Thymosin Alpha-1 Rebound Effects When Stopping: What the Evidence Actually Shows

Is there any evidence of a rebound when thymosin alpha-1 stops?

Rebound means a biological measurement overshooting its pre-treatment level after a drug is withdrawn. None of the trials described below was designed to test for a rebound after a course of thymosin alpha-1, so a rebound can be neither confirmed nor ruled out.

The trials that followed people after treatment tracked hepatitis B markers. In the 98-person trial, thymosin alpha-1 or placebo was added to lymphoblastoid interferon for 24 weeks; 48 people received thymosin alpha-1 and 50 received placebo [3]. The primary endpoint was loss of HBeAg at 72 weeks. HBeAg loss was 45.8% with the combination and 28.0% with interferon alone, a difference of 17.8% (95% CI -1.2 to 35.3; P=0.067) that was not statistically significant [3].

An earlier pilot gave 7 people thymosin fraction 5 or thymosin alpha-1 and 5 people placebo twice weekly for six months. Serum HBV DNA cleared in 6 of 7 (86%) versus 1 of 5 (20%) (P<0.04), and the responders' clinical, biochemical and serological improvement was sustained over 26 ± 3 months of follow-up [4]. The pilot also reported improvements over initial values in peripheral lymphocyte, CD3 and CD4 counts and in interferon-gamma production, but it did not report an overshoot after treatment ended [4]. That pilot enrolled 12 people, and its active group was not thymosin alpha-1 alone.

What the pharmacokinetics say, and what they do not

Thymosin alpha-1 is absorbed quickly after subcutaneous injection and cleared with a short half-life. In healthy volunteers, there was no accumulation after daily dosing for five days [1][2], and a pharmacy review reports that blood levels return to baseline within 24 hours [1]. These data show how long the peptide stays in the blood. They do not show what T-cell or cytokine measurements do over the following weeks, which is the measurement a rebound claim would need.

Safety reports during and after treatment

A 2026 Cochrane review pooled 10 randomized trials in 1,349 adults with chronic hepatitis B. Thymosin alpha-1 may have caused fewer serious adverse events than the comparison treatments (risk ratio 0.72, 95% CI 0.53 to 0.99; 5 trials, 1,056 people; low-certainty evidence) and fewer non-serious ones (RR 0.47, 95% CI 0.27 to 0.83; very low-certainty evidence) [5].

FDA's search of its adverse-event database, dated April 22, 2024, found zero reports for thymosin alpha-1 [2]. Authors tied to the manufacturer report more than 4,400 people enrolled in trials and more than 17 million doses of Zadaxin given after marketing, with adverse experiences described as infrequent and mild. Those are company-linked figures rather than independent ones [2].

What the label covers

Thymosin alpha-1 is not FDA-approved in the United States, and no US-approved dose exists [2]. FDA lists approvals in other countries, including Italy, where the Zadaxin 1.6 mg powder is given subcutaneously or intramuscularly twice weekly for four weeks starting at the first vaccination, repeated from week 8 to week 12 at the second vaccination [2]. The Italian label says the product is not for children, is contraindicated in pregnancy and breastfeeding, and calls for caution in people with allergies or autoimmune disease and in people taking other immune-modulating drugs [2].