What is SS-31 (Elamipretide)? What it does, benefits and what the evidence shows

SS-31 is a synthetic tetrapeptide, also known by its drug name elamipretide, that concentrates in the inner mitochondrial membrane and binds cardiolipin, a lipid involved in energy production. It was developed by Stealth BioTherapeutics (recently renamed Mighty Therapeutics). In September 2025, FDA granted accelerated approval to the company's drug product, Forzinity, to improve muscle strength in Barth syndrome, an ultra-rare genetic mitochondrial disease. The sponsor calls it the first approved mitochondria-targeted therapeutic, and it is the only approval we found for this molecule. It is not approved for aging, energy, exercise performance, or any of the broader uses it's marketed for on the grey market, and several other trials of it, in other conditions, missed their main goals.

What SS-31 is and how it works

SS-31 belongs to a class of amphipathic "Szeto-Schiller" peptides designed to accumulate in mitochondria without needing a transporter, reportedly reaching concentrations 1,000 to 5,000 times higher inside the mitochondrial inner membrane than outside the cell (Mitchell et al., Journal of Biological Chemistry, 2020). That paper's central finding is that SS-31's binding is driven mainly by the membrane's surface charge rather than a specific lock-and-key fit to cardiolipin itself, and that it "modulated the surface electrostatics of both model and mitochondrial membranes" and reduced calcium buildup in that same membrane region. FDA's own approved product label puts the mechanism plainly: Forzinity "is a mitochondrial cardiolipin binder that localizes to the inner mitochondrial membrane and improves mitochondrial morphology and function" (DailyMed, FORZINITY label).

That mechanism is why SS-31 has been studied across a range of conditions where mitochondrial energy production is thought to break down — muscle disease, heart failure, kidney injury, eye disease and aging research generally — but a plausible mechanism in isolated mitochondria is not the same as a proven clinical benefit, and most of that broader program has not produced a second approval.

Who developed it, and what the approval actually covers

Stealth BioTherapeutics has run essentially the entire clinical program behind SS-31/elamipretide; it's not a molecule with multiple competing developers the way some peptides are. The company rebranded to Mighty Therapeutics after the Barth syndrome approval, but the sponsor of record on the approved drug, Forzinity, and on the trials described below, is Stealth BioTherapeutics Inc. (Stealth BioTherapeutics, accelerated approval announcement).

It's worth being precise about the size of what FDA actually approved. Barth syndrome is described in the sponsor's own release as affecting approximately 150 people in the entire United States — an ultra-rare disease, not a broad diagnosis. FDA's approval covers improving muscle strength in that specific, genetically confirmed population weighing at least 30 kg; it does not cover general mitochondrial support, energy, aging, or any use outside that one disease. The company also holds Orphan Drug, Fast Track, Priority Review, and Rare Pediatric Disease designations tied specifically to Barth syndrome, not to elamipretide's other research programs.

What people use it for

What the research actually shows

Barth syndrome — the one approved use

FDA's accelerated approval is based on improvement in knee extensor muscle strength — the strength of the muscle used to straighten the leg at the knee — which the agency determined is "reasonably likely to predict patient benefit, such as an ability to stand more easily or walk farther" (FDA press announcement, September 19, 2025). That basis came from the open-label portion of the company's Phase 2 TAZPOWER trial: participants showed gains of more than 45% in that muscle-strength measure, which the sponsor says "significantly correlated with gains in the six-minute walk test" (Stealth BioTherapeutics, NDA resubmission announcement). This approval covers only patients weighing at least 30 kg with genetically confirmed Barth syndrome, and it's conditional: FDA's letter and the sponsor's own release both note that continued approval depends on a required confirmatory trial verifying real clinical benefit, and the company announced on July 8, 2026 that the first patient had been dosed in that Phase 4 study, called 4TAZPower (Mighty Therapeutics announcement) (FDA; Stealth BioTherapeutics, accelerated approval announcement).

It's worth knowing this wasn't a clean, first-pass approval. FDA had previously issued a complete response letter on this same application in May 2025, after a review that included an advisory committee vote, before the agency and the company agreed on the muscle-strength endpoint as an accelerated-approval path and the company resubmitted in August 2025 (Stealth BioTherapeutics, resubmission announcement).

Primary mitochondrial myopathy — the larger trial that missed

Outside Barth syndrome, elamipretide's best-tested use is primary mitochondrial myopathy, a broader category of inherited mitochondrial muscle disease. In MMPOWER-3, a 218-person, placebo-controlled Phase 3 trial run at 27 sites across seven countries, elamipretide did not separate from placebo on either primary endpoint — six-minute walk distance or a fatigue score — over the full trial period; the published results describe "Class I evidence that elamipretide does not improve the 6MWT or fatigue" in this population as a whole (Karaa et al., Neurology, 2023). The same publication reports the drug was well tolerated, with adverse events mostly mild to moderate. A prespecified subgroup analysis of the same trial found that participants with a nuclear-DNA form of the disease improved on walking distance, and a later post hoc analysis examined genotype-specific effects in more detail. Neither was the trial's primary test, so those findings are hypothesis-generating rather than proof, and they are separate from the Barth syndrome approval, which rests on a different disease and endpoint.

