The answer so far
A medicine can have a real approval for one narrowly defined use while other uses remain unproven. Here is what the Barth syndrome trial measured, why confirmation is required and what that means for broader claims.
Elamipretide, also called SS-31, is a peptide studied for its interaction with mitochondria. In 2025 the US FDA granted accelerated approval to the medicine Forzinity to improve muscle strength in adults and children with Barth syndrome who weigh at least 30 kg. That decision is specific to this rare condition and product; it does not establish that SS-31 improves ageing, energy or longevity in healthy people.1
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What is the biological idea?
Mitochondria are structures inside cells that help make usable energy. Barth syndrome is a rare inherited condition involving abnormal cardiolipin, a fat-like component of the inner mitochondrial membrane. Elamipretide binds cardiolipin. This gives researchers a reason to study it. But binding to cardiolipin does not by itself prove that the medicine improves a person’s symptoms or lifespan. For another mitochondrial-associated peptide with different evidence, see MOTS-c.3
What did the controlled trial find?
The sponsor-funded Barth syndrome study enrolled 12 participants. In a randomised crossover design, each received elamipretide and placebo during separate treatment periods. After 12 weeks of each treatment, neither of the two main outcomes—distance walked in six minutes and a fatigue score—showed superiority to placebo. The small sample and short treatment periods limit what can be concluded. The trial was funded by Stealth BioTherapeutics, and one author reported employment there.3
Why was the medicine approved?
In the longer open-label extension, participants knew they were receiving treatment and there was no concurrent placebo group. FDA described an increase in knee-extensor strength in this period; it was not significantly different during the initial randomised period. FDA considered the later strength measure reasonably likely to predict a meaningful benefit and granted accelerated approval in September 2025. This pathway requires a further controlled study to verify clinical benefit. As checked on 4 November 2026, the FDA lists that requirement as ongoing, with a scheduled completion date in 2030—not proof of a result. Read how approval and confirmatory studies differ.24
What happened in a different mitochondrial muscle disease?
MMPOWER-3 studied a separate group of inherited mitochondrial muscle disorders, not Barth syndrome or healthy ageing. In this 24-week, double-blind trial, 218 participants were randomly assigned elamipretide or placebo. Neither of the two main outcomes—six-minute walking distance and a fatigue score—showed a treatment benefit over placebo. Estimated walking distance increased by 14.1 metres in the treatment group and 17.3 metres with placebo; the estimated between-group difference was −3.2 metres, with a 95% confidence interval from −18.7 to 12.3 metres. That uncertainty leaves room for different effects but does not establish a benefit in this study. Stealth BioTherapeutics funded the trial. This result does not overturn the separate, narrowly scoped Barth approval or settle every genetic subgroup.5
What did the older-adult energy study measure?
A 2021 placebo-controlled trial enrolled 39 older adults selected for low measured mitochondrial function in a hand muscle. After a single intravenous infusion, the percentage change in one measure of the muscle’s capacity to produce ATP—a molecule cells use to transfer energy—differed from placebo immediately; the absolute change did not meet the usual statistical threshold. The difference was not seen at day seven, and the study found no statistically significant improvement in resistance to fatigue in that muscle. A short-lived laboratory measurement is not the same as feeling more energetic, improved daily function or a longer life. Stealth BioTherapeutics, NIH grants and the University of Washington supported the study; the authors reported working with the company on the design and some disclosed paid consulting. For the full story of fatigue, the earlier positive signal and later trials, read SS-31 and energy in human studies.6
What about side effects and product quality?
FDA reports injection-site reactions and allergic reactions, some serious, with Forzinity. In the small crossover study, all 12 participants had a local administration reaction while receiving the medicine; eight also had one during placebo. Small numbers cannot establish every rare or long-term harm. An online vial labelled SS-31 is not automatically the approved medicine: its identity, purity, strength and sterility cannot be assumed from the label. For context, see research-peptide product and storage claims.2
Does SS-31 slow ageing or boost energy?
The Barth syndrome approval does not answer those questions. A finding in a rare disease, a laboratory model or a short biomarker study cannot establish longer life, better energy or improved performance in healthy people. Claims require a defined product, appropriate comparison, outcomes meaningful to participants and adequate safety follow-up. The US decision also does not establish approval in another country. Explore the path from laboratory research to people.13
Sources, kept close.
Selected supporting sources; this is not a complete literature review.
- FDA Grants Accelerated Approval to First Treatment for Barth SyndromeU.S. Food and Drug Administration · US regulatory approval announcement, 2025
- Drug Trials Snapshots: ForzinityU.S. Food and Drug Administration · US regulatory trial summary, 2025
- A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndromeGenetics in Medicine / PubMed Central · Sponsor-funded clinical study, 2020
- Ongoing accelerated approvals and postmarketing requirementsU.S. Food and Drug Administration · US regulator status and postmarketing requirements
- Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical TrialNeurology / PubMed Central · Sponsor-funded phase 3 randomized clinical trial, 2023
- In vivo mitochondrial ATP production is improved in older adult skeletal muscle after a single dose of elamipretide in a randomized trialPLOS ONE · Randomized clinical trial, 2021