The plain answer is that the TB-500 half-life in people is unknown. Half-life means the time it takes the level in the blood to fall by half. No published human study has measured that for the seven-building-block fragment the FDA calls TB-500. The 0.5-to-2.08-hour number repeated online comes from intravenous NL005, the paper's 44-amino-acid recombinant thymosin beta-4 study molecule. It cannot tell us how long injected TB-500 stays in a person's system.
What the TB-500 half-life number really measures
The name is where the confusion starts. Thymosin beta-4 is a naturally occurring peptide made of 43 amino acids, the small building blocks that make up peptides. TB-500 is a synthetic fragment, meaning one short piece of that larger molecule.
Researchers confirmed the distinction by testing material sold as TB-500. They identified it as Ac-LKKTETQ, the seven-amino-acid section spanning positions 17 through 23 of thymosin beta-4 (Esposito et al., Drug Testing and Analysis 2012). Sharing a short sequence does not make the two molecules interchangeable. Size, route, absorption and breakdown can all change how long a substance remains measurable.
| Substance | What it is | Human half-life evidence |
|---|---|---|
| TB-500 | Seven-amino-acid thymosin beta-4 fragment | Not established |
| Recombinant human thymosin beta-4 (NL005) | 44-amino-acid study molecule | About 0.5–2.08 hours after intravenous dosing in one phase 1 study |
That is why a one- or two-hour figure can be accurate for the study it came from and still be the wrong answer for TB-500. When a page gives both molecules the same half-life without explaining this split, it has skipped the part that decides whether the number applies.
What the human thymosin beta-4 studies found
A 2010 phase 1 trial placed 40 healthy volunteers into four groups and studied full-length synthetic thymosin beta-4 given into a vein. Its pharmacokinetics, or how the substance moved through and left the body, showed that blood exposure rose in step with the amount given; the half-life also grew longer as the amount increased (Ruff et al., Annals of the New York Academy of Sciences 2010). This was not a study of the short TB-500 fragment.
A later phase 1 trial tested recombinant human thymosin beta-4 (NL005), which the paper describes as comprising 44 amino acids. Among 54 healthy volunteers in the single-dose part, its intravenous half-life ranged from 0.5 to 2.08 hours. Another 30 volunteers took part in the repeat-dose portion, where researchers found no obvious buildup in the blood (Wang et al., Journal of Cellular and Molecular Medicine 2021).
In plain terms, the 44-amino-acid NL005 study molecule fell quickly in the blood after it was put directly into a vein. But an intravenous result skips the slow absorption that can happen after an injection under the skin. It also says nothing certain about a shorter fragment.
Even for a well-studied drug, time in the blood, time until an effect fades and time until a test can no longer detect it are three different clocks. A half-life is not a dosing schedule or a detection window.
What the eye and wound programs can tell us
The other human research is sometimes folded into claims about TB-500, but those trials also used full-length thymosin beta-4. A small phase 2 dry-eye trial enrolled nine people and applied it as an eye drop, not an injection reaching the whole body (Sosne et al., Cornea 2015). A separate phase 2 program studied topical thymosin beta-4 in 73 people with venous ulcers (Guarnera et al., Annals of the New York Academy of Sciences 2010).
Those studies help map where full-length thymosin beta-4 has been investigated. They do not measure how fast TB-500 leaves human blood. A local eye drop, a gel on a wound and an injection into the bloodstream answer different questions.
People also compare TB-500 with BPC-157 because both are discussed around recovery. The BPC-157 half-life evidence has its own gaps; one uncertain estimate cannot validate another.
What changed in the 2026 FDA review
As of September 6, 2026, the day this article was written, the clearest fresh assessment was the FDA staff briefing prepared for the July 23 Pharmacy Compounding Advisory Committee meeting. The FDA's TB-500 briefing defines the substance as the seven-amino-acid Ac-LKKTETQ fragment and says the agency found no human pharmacokinetic or pharmacodynamic study by any route. Pharmacodynamics means what a substance does to the body.
The briefing also says the naming and chemical characterization in available sources are inconsistent. That matters when a half-life claim is copied from one molecule to another: the label may stay the same while the substance being discussed changes.
The committee review was not a drug approval, and it did not create a human half-life. TB-500 is not FDA-approved. A licensed provider may still prescribe a compounded formulation; that decision is between you and your doctor.
Why forum estimates are still guesses
A confident-looking number usually comes from one of four substitutions: full-length thymosin beta-4 for the fragment, animal data for human data, intravenous delivery for an injection under the skin, or duration of a hoped-for effect for time in the blood. None of those is a small detail.
The familiar rule that about 97% of a substance is cleared after five half-lives only helps when the starting half-life is known. Here it is not. It also cannot predict when a drug test will turn negative, because tests may look for breakdown products rather than the original peptide.
For the broader evidence rather than the timing question, see what studies say about TB-500 benefits and the separate guide to TB-500 side effects and safety gaps.
What this means for a prescription
An unknown half-life does not prove that TB-500 works or does not work. It defines what has not been measured. A responsible prescribing conversation should keep that uncertainty visible instead of converting it into a precise schedule.
At Promise, a licensed provider reviews every request, and not everyone qualifies. If a prescription is written, a licensed U.S. compounding pharmacy dispenses it, and the provider sets the timing from the individual prescription rather than a forum estimate.
The answer worth keeping
The human TB-500 half-life remains unknown. The best human number nearby is 0.5 to 2.08 hours, but it belongs to the 44-amino-acid NL005 study molecule delivered into a vein. The TB-500 fragment, the usual injection route and the question of how long it remains detectable have not been measured together in people. That shorter answer is less satisfying than a neat number, but it is the one the evidence supports.