-reviewed phase 3 randomized controlled trial, abstract and results read at source. Twenty-two centers in China, September 2016 to December 2020. 1,106 adults aged 18 to 85 with sepsis by Sepsis-3 criteria, randomized 1:1 to subcutaneous thymosin α1 or placebo every 12 hours for seven days, with 1,089 in the modified intention-to-treat analysis. Primary outcome 28 day all-cause mortality: 127 participants (23.4%) in the thymosin α1 group against 132 (24.1%) on placebo, with no statistically significant difference. The authors conclude there is no clear evidence that thymosin α1 decreases 28 day all-cause mortality in adults with sepsis. A benefit appeared in the diabetic subgroup with a significant test for interaction. Trial registration NCT02867267. Note that secondary reports of this trial quote slightly different hazard ratios, most likely adjusted against unadjusted, which is why the mortality percentages are used in the body text rather than a ratio.
[^2]: Efficacy of thymosin α1 for sepsis: a systematic review and meta-analysis of randomized controlled trials. Frontiers in Cellular and Infection Microbiology, 2025. Peer-reviewed systematic review and meta-analysis, abstract read at source. Eleven randomized controlled trials, 967 patients in the thymosin α1 arm and 960 controls. The overall pooled analysis found a significant reduction in 28 day mortality, odds ratio 0.73, 95% CI 0.59 to 0.90, P=0.003. Restricted to high-quality trials the effect was not significant, odds ratio 0.82, 95% CI 0.65 to 1.03, P=0.09, and restricted to multicenter trials it was also not significant, odds ratio 0.86, 95% CI 0.68 to 1.08, P=0.20. This divergence between the pooled result and the higher-quality subgroups is the basis for the small-study bias point in the body text.
[^3]: Wu J, Zhou L, Liu J, et al. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Critical Care 2013;17:R8. doi:10.1186/cc11932. Peer-reviewed randomized controlled trial, read at source. Six tertiary teaching hospitals in China, May 2008 to December 2010. 28 day all-cause mortality 26.0% in the thymosin α1 group against 35.0% in controls, relative risk 0.74, with P=0.062 on the non-stratified analysis and P=0.049 by log-rank. Single-blind. Cited here as the earlier and smaller trial that TESTS did not replicate.
[^4]: Scale of thymosin alpha-1 clinical use and approval. Reported consistently across four independent sources, all of which have a commercial position in the peptide market. More than 30 clinical trials enrolling over 11,000 subjects, post-marketing surveillance covering more than 600,000 treated patients, and approval in approximately 35 countries for chronic hepatitis B, hepatitis C and immunodeficiency states under the name Zadaxin or thymalfasin, with no FDA approval. A regulatory listing or the prescribing information from an approving country should replace these once located, since the scale figures are doing real work in the opening.
[^5]: 2023 FDA restriction of thymosin alpha-1 from compounding. Reported in secondary sources and consistent with the wider regulatory picture our guides on the July 2026 vote and the February 2027 panel describe. The FDA listing or Federal Register entry should be located and cited directly, since the contrast between a large international safety record and a US compounding restriction is one of the more striking claims on this page.
[^6]: Evidence levels for LL-37, KPV and thymalin. Reported across industry sources. LL-37 has early clinical work concentrated in topical wound and biofilm applications. KPV has preclinical evidence only, with no completed human trials, in colitis and skin inflammation models. Thymalin is a thymus extract studied in Russian clinical work associated with the Khavinson research tradition, largely unblinded. See also our February 2027 panel preview, which carries the LL-37 safety material with primary citations.