Is thymosin alpha-1 FDA approved?

No. It is approved in many other countries as Zadaxin, mainly for hepatitis B, but not in the US.

Does thymosin alpha-1 prevent colds?

We found no trial showing that. Its trials were in hepatitis B, hepatitis C and sepsis, and even there the benefit is uncertain.

Is LL-37 an immune booster?

Its human trials used it as a gel on leg ulcers. FDA notes animal and lab findings that raise concern about injected use, and there are no human trials of it as an immune booster.

Thymosin Alpha-1 and LL-37 for Immunity: The Evidence

Short answer: Thymosin alpha-1 (Zadaxin) is approved in many countries, though not the US, mainly for hepatitis B. A 2026 Cochrane review of 10 trials rated that evidence low to very low certainty, and a 1,106-patient sepsis trial found no survival benefit. LL-37 was tested as a wound gel, with mixed results. No trial we found shows either prevents colds or boosts immunity in healthy people.

What people discuss for immune support

What people discuss, and the strongest evidence we found for this goal
Compound	Best evidence	What it shows
Thymosin alpha-1 (Zadaxin)	Human trials	Approved in over 35 countries for hepatitis B and C and as an immune stimulant, per a 2009 review. Not FDA approved. Trials in hepatitis B and sepsis show uncertain or no benefit.
LL-37	Small or early human studies	Human trials used it as a gel on venous leg ulcers, not as an immune booster. A 148-person trial found no overall healing benefit.
KPV	Animal or lab studies only	Reduced inflammation in mouse colitis. FDA found no human studies.

"Animal or lab studies only" means no published human trial for this goal that we could find. Results in rats or cells often do not carry over to people. Compounds that are not FDA-approved have no tested label for dose, purity or safety.

What has thymosin alpha-1 shown in trials?

Thymosin alpha-1 is a 28-amino-acid peptide first found in the thymus, the gland where T cells mature. Its drug form is called thymalfasin, sold as Zadaxin. A 2009 review says it is approved in over 35 countries for hepatitis B and C and as an immune stimulant and vaccine helper. It is not approved in the US.

Hepatitis B. A 2026 Cochrane review pooled 10 randomized trials with 1,349 adults. Thymosin alpha-1 may reduce deaths (3 trials, 907 people), but the authors rated almost every result very low certainty and said they are not sure it helps.
Sepsis. The TESTS trial (BMJ 2025) randomized 1,106 adults in 22 Chinese hospitals to injections every 12 hours for 7 days or placebo. Deaths at 28 days were 23.4% vs 24.1%, no real difference.
Older trials. A 2001 review called hepatitis B and C results "mixed" and said effects on illness and death "remain to be seen."

Side effects in trials were mostly mild injection-site irritation. The sepsis trial found no difference in safety outcomes between groups.

LL-37: a wound gel, not an immune shot

LL-37 is a 37-amino-acid antimicrobial peptide your own body makes, part of a protein called cathelicidin. The human trials put it directly on chronic venous leg ulcers twice a week.

2014, 34 people: the two lower strengths shrank ulcers more than placebo over 4 weeks; the highest strength did no better than placebo.
2021, 148 people (phase 2b): no significant healing benefit over placebo in the whole group. A benefit appeared only in an after-the-fact look at large ulcers.

Both trials found it well tolerated on the skin. That tells us nothing about injecting it. FDA lists LL-37 with possible immune reactions and notes that animal and lab findings suggest harm to male reproduction and possible tumor-promoting effects in some tissues.

What these peptides have not been shown to do

We found no published randomized trial showing that thymosin alpha-1, LL-37 or KPV:

prevents colds, flu or other infections in healthy adults,
shortens a common illness,
or "boosts" immunity in a way that leads to better health in people who are not sick.

A lab change, like more active T cells, is not the same as fewer infections. For thymosin alpha-1, FDA's compounding page notes possible immune reactions and says safety information is inadequate to judge the risk. If you get infections often, a clinician can look for causes such as diabetes, low antibody levels or medicines.

This page is education, not medical advice. A clinician should diagnose the problem and make any prescription decision with you.

What is worth tracking

Doses with date and time. Thymosin alpha-1 trials used fixed schedules; your log shows what you actually did.
Illness days. Log each cold or infection as a symptom with start and end dates. After months, you can compare with the same season last year.
Injection sites. Record and rotate; redness and irritation were the most common trial side effect.
Lab results. Store blood counts, liver tests or viral load results with dates, especially if a specialist is treating hepatitis.
Other changes. Sleep, stress and new medicines affect how often you get sick. Note them in the daily check-in.

Terms on this page: Peptide · Injection site rotation · Compounding pharmacy

This page is education, not medical advice, and it is not a recommendation to use any compound. It does not cover dosing. Talk to a licensed clinician before starting, changing or stopping anything, and get diagnosed before treating a symptom.

Frequently asked questions

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