SS-31 (Elamipretide): What It Is, What the Trials Showed, and Its Legal Status

SS-31 (also written SS31 or SS 31), also called elamipretide, is a small lab-made peptide used for weight loss, and designed to stick to the inner wall of mitochondria, the parts of your cells that turn food into usable energy.

As of October 2026, it’s FDA-approved as Forzinity, a prescription drug for Barth syndrome, a rare inherited disease that weakens the heart and muscles, in patients who weigh at least 30 kg (about 66 pounds) (FDA). It isn’t approved for energy, aging, or anything else.

That makes SS-31 the most clinically tested “mitochondrial peptide” you’ll hear about — it has real human trials and one real FDA approval, which almost no other peptide in the biohacking world can claim. But its bigger trials in other conditions mostly missed their main goals. Here’s what it is, what those trials actually found, and what you’re really getting depending on where it comes from.

What is SS-31 (elamipretide)?

SS-31 is a short chain of four amino acids (the building blocks of proteins) that a drug company developed to target mitochondria directly. The company behind it is Stealth BioTherapeutics, which holds the approval for Forzinity (FDA).

Also known as: SS-31, SS31, elamipretide, MTP-131, Bendavia, and Forzinity (the brand name of the approved drug).

The “SS” stands for Szeto-Schiller, after the two researchers who discovered this family of peptides: Hazel Szeto at Weill Cornell Medical College and Peter Schiller at the Montreal Clinical Research Institute (IRCM) (IRCM). SS-31 grew out of a compound from Schiller’s lab that turned out to collect inside mitochondria, and IRCM and Cornell share patents on these compounds (IRCM).

Regulatory status: Elamipretide (SS-31) is FDA-approved only as Forzinity, to improve muscle strength in people with Barth syndrome who weigh at least 30 kg, under accelerated approval, as of October 2026 (FDA label). It is not approved for any other use. For live updates, see our Regulatory Tracker.

What does SS-31 do? Mitochondria and cardiolipin

SS-31 is built to latch onto cardiolipin, a fat molecule in the inner wall of mitochondria that holds the energy-making machinery in place (Szeto lab, Br J Pharmacol 2014). The idea is that by protecting cardiolipin, SS-31 helps that machinery keep running smoothly when cells are under stress.

In lab experiments on mitochondria pulled out of cells, SS-31 raised oxygen use and boosted output of ATP, the molecule your cells spend as energy currency (Szeto lab, Br J Pharmacol 2014).

The longevity buzz comes mostly from mice. In older mice treated long-term, SS-31 was linked to better heart function and healthier aging (mouse study, PubMed 36250163). In one of the best-known studies, 8 weeks of SS-31 given to 2-year-old mice (elderly, for a mouse) improved how well their hearts relaxed and refilled, moving it back toward young-mouse levels; the benefit held 2 weeks after stopping but had dropped by about half by 4 weeks (eLife 2020). Those results are real and are a big reason people are curious about it, but they come from mice, and no controlled human trial has yet shown a matching lasting benefit for aging (see below).

So the honest summary of the mechanism is this: there’s a clear, well-studied theory of how SS-31 works inside mitochondria. The closest human evidence is one small study in older adults, where a single dose briefly raised the energy output of their muscle mitochondria (more below). Whether SS-31 produces a lasting “repair your mitochondria” effect in a healthy human hasn’t been shown.

SS-31 benefits: what the human trials actually found

Short answer: the only human benefit strong enough to earn FDA approval is a strength gain in Barth syndrome, and even that result is still being confirmed. Smaller studies have turned up real early signals in other conditions, but its larger controlled trials missed their main goals. Here’s the scorecard.

A quick vocabulary note before the table. A trial’s “main goal” (its primary endpoint) is the one result the researchers committed to measuring in advance, which is the fairest test of whether a drug works. A “placebo” is a dummy treatment that lets researchers compare the drug against nothing. A result is “statistically significant” when it’s unlikely to be a fluke of chance, by the cutoff researchers set in advance.

Mitochondrial myopathy: an early win that didn’t hold up

The first placebo-controlled trial, MMPOWER, gave 36 adults SS-31 or placebo for 5 days. The highest-dose group walked about 64 meters farther on a 6-minute walk test, versus about 20 meters on placebo, a result that just missed statistical significance (Neurology 2018). That’s a fair point in SS-31’s favor, but the trial was small and short.

