Sermorelin + Thymosin Alpha-1 Stack: Evidence, Mechanism Overlap, and Protocol

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What sermorelin does

Sermorelin is a 29-amino-acid analogue of human growth hormone-releasing hormone and the shortest synthetic peptide with full GHRH activity; it is not growth hormone itself [3]. It acts on the pituitary GHRH receptor to trigger release of the body's own growth hormone. In a study of 30 healthy men given GHRH(1-29)-NH2 intravenously or intranasally, the peptide was eliminated quickly after intravenous injection, yet growth hormone stayed raised for about 3 hours [6].

Geref was approved for two narrow uses: testing the pituitary's ability to secrete growth hormone, and treating idiopathic growth hormone deficiency in children with growth failure [1]. The maker discontinued it in 2008 and FDA withdrew both approvals effective June 18, 2009, later determining it was not withdrawn for safety or effectiveness reasons [1]. It was never approved for adult or anti-aging use [1]. Sermorelin sold in the US today is compounded, and compounded drugs are not FDA-approved [7].

What thymosin alpha-1 does

Thymosin alpha-1 is a 28-amino-acid peptide originally isolated from thymosin fraction 5; thymalfasin is the synthetic version, and Zadaxin is the finished injection sold outside the US at 1.6 mg per vial [2]. It is thought to modulate the immune system by augmenting T-cell function [8]. After subcutaneous injection it peaks at about 0.7 to 2 hours and clears with a half-life of about 1.8 to 2.6 hours [2]. It is not FDA-approved in the US; its four US orphan designations are not approvals [2]. It is approved as a medicine in some other countries, including Italy, India and Hong Kong [2].

What the separate evidence shows

For thymosin alpha-1, a 2026 Cochrane review of 10 randomized trials in 1349 adults with chronic hepatitis B found possible lower all-cause mortality (risk ratio 0.53, 95% CI 0.29 to 0.96; 3 trials, 907 people) on very low-certainty evidence; the reviewers said they were not sure it helps [9]. For sermorelin, a 1-year open-label study treated 110 previously untreated prepubertal children with growth hormone deficiency, and 86 were evaluable for efficacy. Average height velocity rose from 4.1 cm/yr at baseline to 8.0 cm/yr at 6 months and 7.2 cm/yr at 12 months [10].

Side effects and status

In that pediatric study, no adverse changes appeared in biochemistry or hormone tests, fasting glucose did not change, and sermorelin was well tolerated [10]. FDA's April 22, 2024 adverse-event database search found zero reports for thymosin alpha-1 [2]. Sermorelin is named on WADA's 2026 and 2027 Prohibited Lists (S2.2.4), prohibited at all times; thymosin alpha-1 is not named on either [4][5]. FDA's safety-risk page lists thymosin alpha-1 as nominated but withdrawn, with concerns about immunogenicity for certain routes, peptide impurities and safety information the agency called inadequate [11].

What protocol exists?

No combined protocol exists. Published single-agent examples include sermorelin 30 micrograms per kg at bedtime in children with growth hormone deficiency [10], sermorelin 0.5 mg or 1 mg twice daily for 14 days in a short study of healthy older men [12], and the Italian Zadaxin label's 1.6 mg subcutaneously or intramuscularly twice weekly for 4 weeks from the first vaccination, repeated weeks 8 to 12 at the second vaccination, as an adjuvant to influenza vaccine [2]. Another thymosin alpha-1 schedule, 1.6 mg twice weekly for 8 weeks, was used in a preliminary report in dialysis patients [13]. None of these is a combined protocol or a recommended dose.