What SS-31 is
SS-31, developed as elamipretide, is a four-amino-acid peptide that concentrates in the inner mitochondrial membrane and binds cardiolipin — a phospholipid unique to that membrane and essential to the structure of the electron transport chain. The theory is that stabilising cardiolipin makes mitochondria run more efficiently, producing more ATP with less oxidative damage.
The mechanism is genuinely elegant and genuinely well supported in laboratory work. What happened next is the useful part.
A rare thing: it was properly tested
Almost every compound on this site suffers from an absence of human trials. SS-31 is the exception. Stealth BioTherapeutics ran a substantial clinical programme across several indications. The results were mixed, and mixed results are far more informative than silence.
What succeeded
In September 2025 the FDA granted accelerated approval to elamipretide, as Forzinity, for Barth syndrome — an ultra-rare X-linked genetic disorder affecting cardiolipin metabolism. The approval was based on improvement in knee extensor muscle strength as an intermediate clinical endpoint, with a confirmatory post-marketing trial required. This made it the first approved therapy directed at mitochondria.
What failed
MMPOWER-3, a phase 3 trial in primary mitochondrial myopathy, missed its primary endpoints on the six-minute walk test.
ReCLAIM, in dry age-related macular degeneration, did not meet its primary endpoints.
Heart failure results have been mixed across the programme. These are not gaps in the evidence — they are well-powered trials that returned negative answers.
Why the failures matter more than the approval
The approval covers an ultra-rare genetic disease in which cardiolipin metabolism is specifically broken. That is the population where a cardiolipin-binding drug would be expected to help, and it did.
SS-31 works where mitochondrial dysfunction is the identified, specific, genetic cause. Where it was tested against broader mitochondrial dysfunction, it largely did not.
The grey market sells SS-31 for energy, performance, longevity and general "mitochondrial health" in people with no diagnosed mitochondrial disease. That population is further from Barth syndrome than the populations in which the drug was tested and failed.
It is difficult to construct an evidence-based argument for that use. The best available human data suggests that when SS-31 was given to people with genuine but non-specific mitochondrial impairment, it did not produce a measurable functional benefit.
Safety
One genuinely reassuring finding: across the trial programme SS-31 was generally well tolerated. Injection-site reactions — redness, pain, induration — were the hallmark adverse effect, alongside headache and nausea. This is a better-characterised safety profile than almost anything else discussed on this site, precisely because real trials were run.
That said, tolerability in supervised trials of a manufactured product is not the same as safety of an unregulated vial used indefinitely without monitoring.
Regulatory position
United States: FDA accelerated approval (September 2025) for Barth syndrome only, with confirmatory trial obligations outstanding.
United Kingdom: not licensed. No UK marketing authorisation for any indication.
Where this leaves you
SS-31 is a real drug with a real approval and a real evidence base. That evidence base says it helps in one specific genetic disease and did not help in several broader conditions that were properly tested. Anyone selling it for general energy or longevity is selling you the mechanism, not the results.