SS-31 (Elamipretide): Real human trials, mostly failed endpoints, one narrow approval

SS-31, now approved as the drug elamipretide, is one of the more useful case studies on this site. It isn’t some mystery ingredient dreamed up by supplement marketers — it’s a genuine molecule that went through real, careful clinical trials. The honest lesson lies in what those trials found: in its three largest and best-designed studies, the compound failed to hit its main goals, and its single FDA approval is a narrow, conditional one for an extremely rare genetic disease. There’s a big gap between that reality and the way SS-31 gets sold online for “mitochondrial health” and anti-aging.

What it is

SS-31 is a synthetic, cell-penetrating tetrapeptide (a tiny protein made of just four amino acid building blocks) — written out as D-Arg-Dmt-Lys-Phe-NH2. Two of its building blocks are slightly tweaked versions (a “D” form of arginine and a methyl-modified tyrosine), which keeps the body’s enzymes from chopping it up quickly. The way its parts alternate between water-loving and oil-loving chemistry also lets it slip across cell membranes. Its standout feature is where it ends up and what it grabs onto: it gathers selectively inside the inner membrane of mitochondria (the tiny “power plants” inside our cells) and latches onto cardiolipin, the signature fat molecule of that membrane.

By holding cardiolipin steady, SS-31 is thought to keep the folded internal structure of mitochondria intact, help build the machinery that generates energy, raise ATP output (ATP is the cell’s main energy currency), cut down on reactive oxygen species (unstable molecules that can damage cells), and keep a protein called cytochrome c anchored in place (which helps hold back a key trigger for cell self-destruction). Researchers understand this mechanism well from lab work. It also explains why Barth syndrome makes the most sense as a target: Barth is caused by mutations in the TAFAZZIN gene that mess up how cardiolipin is maintained, so a drug that stabilizes cardiolipin has a clear, logical reason to help there.

The claims

There are two very different sets of claims here. The legitimate clinical program (run by Stealth BioTherapeutics) tested elamipretide for Barth syndrome, primary mitochondrial myopathy, dry age-related macular degeneration (a vision-loss condition, also called geographic atrophy), heart failure, and acute heart attack.

The gray-market story is much broader and mostly unsupported. Research-peptide sellers and biohackers pitch SS-31 for “mitochondrial health,” anti-aging, more energy, faster workout recovery, and heart or brain “optimization.” None of these healthy-person performance or longevity uses are backed by controlled trials in people.

The evidence

One pattern is worth pointing out right away: in its three largest, best-designed trials, elamipretide failed to hit its main goals. Its only approval rests on a strength signal from an open-label study (one with no placebo group for comparison) in roughly ten patients. And most of the positive data come from studies the company itself funded.

Barth syndrome (TAZPOWER) — the basis for approval. This was a randomized, double-blind, placebo-controlled crossover trial (the gold-standard design, where neither patients nor doctors know who gets the real drug) in just 12 patients, followed by an open-label extension (a follow-on phase where everyone knowingly takes the drug). The carefully controlled phase failed both main goals: how far patients could walk in six minutes differed by only −0.8 meters (p=0.97, meaning the result is statistically meaningless), and a fatigue score barely budged (p=0.89). The improvements that backed the approval showed up afterward, in the uncontrolled open-label extension, where patients walked a cumulative 96.1 meters farther by week 168 and their knee strength improved. But that single-arm design has no placebo group to compare against and is prone to bias. The FDA accepted the knee-strength result as an intermediate clinical measure under accelerated approval, and clearly required a follow-up confirmatory trial.

Primary mitochondrial myopathy (MMPOWER-3). This was the big Phase 3 trial: randomized, double-blind, placebo-controlled, 218 patients (109 on the drug, 109 on placebo), over 24 weeks. It was negative. The six-minute walk distance differed by only −3.2 meters between the groups (not a meaningful difference), and there was no benefit for fatigue. A post-hoc subgroup analysis (a slice of the data examined after the fact, rather than planned in advance) — looking at patients with a certain genetic variant — hinted at a possible benefit, but that kind of finding can only suggest ideas to test later; it can’t make up for missing the main goal.

Dry AMD / geographic atrophy (ReCLAIM-2). This Phase 2 randomized trial also missed its main goals (a low-light vision test and the growth of the eye lesion). One secondary finding — roughly a 43% reduction in the thinning of a particular retinal layer compared with placebo — was called promising, but it doesn’t confirm anything on its own.

Cardiology (Bendavia era). A trial called EMBRACE-STEMI tested elamipretide given through an IV in 118 people having a heart attack, and it was negative on its main goal; it didn’t shrink the area of heart damage. The heart-failure program (PROGRESS-HF) also didn’t meet its main goal of reducing the size of the heart’s main pumping chamber.