The follow-up, MMPOWER-2, had 30 adults take SS-31 for 4 weeks and placebo for 4 weeks, in random order. Walking distance again didn’t clear the bar for statistical significance, but participants reported noticeably less fatigue on the drug (J Cachexia Sarcopenia Muscle 2020). The Stealth-funded researchers themselves treated the fatigue result as exploratory, a lead to test in a bigger trial (same study).

That bigger trial was MMPOWER-3. It followed 218 adults for 24 weeks, and SS-31 didn’t beat placebo on either walking distance or fatigue (Neurology 2023).

A 2024 re-analysis looked at people whose disease comes from defects in the genes that copy mitochondrial DNA (most often a gene called POLG). They trended toward walking farther than those on placebo, and one smaller group within them, people whose eye muscles were also weakening, improved clearly (Orphanet J Rare Dis 2024). That’s worth knowing. Stealth has also run a Phase 3 trial, NuPower, in patients with this kind of genetic cause; the registry lists it as completed, but we found no published results (ClinicalTrials.gov). The re-analysis itself is post-hoc, meaning researchers went back after the trial ended and sliced the data into subgroups, and that kind of look can point to a new question without proving the drug works.

Barth syndrome: real gains, with real caveats

In TAZPOWER, 12 people with Barth syndrome were first compared against placebo in a randomized phase, meaning a coin-flip decided who got the drug first. That phase missed its main goals (FDA Snapshot).

Patients then continued into an open-label extension, where everyone got SS-31 and knew it. In that company-sponsored extension, strength and heart function improved and held up over 168 weeks (Genet Med 2024). That’s real data from a disease with no other approved treatment. It also had no placebo group for comparison, only 8 patients remained at week 168, and it was run by the company that makes the drug, so the FDA approved it on the condition that it be confirmed (more below).

Supporters point to a second analysis that compared the 8 treated patients against 19 untreated Barth patients tracked over time. In that Stealth-funded study, which included Stealth employees as authors, the treated group walked about 80 to 90 meters farther and gained more muscle strength than the untreated group (Orphanet J Rare Dis 2022). FDA reviewers judged it couldn’t prove the drug worked: patients who know they’re on a drug may simply try harder on effort-based tests, and the untreated group wasn’t randomly chosen (FDA briefing document).

Healthy older adults: a brief, measurable effect

In a 2021 placebo-controlled study funded by Stealth BioTherapeutics (with additional NIH grant support), in which three authors had served as paid consultants to the company, a single infusion of SS-31 in 39 healthy 60- to 85-year-olds briefly raised their muscles’ energy-making capacity, measured by scans inside the living muscle (PLoS One 2021; disclosures). That’s a genuine, measurable effect in people, which is more than most “longevity” peptides have. But it didn’t improve muscle fatigue, and the effect was gone by day 7 (PLoS One 2021).

Heart failure and eye disease

In people with heart failure, where the heart pumps too weakly, an early 36-patient safety study found that a single infusion at the highest dose tested reduced the volume of the heart’s main pumping chamber right after treatment, which the authors called a favorable change (Circ Heart Fail 2017). But when the 71-patient PROGRESS-HF trial tested 4 weeks of treatment, SS-31 didn’t improve that same chamber-size measure, its main goal, though it was well tolerated (J Card Fail 2020).

Dry age-related macular degeneration (dry AMD) is a common cause of vision loss in older adults, where the center of the retina slowly wears away. In the ReCLAIM-2 trial of 176 patients, SS-31 missed its main goals, which measured vision in dim light and the growth of damaged retina (Ophthalmology Science 2024). As a secondary result, scans showed about 43% slower loss of the retina’s light-sensing layer compared with placebo, and that scan measure is now the main goal of a Phase 3 trial (ReCLAIM-2 paper; ClinicalTrials.gov). Until that trial reports, the eye benefit is unproven.

