Thymosin Alpha-1 + MOTS-c Stack: When to Pick One Over the Other

What each peptide actually is

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 injection sold outside the US at 1.6 mg per vial — not interchangeable with bulk powder [2]. It is an immune modulator that augments T-cell function [10]. In a cell and mouse study of antifungal immunity, it matured fungus-pulsed dendritic cells through myeloid differentiation factor 88-dependent Toll-like receptor signaling [11]. After subcutaneous injection in healthy volunteers it peaks at about 0.7 to 2 hours, clears with a half-life of about 1.8 to 2.6 hours, and does not accumulate over five days of daily dosing [2].

MOTS-c is a 16-amino-acid peptide, molecular weight about 2,174.6 g/mol, encoded in mitochondrial DNA and first reported in 2015 by Lee et al. in Cell Metabolism [1][12]. 'MOTS-c' is a common name, not an official drug name [1]. All pharmacology of administered MOTS-c comes from cell and rodent studies [1][12][4].

Does MOTS-c have any human treatment data?

No study has given MOTS-c to a person as a treatment. For its July 2026 review FDA searched PubMed, Embase, the Cochrane library and other sources and found no clinical study in which MOTS-c was given to humans; the 12 papers the nominator cited were not clinical studies [1].

Human studies have measured the body's own MOTS-c instead. Reynolds et al. (2021) found that exercise raises endogenous MOTS-c in skeletal muscle and in circulation; participants were not given the peptide [4]. von Walden et al. (2021) randomized 30 healthy adults to endurance exercise (n=10), resistance exercise (n=10) or control (n=10); circulating MOTS-c showed only a trend to rise after endurance exercise, and plasma MOTS-c was not related to fitness [5].

The effects that drive the marketing come from mice. Injected MOTS-c reduced weight gain on a high-fat diet — about 20% lower body weight after 8 weeks at 0.5 mg/kg/day intraperitoneal — and improved insulin sensitivity and exercise performance [1][12]. In one study a late-life schedule (15 mg/kg three times weekly from 23.5 months of age) lengthened median lifespan from 912 to 970 days [1][4]. FDA notes that the molecular targets are unknown and that no dose-response relationships were assessed [1].

Does thymosin alpha-1 have human data?

Yes, in chronic hepatitis B, and the results are modest.

A 2026 Cochrane review pooled 10 randomized trials with 1,349 adults, published 1991 to 2018 and run in Bangladesh, China, Italy, Korea, Singapore and Taiwan. It found possibly lower all-cause mortality with thymosin alpha-1 (risk ratio 0.53, 95% CI 0.29 to 0.96; 3 trials, 907 people) at very low certainty, little to no effect on quality of life or liver histology, and a very uncertain effect on hepatitis B-related illness. The reviewers said they are 'not sure' it helps [3].

Individual trials are small. Lim et al. (2006) gave 98 adults 24 weeks of lymphoblastoid interferon plus thymosin alpha-1 (n=48) or placebo (n=50); HBeAg loss at 72 weeks was 45.8% versus 28.0%, a difference of 17.8% (95% CI -1.2 to 35.3, P=0.067) that was not statistically significant [13]. Mutchnick et al. (1991) treated 12 people, 7 with thymosin fraction 5 or thymosin alpha-1 and 5 with placebo twice weekly for 6 months; serum HBV DNA cleared in 6 of 7 (86%) versus 1 of 5 (20%), P<0.04 [14]. A 2001 pharmacy review reported a randomized trial in which HBV DNA cleared in 40.6% after 6 months and 25.6% after 12 months of thymosin alpha-1, versus 9.4% of untreated controls [10].

Which one to pick, and on what basis

The deciding factor is not mechanism. It is what has been measured in people.

If the question is chronic hepatitis B, thymosin alpha-1 is the only one of the two with human trials, though the 2026 Cochrane evidence is very low certainty and the reviewers were unsure whether it helps [3]. If the question is metabolic, MOTS-c has mouse data and no human treatment data. FDA also found no animal toxicity studies and no immunogenicity data, and noted an in-vitro study in which MOTS-c is quickly broken down in human blood, which raises the question whether injecting it can produce active levels in a person [1].

There are no pharmacokinetic data for MOTS-c in humans or in live animals [1]. Thymosin alpha-1 has human pharmacokinetic data [2].

The regulatory status is not the same

Thymosin alpha-1 is not FDA-approved. It holds four US orphan drug designations, and orphan designation is a separate process from approval [2]. FDA lists national authorizations in Italy (Zadaxin as an adjuvant to influenza vaccination in immunocompromised people), India (chronic hepatitis B in adults 18 and older with compensated liver disease and viral replication) and Hong Kong [2]. Two reviews by authors tied to the Zadaxin developer say it is approved in 'over 35 countries' (2009) or 'more than 30 countries' (2007) [16][17]. The European Medicines Agency has granted only an orphan designation for hepatocellular carcinoma, which is not a marketing approval [2].

