SS-31

SS-31 is elamipretide, and as of 19 September 2025 it is an FDA-approved medicine. The product is Forzinity, and the approved indication is narrow: to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg. Barth syndrome is a rare inherited mitochondrial disease that mainly affects males, often begins with severe heart failure in infancy, and leaves survivors with fatigue, poor stamina and exercise intolerance. Forzinity is the first treatment the FDA has approved for it.

The approval is accelerated, which matters. It rests on improved strength in the knee extensor, the muscle that straightens the leg, a measure the FDA considers reasonably likely to predict real benefit such as standing more easily or walking farther. The agency is requiring a randomized, placebo-controlled confirmatory trial to show that the strength gain actually becomes that benefit. Continued approval may depend on it.

Online, SS-31 is sold as a mitochondrial energy, anti-aging and recovery peptide. None of that is what was approved. Outside Barth syndrome, a 218-person Phase 3 trial in primary mitochondrial myopathy missed both primary endpoints, and the human study in older adults tested a single dose and measured it the same day.

Elamipretide, also called SS-31, MTP-131 and Bendavia in earlier papers, is a synthetic four-amino-acid peptide with the sequence D-Arg-Dmt-Lys-Phe-NH2. Two of those residues, D-arginine and lysine, carry a positive charge, and the inner mitochondrial membrane is rich in negatively charged cardiolipin. That electrostatic pull is how the peptide gets into cells and concentrates inside mitochondria.

Stealth BioTherapeutics developed it and holds the approval. The name people search for, SS-31, and the name on the prescription, Forzinity, refer to the same molecule. A vial labelled SS-31 from a peptide vendor is not the same product, because the approval covers a manufactured drug made and tested to FDA standards, not the sequence.

Cardiolipin Is the Target

Cardiolipin is a phospholipid found almost exclusively in the inner mitochondrial membrane. It holds the folded cristae in shape and supports the electron transport chain complexes that make ATP. When mitochondria are under stress, cardiolipin oxidizes, and energy production and cell survival both suffer.

Elamipretide binds cardiolipin selectively. The proposed consequences, laid out in a 2025 review, follow from that one interaction: more stable cristae, a more efficient electron transport chain, less reactive oxygen species production, and higher ATP output. Downstream, the review describes inhibition of the mitochondrial permeability transition pore and less release of cytochrome c, the signal that starts caspase-driven cell death.

Why Barth Syndrome Was the Right Test

Barth syndrome is caused by defects in the TAZ gene, whose enzyme remodels cardiolipin. The disease is, in effect, a cardiolipin disorder, which makes it the most direct possible match between what the drug binds and what is broken. That fit explains why the approval landed there. It also explains why results in Barth syndrome do not carry over automatically to conditions where cardiolipin is not the primary fault.

Approved does not mean approved for you. Forzinity is approved for one inherited cardiolipin disease, on a strength measure, in people weighing at least 30 kg. Marketing that calls SS-31 "FDA-approved" next to claims about energy or longevity is attaching a real approval to uses it never covered.

The aging study is a lab reading, not a benefit. Thirty-nine older adults got one dose, and a spectroscopy measure of ATP capacity rose for a few hours. It was gone a week later, and the participants did not resist fatigue any better. That is the mechanism showing up in human muscle. It is not evidence that anyone felt or performed differently.

The bigger trials are the ones that failed. MMPOWER-3 was the largest and best-controlled test of the energy idea in people with genuinely impaired mitochondria, and the walk-test difference was effectively zero. A post hoc analysis found a possible benefit in a subgroup with mitochondrial DNA replisome disorders, and a follow-up Phase 3 trial, NuPOWER, is aimed at that group. A subgroup signal found after the fact is a reason to run another trial, which is what is happening, not a result.

The FDA lists mild-to-moderate injection site reactions as the most common side effect in clinical trials, and notes that serious reactions have also been reported. In MMPOWER-3, most adverse events were mild to moderate and the drug was described as well tolerated. A 2025 review of the trial program adds headache, dizziness, nausea, abdominal pain and fatigue as less common events and rare urticaria as the more severe one, with no clinically significant changes in vital signs, labs or ECG, including through the 168-week Barth syndrome extension.

For tested athletes, elamipretide is not on the 2026 WADA Prohibited List. The S0 class covers substances with no current approval for human therapeutic use, and the September 2025 approval took elamipretide out of that class. Research-grade material sold online is still an unapproved product even though the substance itself now has an approval.

Two things are true at once. Elamipretide is a real medicine, with a clear mechanism, an FDA approval and a tolerability record from years of trials. And the approval covers a single rare disease, granted on muscle strength and still waiting on a confirmatory trial.

On-label, the evidence supports this: in people with Barth syndrome, longer-term treatment was followed by better walk distance, strength and cardiac measures, and the FDA judged the strength gain reasonably likely to predict benefit. Off-label, it does not support claims about energy, endurance, recovery or aging. The controlled trials that tested the broader energy idea, in mitochondrial myopathy, heart failure and macular degeneration, missed their primary endpoints.

If you or someone you care for has Barth syndrome, this is a conversation for a specialist, and Forzinity is the product that conversation is about. For anyone else, SS-31 is worth understanding as a well-built drug for a specific disorder, not as something to add to a routine.

Not approved for mitochondrial support, energy or aging. The only approved use is improving muscle strength in people with Barth syndrome who weigh at least 30 kg. Every other use is off-label and untested at the level the approval required.

Not a proven treatment for mitochondrial myopathy, heart failure or macular degeneration. Each was tested in a controlled trial, and each missed its primary endpoint.

Not the same thing as Forzinity when bought as a research peptide. The approval applies to the manufactured, FDA-reviewed product. Vendor material shares a sequence, not a quality standard.

Not a fully confirmed approval yet. Accelerated approval was granted on a strength measure, and the FDA requires a post-approval randomized trial to show that strength translates into patient benefit.

I like this entry because the drug did exactly what a good targeted drug should. It binds a lipid that breaks down in sick mitochondria, and it was approved in the one disease where that lipid is the root problem. The approval in Barth syndrome is the payoff, and I want to give it full credit.

What I will not do is let that credit spread. The same molecule went through a Phase 3 trial in 218 people with mitochondrial myopathy and walked them no farther than placebo. That result is the clearest guide to what SS-31 does for a tired, healthy adult, and it is not encouraging. The research-peptide market quotes the approval and skips the trial.

So my read is simple. It is a legitimate prescription medicine for one disease, with an open question still being answered by a confirmatory trial. For everyone else, the best evidence says the energy story does not hold up, and a vendor vial is not the drug that earned the approval anyway.