Sermorelin + Thymosin Alpha-1 Stack: When to Pick One Over the Stack

What each peptide actually is

Sermorelin is a 29-amino-acid analogue of human growth hormone-releasing hormone and the shortest synthetic peptide that keeps the full biological activity of GHRH [6]. It acts on the pituitary's GHRH receptor to make the body release its own growth hormone, and it is not growth hormone itself [6]. After an intravenous dose in healthy men it was eliminated quickly, yet growth hormone stayed raised for about 3 hours [11].

Thymosin alpha-1 is a 28-amino-acid peptide with an acetylated N-terminal end, originally isolated from thymosin fraction 5, a thymus extract; thymalfasin is the synthetic version with the same sequence, and Zadaxin is the brand of the finished 1.6 mg injection sold outside the US [2]. It is described as an immune modulator that augments T-cell function [12] and it signals through Toll-like receptors on myeloid and plasmacytoid dendritic cells [13]. After subcutaneous injection in healthy volunteers it peaks within about 2 hours, has a serum half-life of about 2 hours, and showed no accumulation after five daily doses [12][2].

Is there any evidence for the stack itself?

The trials described on this page tested each peptide on its own. The case for stacking is mechanistic: sermorelin works on pituitary cells and thymosin alpha-1 works on immune cells, so the two do not act on the same receptor [6][13]. Their different mechanisms do not establish that the combination works better than either peptide alone or that it is safe to combine.

Dosing is where that gap bites. Sermorelin has no current approved dose because no sermorelin product is FDA-approved [7], and thymosin alpha-1 has no US-approved dose at all [2]. A stack built from two products with no approved dose has no reference point. The thymosin alpha-1 combination trials described here paired it with hepatitis B treatments, not GHRH analogues: one double-blind trial added it to 24 weeks of interferon [14], and the Cochrane review pooled trials where it was given alone or with interferon, lamivudine, entecavir or tenofovir [5].

Where sermorelin's evidence sits

The key sermorelin trial was in children, not adults. In an open-label, multicenter, 1-year study, 110 previously untreated prepubertal children with growth hormone deficiency received 30 micrograms/kg subcutaneously at bedtime, and 86 were evaluable. Mean 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, and 74% were good responders at 6 months. No adverse changes appeared in general biochemistry or hormone tests, fasting glucose did not change, IGF-I was not excessively raised, and the peptide was well tolerated overall [3].

The adult data are much thinner. Ten healthy older men (mean age 68) took 0.5 mg and 1 mg of GHRH(1-29) subcutaneously twice daily for 14 days each, compared with nine young men (mean age 26). At the high dose, 24-hour growth hormone and IGF-I rose to levels not significantly different from the young men. That was a short physiology study, not a trial of clinical benefit [4].

It is also worth being precise about what sermorelin was ever cleared for. Geref's approved uses were treating idiopathic growth hormone deficiency in children and testing the pituitary's ability to secrete growth hormone. It was never approved for adults, anti-aging, weight loss or athletic performance. Geref was discontinued in 2008, and its approvals were withdrawn in 2009; sermorelin products available in the US today are compounded and are not FDA-approved [7][1].

Where thymosin alpha-1's evidence sits

The main evidence is in hepatitis B. A 2026 Cochrane review pooled 10 randomized trials with 1,349 adults published between 1991 and 2018. All-cause mortality may have been lower with thymosin alpha-1 (risk ratio 0.53, 95% CI 0.29 to 0.96; 3 trials, 907 people), but the certainty was very low; there was little to no effect on quality of life or on liver histology, and the reviewers said they were not sure it helps [5].

Individual trials show why the picture stays muddy. In a double-blind trial of 98 adults with HBeAg-positive hepatitis B, adding thymosin alpha-1 three times weekly to 24 weeks of interferon produced HBeAg loss of 45.8% versus 28.0% (difference 17.8%, 95% CI -1.2 to 35.3, P=0.067), which fell short of statistical significance [14]. A 1991 pilot in 12 people, in which 7 received thymosin fraction 5 or thymosin alpha-1 and 5 received placebo, found hepatitis B virus DNA cleared in 6 of 7 treated patients versus 1 of 5 on placebo (P<0.04) — a result far too small to generalize from [15].

Thymosin alpha-1 is approved as a medicine in some countries. FDA lists Italy, where Zadaxin is authorized as an adjuvant to influenza vaccination in immunocompromised people, and India, for chronic hepatitis B in adults with compensated liver disease, plus Hong Kong. It is not approved in the United States [2].

So when would you pick one over the stack?

Start with the condition, not the peptide.

If the question is growth hormone deficiency in a child, the growth data discussed here are for sermorelin, and they come from a single open-label study [3]. If the question is chronic hepatitis B, the trial data discussed here are for thymosin alpha-1, and the Cochrane review's mortality finding was very low certainty [5]. For general goals such as energy, recovery or immune support in otherwise healthy adults, the trials above do not establish a useful dose or benefit for either peptide [3][4][5].

When neither condition applies, the available studies do not show that using both peptides improves outcomes over using one, and they do not establish a benefit for either peptide in those general goals [3][5]. HealthRX.com offers sermorelin by prescription.

The regulatory difference between the two

Sermorelin does not appear on FDA's page of bulk substances that may raise significant safety risks when compounded [9]. On FDA's list for outsourcing facilities it sits in category 1, under evaluation, marked as a component of approved drugs [8]. It is absent from FDA's list for state-licensed pharmacies and physicians and is not one of the six peptides in HHS's October 2026 draft enforcement-discretion policy [16][17].

Thymosin alpha-1 is treated more cautiously. It appears in none of the three categories on FDA's list for state-licensed pharmacies and is category 3, nominated without adequate support, for outsourcing facilities [16][8]. FDA moved it to the nominated-but-withdrawn table after nominators withdrew, but its stated concerns are still posted: significant risk of immunogenicity for certain routes of administration, peptide-impurity and characterization problems, and safety information inadequate to understand the extent of any safety issues [9]. In December 2024 FDA's Pharmacy Compounding Advisory Committee voted 4 yes and 17 no, with no abstentions, on both the free base and the acetate, with members who voted no citing no compelling evidence of clinical effectiveness and safety [18]. It is not in the HHS draft policy either [17].

What both have in common: compounded drugs are not FDA-approved, and FDA does not review their safety, effectiveness or quality before they are marketed [1].

Banned in sport or not

WADA's 2026 Prohibited List names sermorelin in S2.2.4 among growth hormone releasing factors, prohibited at all times in and out of competition, and it is also named in the 2027 list [10][19]. Thymosin alpha-1 is not named on either list. WADA does name thymosin-beta-4 and its derivatives such as TB-500, which is a different peptide [10].

What to watch for

The Italian Zadaxin label, which covers a finished product sold outside the US and not compounded powders, says it is not for children, is contraindicated in pregnancy and breastfeeding, and calls for caution in people with allergies or autoimmune disease and with other immune-modulating drugs [2].

Adverse-event information for thymosin alpha-1 is thin. FDA's search of its adverse-event database in April 2024 found zero reports [2]. Authors tied to the manufacturer report more than 4,400 people enrolled in trials and over 17 million doses of Zadaxin given after marketing, with adverse experiences described as infrequent and mild — company-linked data [2]. The 2026 Cochrane review found 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) and fewer non-serious events (RR 0.47, 95% CI 0.27 to 0.83; very low certainty) [5].

For sermorelin, the 1-year pediatric study reported no adverse changes in general biochemistry or hormone tests and no change in fasting glucose [3].