For compounding, thymosin alpha-1 sits in FDA's 'nominated but withdrawn' table, and the agency's concerns are still posted: significant immunogenicity risk for certain routes of administration, peptide impurity and characterization problems, and safety information that is inadequate [15]. It is in none of the three categories on FDA's list for state-licensed pharmacies and physicians (updated May 14, 2026), and on the outsourcing-facility list (updated March 21, 2025) it is category 3, 'nominated without adequate support'; FDA's category 1 policy does not apply to it [18][19]. On December 4, 2024 FDA's Pharmacy Compounding Advisory Committee voted 4 yes and 17 no, with no abstentions, on both the free base and the acetate [8]. Thymosin alpha-1 is not one of the six peptides in HHS's October 2026 draft enforcement-discretion policy [20].

MOTS-c is not FDA-approved either, and FDA's July 2026 review found no USP or National Formulary monograph, no listing in the European or Japanese pharmacopeias, and no product authorized by the European Medicines Agency [1]. It is also in the 'nominated but withdrawn' table, with FDA's concerns posted: no identified human exposure data by any route of administration, missing safety information, and immunogenicity risk from aggregation and impurities [15]. Removal from category 2 was not a finding that it is safe. FDA's category 1 policy does not cover it [18][19][21].

On July 23, 2026 the same advisory committee voted in favor of adding MOTS-c to the bulks list, against FDA staff's recommendation. That vote is advice only and is not binding; MOTS-c is not on the list, and adding it takes formal rulemaking [9][22]. FDA staff had proposed that neither the free base nor the acetate be added, citing inconsistent naming, no impurity, aggregate or endotoxin data, no human safety or effectiveness data, no animal toxicity studies, and the existence of FDA-approved treatments for obesity and osteoporosis [1]. HHS's draft enforcement-discretion policy for six peptides, filed in October 2026, names MOTS-c and covers the acetate given subcutaneously or intramuscularly; HHS says it 'does not intend to take enforcement action' against a state-licensed pharmacy, federal facility or licensed physician that compounds with it if conditions are met, such as a single active ingredient, a patient-specific prescription, an FDA-registered manufacturer with a certificate of analysis, and a label reading 'This is a compounded drug product. Not for resale.' The notice says it creates no rights and is not binding on HHS or the public, and it is not an approval or a listing. Compounded drugs are not FDA-approved [22][20].

In sport, one of the two is banned

MOTS-c is named on WADA's Prohibited List in S4.4.1 as an AMPK activator and is prohibited at all times, in and out of competition, as a non-Specified Substance. It is on the 2026 List (in force since January 1, 2026) and stays on the 2027 List [6][7]. Thymosin alpha-1 is not named on either list [6][7]. WADA does name thymosin-beta-4 and its derivatives such as TB-500 in S2.3, which is a different peptide [6][2].

What the Italian Zadaxin label says

Only thymosin alpha-1 has a label to describe, and it is Italy's Zadaxin 1.6 mg. According to that label, it is given subcutaneously or intramuscularly twice weekly for 4 weeks starting at the first vaccination, repeated from week 8 to week 12 at the second vaccination. The label 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]. That label covers the Italian vaccine-adjuvant authorization, not hepatitis B and not any US use.

What the safety numbers show

In the 2026 Cochrane review, 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 adverse events (risk ratio 0.47, 95% CI 0.27 to 0.83; very low-certainty evidence) [3]. FDA's search of its adverse-event database on April 22, 2024 found zero reports for thymosin alpha-1; authors tied to the manufacturer report over 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' [2].

For MOTS-c, FDA's adverse-event database search through March 9, 2025 retrieved no reports [1]. That is not evidence of safety: FDA found no human exposure data by any route, so its safety in humans is unknown [1].

The combination itself

FDA found no study in which MOTS-c was given to a person, so no human study of the combination exists [1]. Having two different mechanisms — an immune modulator versus mouse metabolic signaling that involves AMPK — is not evidence that combining them does anything useful. A stack pairs one peptide with very-low-certainty human trial evidence and one with no human dosing study [3][1].

Dosing, as far as anything is documented

Thymosin alpha-1 has no US-approved dose [2]. The only label dose is the Italian 1.6 mg twice weekly schedule for the vaccine-adjuvant indication [2], and Lim et al. added thymosin alpha-1 to interferon for 24 weeks without a statistically significant HBeAg benefit [13].

MOTS-c has no approved dose and no human dosing study. The nominated products were 5 mg and 10 mg subcutaneous injections, which describes what was nominated, not a recommendation [1]. The rodent doses used (0.5 to 15 mg/kg) cannot be converted into human doses [1].