Animal data. There’s a large pile of early lab work in mice and larger animals modeling heart failure, injury from restored blood flow, kidney disease, brain degeneration, and the mitochondrial decline that comes with aging. This work supports the mechanism and is what justified moving into human trials — but, as those human trials show, it has carried over to people inconsistently.

What’s missing. There’s no placebo-controlled confirmation yet of the Barth strength benefit (that confirmation is a requirement the company must meet after marketing begins). There’s no successful Phase 3 trial in any common condition. And there is zero controlled human evidence for anti-aging, general energy, athletic performance, or thinking and memory — the very uses pushed in the gray market.

Safety and side effects

In the clinical trials, elamipretide was generally well tolerated, and the most common side effect was injection-site reactions — redness, hardening, bruising, itching, and pain — usually mild to moderate. The approved FORZINITY label lists injection-site reactions as the most common adverse reactions and notes the product contains benzyl alcohol (a preservative that’s a concern in newborns); no boxed warning (the FDA’s strongest safety alert) was found.

Two important caveats. First, long-term safety data are limited, because the groups studied were small, people took the drug for relatively short periods, and most were rare-disease patients. Second, safety in otherwise healthy people using gray-market “research” product is simply unknown. That gray-market material is not the approved drug — it isn’t made to pharmaceutical standards, and its purity, strength, and possible contaminants aren’t verified.

Legal and regulatory status

On September 19, 2025, the FDA granted accelerated approval to FORZINITY (elamipretide HCl) injection, cleared to improve muscle strength in adult and pediatric Barth syndrome patients weighing at least 30 kg. It’s the first FDA-approved treatment for Barth syndrome and the first FDA-approved drug aimed at mitochondria. Keeping that approval depends on a confirmatory trial.

Getting there was contested, which matters for credibility. An FDA advisory committee voted only 10–6 (October 10, 2024) that the drug was effective for Barth. The agency then sent a Complete Response Letter (a formal “not yet” — May 15, 2025) before the company resubmitted (August 15, 2025) based on the knee-strength intermediate measure, after which accelerated approval followed.

Outside the US, there’s no approval anywhere — no EMA, MHRA, or other regulator has cleared it. And outside Barth syndrome, every use is off-label or unapproved. The “research peptide” SS-31 sold online is not FORZINITY.

On anti-doping, the status is genuinely murky. Elamipretide isn’t named individually on the WADA Prohibited List. Before the 2025 approval, it likely fell under category S0 (substances not approved by any health authority). Now that it has FDA approval, the S0 definition arguably no longer fits — yet it isn’t listed in any other category either. An athlete should check directly with their anti-doping authority and consider a therapeutic use exemption if it’s prescribed.

Bottom line

SS-31 / elamipretide is a real drug with a sensible mechanism, a deep foundation of animal data, and genuine human trials — which is more than most compounds on this site can say. But the verdict from those human trials is sobering: the large pivotal studies in mitochondrial myopathy, heart failure, and dry AMD all failed their main goals, and the one approval is a narrow, accelerated, ultra-rare-disease use built on an open-label strength signal in about ten patients, still awaiting confirmation. For Barth syndrome there’s now regulator-vetted human evidence. For everything else the data are preliminary at best, and for anti-aging or performance there’s no credible human evidence at all. Nothing here is medical advice.

Evidence grade: 6/10 · Preliminary. (and “Unsupported” (grade 1) for any longevity or performance use).

Sources

• Mechanism and structure review, Int J Mol Sci, 2025 (PMC11816484)
• Same review (publisher version, MDPI)
• TAZPOWER randomized phase (negative on primary endpoints) (PMC7935714)
• TAZPOWER 168-week open-label extension, Genetics in Medicine, 2024
• MMPOWER-3 Phase 3 (negative), Neurology, 2023 (PMC10382259)
• MMPOWER-3, Neurology (publisher version)
• MMPOWER-3 post-hoc subgroup analysis (PMC11583740)
• ReCLAIM-2 (dry AMD, missed primary endpoints), Ophthalmology Science, 2024
• ReCLAIM-2 (PMC11599447)
• EMBRACE-STEMI results (negative) — American College of Cardiology
• EMBRACE-STEMI trial design (PubMed)
• FDA grants accelerated approval, first treatment for Barth syndrome (Sept 2025)
• FDA approval package, FORZINITY (NDA 215244)
• FDA advisory committee meeting (Oct 10, 2024, 10–6 vote)
• Stealth BioTherapeutics FORZINITY approval announcement
• Regulatory timeline — The Cardiology Advisor
• US-only approval context — Labiotech
• WADA Prohibited List
• WADA Prohibited List context — BSCG
• Discovery and history — Weill Cornell Medicine