Dry age-related macular degeneration — missed primary endpoints, a secondary signal

In ReCLAIM-2, a Phase 2 trial in geographic atrophy from dry AMD, the company's own results announcement states plainly that "the trial's primary endpoints of mean change in LLVA and GA progression were not met." Secondary analyses did find elamipretide slowed decline of the ellipsoid zone (a mitochondria-rich retinal layer) and produced a "categorical" improvement of more than two lines in low-luminance visual acuity in a subset of patients, and the company describes elamipretide as "generally well tolerated," with a lower rate of new fluid buildup in the study eye than on placebo (Stealth BioTherapeutics, ReCLAIM-2 data announcement). A secondary finding in a Phase 2 trial that missed its main goals is not the same as proof the drug slows vision loss, and we found no completed Phase 3 confirming it for this use.

Aging and general "cellular energy" claims

We found no published human trial testing SS-31/elamipretide for aging, general energy, or athletic performance in otherwise healthy people. Its mitochondrial mechanism is why it gets discussed alongside compounds marketed for longevity, but that's a mechanistic hypothesis carried over from the approved-disease research, not a tested claim in its own right.

Side effects and risks

In the trials behind the Barth syndrome approval and the mitochondrial myopathy study, the most commonly reported side effects were injection-site reactions — redness, pain, firmness, itching, bruising and hives at the injection site — which FDA's label says are typically managed with oral antihistamines or topical corticosteroids (Stealth BioTherapeutics, accelerated approval announcement). The approved label also lists increases in eosinophil counts in longer studies, without associated symptoms, and a contraindication for serious hypersensitivity to the drug or its ingredients (DailyMed, FORZINITY label). The approved formulation contains benzyl alcohol as a preservative, and the label specifically states it is not approved for use in neonates because of a known risk of serious toxicity from that ingredient in that age group.

None of that safety data comes from healthy adults using the compound for general wellness — it comes from small trials in people with a specific, serious diagnosis, over trial periods measured in months, not years. There's no long-term safety data in healthy users at all.

There's also a structural gap worth naming directly: Forzinity is manufactured under FDA-inspected conditions, dispensed through a pharmacy, and administered under a physician's supervision for a genetically confirmed diagnosis. A vial sold on the grey market under the research name "SS-31" carries none of that — no confirmed manufacturing standard, no clinical oversight, and no diagnosis-specific dosing rationale behind it, because the approval itself doesn't extend to any use outside Barth syndrome. The trial safety data above describes what happened to a small, closely monitored patient population, not what happens when the same peptide is self-administered outside medical supervision for an unapproved reason.

On the grey-market side, our figures below show SS-31 sold outside a prescription is mostly measuring close to label on both purity and quantity, though with more spread on dose than the tightest compounds in our data, and 1 of 17 vials failed our screen outright.

What the lab tests show

As of September 30, 2026, our database includes 17 SS-31 vials from 10 vendors, drawn from 14 lab reports dated January 2025 through September 2026. 16 vials had a purity result; 14 came in at 99% or higher, 0 measured under 95% pure, and the median purity across them was 99.77%. Dose-versus-label ran a median of 3.2% over label, but the spread was wider than the median suggests: 7 of 16 measured vials landed within 5% of label, while 0 measured more than 20% under label and 4 measured more than 20% over. 1 of the 17 vials failed our definition outright (empty, under 95% pure, more than 20% under label, or more than 50% over).

That's a modest sample — a single vendor's results can move these numbers noticeably — and the spread on dose is worth paying attention to given that the median (+3.2%) looks tighter than the range of individual results actually is. Here's how the ranked vendors for SS-31 look right now; the table shows current verdicts and flags any vendor whose failed-vial rate is above 5%:

Buying smarter

If you're evaluating a specific vendor's SS-31, check their full record on our SS-31 results page rather than relying on a handful of reports — a vendor needs five or more lab reports before we rank them, and the spread in our dose data above means the number of reports behind a score matters as much as the score itself. Whatever COA a vendor posts is worth treating as a starting point, not proof: verify it against how to read a peptide COA and check for the red flags in fake peptide COAs.

Given the wider dose spread we're seeing for this compound compared with its tight median, a quantity test on your own vial is worth the money even when a vendor's posted purity numbers look clean. How to get a peptide tested walks through ordering independent identity, purity and quantity testing from a lab.

If your own report comes back with a fail, stop using that batch, tell anyone else who bought from the same run, bring the report to the vendor, and submit it to VendorVerify so the next buyer checking that vendor's SS-31 record has one more data point than you did.

Database figures without a date update automatically from VendorVerify's lab results; dated figures are snapshots. This isn't medical advice. Forzinity (elamipretide) is an FDA-approved prescription drug for a single indication, Barth syndrome, under a licensed physician's care; SS-31 sold outside that context is unregulated and sold for research use only.