What’s still being tested

• ReNEW (Phase 3, dry AMD): active and no longer recruiting, with primary completion expected in August 2027 (NCT06373731).
• Barth syndrome confirmatory study (Phase 4): recruiting (NCT07531251).
• Healthy aging and physical function (Phase 2): recruiting (NCT07275424). So it’s not accurate to say SS-31 has never been tested for healthy aging — it’s being tested now, with no results yet.

The verdict: SS-31’s only FDA-accepted human benefit is a strength gain in Barth syndrome, approved early, with confirmation still required. Everything else — energy, aging, heart failure, eye disease, mitochondrial myopathy overall — has early signals in small or secondary results, but is unproven or negative in the larger controlled trials so far.

Is SS-31 FDA-approved? What Forzinity’s approval does and doesn’t mean

Yes, but narrowly. On September 19, 2025, the FDA granted Forzinity (elamipretide) accelerated approval, which is an approval based on an early sign that a drug works, given on the condition that the company runs another trial to confirm the benefit (FDA). The early sign here was improved strength in the knee-extensor muscles, the muscles on the front of your thigh that straighten your leg (FDA). Stealth must run that confirmatory trial for the approval to stand.

The label limits use to people with Barth syndrome who weigh at least 30 kg (FDA label). Before that decision, an FDA advisory committee — a panel of outside experts the agency consults — reviewed elamipretide on October 10, 2024, and voted 10 to 6 that it was effective for Barth syndrome (FDA meeting minutes). The “yes” voters cited large effects that seemed to last over time and the flexibility FDA can show for a rare disease. The “no” voters pointed out that the placebo-controlled phase showed no effect and that the rest of the evidence couldn’t settle the question (FDA meeting minutes).

That approval matters. It means SS-31 has been through real regulatory scrutiny, which puts it in a different league from peptides supported only by animal studies. But it applies to one product, for one disease.

Key distinction: Forzinity is the approved drug — made under FDA oversight, with a verified dose and a label listing its known risks, for Barth syndrome only. “SS-31” sold online uses the same molecule’s name but carries none of those guarantees. The approval covers Forzinity, not every vial labeled SS-31.

If you want the bigger picture of how these categories work, see compounding-eligible vs. FDA-approved and how the FDA classifies peptides.

SS-31 side effects and safety

The most common side effect in the Barth syndrome trial was a reaction where the shot goes in. During the placebo-controlled phase of that 12-person trial, every patient on the drug had redness at the injection site, compared with 25% on placebo, and 75% had injection-site pain, compared with 42% on placebo (FDA Snapshot).

The FDA also notes some allergic reactions serious enough to need emergency care (FDA Snapshot). The label says Forzinity shouldn’t be used by anyone with a serious allergy to any of its ingredients (FDA label). It also contains benzyl alcohol, a preservative that can be dangerous for newborns, so it isn’t for use in newborn babies (FDA label).

In the larger trials, the main reported side effects were also injection-site reactions (MMPOWER-3; ReCLAIM-2). The MMPOWER-3 researchers described the drug as well tolerated, with most side effects mild to moderate (MMPOWER-3 abstract). In ReCLAIM-2, which ran 48 weeks, 86% of people on SS-31 reported a side effect of some kind, versus 71% on placebo, and the most common were injection-site reactions such as itching, pain, bruising, or redness (ReCLAIM-2 paper).

Here’s what the safety data doesn’t cover. The trials above enrolled people with specific diseases, plus one single-dose study in healthy older adults, so we found no published data on long-term use in healthy people. None of these findings tell you anything about products sold as “research chemicals,” whose contents haven’t been checked by the FDA.

Is SS-31 legal to buy? Prescription vs. “research use only”

SS-31 reaches people through three very different channels, and they don’t come with the same protections.

A doctor’s prescription, filled at a pharmacy. This is how Forzinity is sold, for Barth syndrome (FDA). You get FDA-reviewed manufacturing, a verified dose, a label with known risks, and a prescriber and pharmacist who are accountable — but no proof it helps anything other than Barth syndrome.

Clinics and compounding pharmacies. Some telehealth and longevity clinics advertise compounded SS-31, a version mixed by a pharmacy rather than made by the drug’s manufacturer — a claim we haven’t verified. A compounded product is not Forzinity and isn’t FDA-approved. A licensed pharmacist is accountable for what’s in the vial, but its purity and strength aren’t verified by the FDA the way an approved drug’s are. Some compounding pharmacies also hold outside accreditation, such as PCAB, which checks their processes against national quality standards for compounded drugs (ACHC); that’s a sign of process quality, not FDA approval.

