How long does Thymosin α-1 stay in your system?
Thymosin α-1 has a published half-life of approximately 2 hours (serum). By the standard pharmacology rule of thumb, a drug is about 97% eliminated after roughly five half-lives, which here works out to about 10 hours. This value comes from a review article rather than a primary pharmacokinetic trial.
Source: Billich, Am J Health Syst Pharm 2001 (PMID 11381492) · Review article · measured: subcutaneous · dataset reviewed September 23, 2026
What is the half-life of Thymosin α-1?
Thymosin alpha-1, also known as thymalfasin and marketed outside the United States as Zadaxin, has a reported serum half-life of approximately 2 hours after subcutaneous injection. The figure comes from a 2001 review, which also describes peak serum concentrations within two hours and a return to baseline within 24 hours.
Thymosin alpha-1 occurs naturally, as a fragment of a thymic protein called prothymosin alpha. That is why the useful endpoint is a return to baseline rather than to zero: there is an endogenous level for injected peptide to fall back to.
How much Thymosin α-1 is left after each half-life?
Each half-life removes half of whatever is left, so the percentages are pure arithmetic and the same for any compound; only the clock changes. For Thymosin α-1, one half-life is 2 hours. The rows below run that forward for a single amount with nothing added.
| Half-lives elapsed | Time elapsed | Remaining | Cleared |
|---|---|---|---|
| 1 | 2 hours | 50% | 50% |
| 2 | 4 hours | 25% | 75% |
| 3 | 6 hours | 12.5% | 87.5% |
| 4 | 8 hours | 6.25% | 93.75% |
| 5 | 10 hours | 3.13% | 96.88% |
| 6 | 12 hours | 1.56% | 98.44% |
| 7 | 14 hours | 0.78% | 99.22% |
Why do the numbers differ between sources?
Thymosin alpha-1 is not FDA-approved, so there is no US label to check the review's figure against. A review summarises earlier work rather than reporting a new measurement, which puts this value a step further from primary data than most in our dataset.
Numbers quoted elsewhere may come from the review, from the studies it drew on, or from no source at all. The approximately 2 hour figure is consistent with the review's own description of peak levels within two hours and baseline within 24, which is reassuring, but it is still best treated as approximate.
What does a half-life not tell you?
Thymosin alpha-1 is an immune modulator that acts through Toll-like receptors on dendritic cells and other antigen-presenting cells. Effects on immune cells, such as dendritic cell maturation or T-cell responses, unfold over a different timescale from the peptide's presence in serum, and the half-life cannot tell you how long they last.
It also says nothing about the clinical evidence. The largest sepsis trial missed statistical significance on its primary mortality endpoint, and outcome questions like that sit entirely outside pharmacokinetics.
Can you run the numbers yourself?
The estimator applies the same decay arithmetic to any half-life, starting amount and time you enter. It opens with Thymosin α-1's published value loaded. Type another figure to see how sensitive the result is to it.
Published value: Approximately 2 hours (serum) (Subcutaneous) · Billich, Am J Health Syst Pharm 2001 (PMID 11381492) · Compound profile
Show the math
What are the limits of this estimate?
Everything above is a single-compartment approximation: one pool of drug, falling by the same fraction in every interval. Real elimination varies with kidney and liver function, body composition, formulation and route, and a published figure describes only the route and the people it was measured in. Many compounds also show a fast distribution phase before the slower elimination the table describes.
A half-life is not a dosing schedule. Labelled intervals come from clinical trials that measured outcomes and tolerability, and nothing on this page suggests when to take, repeat or stop anything. Whether Thymosin α-1 is out of your system for drug-testing purposes is a different question again. Tests often look for metabolites rather than the parent compound, and the answer depends on the assay, its sensitivity and the sample type, none of which a plasma half-life captures.
FAQ.
Where does the thymosin alpha-1 half-life come from?
From a 2001 review article rather than a primary pharmacokinetic trial; the citation is linked on this page. Because thymosin alpha-1 is not FDA-approved, no US label exists to check the figure against.
Is thymosin alpha-1 naturally present in the body?
Yes. It corresponds to a fragment of prothymosin alpha, a protein produced in the thymus. After an injection, levels return to baseline rather than to zero, which the review describes as happening within 24 hours.
Is Zadaxin the same as thymosin alpha-1?
Zadaxin is a brand of thymalfasin, the international nonproprietary name for synthetic thymosin alpha-1. It is approved as a prescription drug in a number of countries but not by the US FDA. Material sold as research-use thymosin alpha-1 is not the approved product.
For what the evidence on Thymosin α-1 actually shows, read the Thymosin α-1 profile. For approval and regulatory position, see is Thymosin α-1 legal. The half-life reference lists every sourced value side by side, including the compounds where a number circulates online with no reliable human data behind it.