“Research use only” websites. These sell SS-31 directly to consumers with no prescription and no medical oversight. That means no FDA manufacturing oversight, no guarantee the vial contains what the label says, no one supervising your use, and no legal protection if something goes wrong. On August 24, 2026, the FDA sent a warning letter to one such seller naming “SS-31 (Elamipretide)” as an unapproved new drug — the FDA’s term for a product marketed for human use without approval — and rejected the “research use only” label as a defense (FDA warning letter; our Oct 1 news roundup).

Some research-use vendors publish independent third-party lab results, and that’s more than many sellers offer. A 2026 analysis of more than 6,000 such lab reports for 14 other popular peptides found the typical sample was close to its labeled purity and amount, yet 42% to 71% failed quality cutoffs the authors modeled on pharmacy and drug-manufacturing standards, and fewer than 4% had been tested for bacterial toxins at all (preprint, not yet peer-reviewed). It didn’t include SS-31. Either way, a lab certificate doesn’t make a product verified, safe, legal, or the same as Forzinity.

The Alliance for Pharmacy Compounding, a trade group whose members would profit from more peptide compounding, argues that prescription-based compounding is a safer route than research-use sellers because it requires a prescription, quality standards, and inspections (APC letter to FDA, July 2026). That’s a fair description of what the compounding channel requires, but it’s an industry position, and the letter doesn’t address SS-31 specifically. For the broader picture, see are peptides legal? and the sourcing section of our energy peptides hub.

SS-31 vs. MOTS-c

Both get called “mitochondrial peptides,” but they’re different things. SS-31 is a lab-designed drug that sticks to the mitochondrial wall, with human trials and one FDA approval. MOTS-c is a peptide your own mitochondria make, studied mostly in mice so far. Their FDA status differs too: MOTS-c sits in the withdrawn table of the FDA’s Category 2 list of compounding ingredients flagged for possible safety risks, and it was discussed at the July 2026 meeting of the PCAC, the FDA’s outside advisory panel on compounding. For a side-by-side, see our energy peptides hub, and for SS-31 in the context of aging, our longevity hub.

Key sources

1. U.S. Food and Drug Administration. “FDA Grants Accelerated Approval to First Treatment for Barth Syndrome.” September 19, 2025. https://www.fda.gov/news-events/press-announcements/fda-grants-accelerated-approval-first-treatment-barth-syndrome
2. Forzinity (elamipretide) prescribing information, Drugs@FDA, revised 9/2025. https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/215244s000lbl.pdf
3. FDA Drug Trials Snapshot: Forzinity. https://www.fda.gov/drugs/drug-trials-snapshots/drug-trials-snapshots-forzinity
4. Karaa A, et al. MMPOWER-3 Phase 3 trial of elamipretide in primary mitochondrial myopathy. Neurology 2023;101(3):e238. doi:10.1212/WNL.0000000000207402. https://pubmed.ncbi.nlm.nih.gov/37268435/
5. Thompson WR, et al. Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER. Genetics in Medicine 2024;26(7):101138. https://pubmed.ncbi.nlm.nih.gov/38602181/
6. ReCLAIM-2 Phase 2 trial of elamipretide in dry age-related macular degeneration. Ophthalmology Science 2024;5(1):100628. https://pubmed.ncbi.nlm.nih.gov/39605874/
7. Roshanravan B, et al. Single dose of elamipretide in older adults. PLoS One 2021. https://pubmed.ncbi.nlm.nih.gov/34264994/
8. ClinicalTrials.gov. ReNEW Phase 3 trial (NCT06373731). https://clinicaltrials.gov/study/NCT06373731
9. FDA warning letter naming SS-31 (Elamipretide), August 24, 2026. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/warning-letters/royal-peptides-llc-734884-08242026
10. FDA Cardiovascular and Renal Drugs Advisory Committee, final summary minutes, October 10, 2024 (elamipretide, NDA 215244). https://www.fda.gov/media/187179/download

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