MuscleLedger

SS-31 (Elamipretide) Became an Approved Drug in 2025 — for Muscle Strength, in a Disease Almost Nobody Has

Alone among the compounds sold to lifters, SS-31 now holds an FDA approval whose endpoint is literally muscle strength. Reading what that approval covers is more sobering than the headline.

Leon H · Published 2026-09-08

Every so often a compound sold in the grey market turns out to have a real drug behind it. SS-31 is that case, and the details are more interesting — and more limiting — than either its sellers or its sceptics tend to report.

This is research journalism. It contains no usage guidance, this site publishes no dose figures, and dosing questions are answered by Peptifact rather than here.

It is not a secretagogue and not a hormone

SS-31, developed as MTP-131 and Bendavia and now approved as elamipretide, is a small aromatic-cationic tetrapeptide that concentrates in the inner mitochondrial membrane and binds cardiolipin — the phospholipid that holds the respiratory chain in working order. FDA's own label calls it a "mitochondrial cardiolipin binder."

That mechanism has nothing to do with growth hormone. It shares a shelf with ipamorelin and CJC-1295 because vendors sell it there, not because it belongs to the secretagogue class. The pitch made for it — more cellular energy, better endurance, better recovery — follows from the mechanism rather than from the hormone story, which makes it a genuinely different claim to evaluate.

The approval, and its four limits

On 19 September 2025, FDA approved FORZINITY (elamipretide hydrochloride injection) under NDA 215244 to Stealth BioTherapeutics. From the label:

FORZINITY is indicated to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg.

Read literally, an approved drug in this market now carries a muscle strength indication. No other compound covered by this publication can say that. It is worth stating plainly before the qualifications, because it is true and it is unusual.

Now the qualifications, all four of which are on the label itself:

  1. The population is a genetic disease. Barth syndrome is an ultra-rare X-linked disorder in which a defect in cardiolipin remodelling impairs mitochondrial function, causing cardiomyopathy, neutropenia and skeletal muscle weakness. It is precisely the disease this drug's mechanism was built for.
  2. It is an accelerated approval. The label states the indication "is approved under accelerated approval based on an improvement in knee extensor muscle strength, an intermediate clinical endpoint."
  3. The approval is conditional. "Continued approval for this indication may be contingent upon verification and description of clinical benefit in a confirmatory trial."
  4. A weight floor is written into the indication — at least 30 kg — which is the kind of detail that only appears when a paediatric population is central to the trial.

An accelerated approval on an intermediate endpoint in an ultra-rare disease is a meaningful regulatory achievement and a narrow one. Restoring strength that a cardiolipin defect has removed is not evidence of adding strength to a person whose cardiolipin is fine.

The muscle trial in older adults

This is the trial nobody selling SS-31 mentions, and it is the closest thing to a test of the performance claim.

NCT02245620 — "A Phase 2 Study to Evaluate the Impact of MTP-131 (Bendavia) on Skeletal Muscle Function in Elderly." Sponsor Stealth BioTherapeutics. 41 participants. Ran January 2015 to July 2016. Results are posted to the registry, which puts it ahead of most trials in this market.

The primary endpoint was change from baseline in ATPmax, the maximal ATP synthetic rate per unit muscle volume measured by a muscle fatigue test — a direct measurement of mitochondrial capacity in working muscle, and exactly the right endpoint for this mechanism.

Timepoint Elamipretide Placebo
Day 1, 2 hours (change in ATPmax, mM/sec) 0.17 (SD 0.196) 0.12 (SD 0.244)
Day 7 (change in ATPmax, mM/sec) 0.09 (SD 0.142) 0.06 (SD 0.172)
Day 7, phosphate/oxygen ratio 0.32 (SD 0.473) 0.51 (SD 0.788)

Both ATPmax differences favour the drug and both are smaller than their own standard deviations. On the secondary measure of mitochondrial coupling efficiency, the placebo group moved further than the treated group at day 7.

We are not calling that a definitive negative and neither should anyone else: 41 participants, a one-week exposure, and wide variance. What it is, is the only published measurement of what this compound does to skeletal muscle energetics in ordinary older humans, and it did not separate.

The rest of the programme

Elamipretide has been through a real development pipeline, and reporting it honestly means reporting both directions:

  • 21 registrations on ClinicalTrials.gov, retrieved 2026-09-08, including a phase 3 in primary mitochondrial disease from nuclear DNA mutations (NCT05162768, 102 participants, six-minute walk test as primary endpoint, completed September 2024), a second phase 3 still active (NCT06373731, 313 participants), and phase 2 programmes in cardiac reperfusion injury (300 participants), heart failure (308), and dry age-related macular degeneration (176).
  • 522 PubMed records, of which 16 are indexed randomised controlled trials.
  • An expanded-access protocol, which is the mechanism used to supply an investigational drug to patients outside a trial — a marker of a serious programme in a disease with no alternatives.

Compare that to what exists for the compounds it is sold beside: ipamorelin's three registrations, CJC-1295's one terminated registration. On the evidence ledger, SS-31 is in a different tier from its shelf-mates — but the tier is "well-studied in disease", not "shown to work in athletes".

The one trial that would answer the question

NCT07275424 — "Study of Healthy Aging and Physical Function With Elamipretide", 30 participants, sponsored by an academic investigator rather than the manufacturer, recruiting since 26 November 2025 with a primary completion date of 15 March 2026. Its primary endpoint is the number of participants with treatment-related adverse events over four weeks of daily subcutaneous injections.

It is a safety and tolerability study in healthy ageing, not a performance trial, and 30 participants will not settle a functional question. It is nevertheless the closest anyone has come, twenty years into this compound's life, to asking what it does to a person who is not ill.

What to take from this page

SS-31 is the most legitimate compound in this market and the one whose legitimacy transfers least. Its approval is real and its endpoint is muscle strength; the disease it was approved for affects a very small number of people through a mechanism healthy lifters do not share. Its skeletal-muscle trial in ordinary older adults produced no separation. Its trial in healthy people is thirty participants long and is about safety.

The pattern is the same one that runs through this whole category and is set out on the secretagogue class page and in our comparison with anabolic steroids: the mechanism is well described, the disease evidence is sometimes strong, and the trained-adult evidence does not exist.

Sources

  • FDA, Drugs@FDA record for FORZINITY (elamipretide hydrochloride), NDA 215244, Stealth BioTherapeutics, original approval 2025-09-19.
  • FORZINITY prescribing information, DailyMed SPL set id 146bf34c-76f2-48db-ac07-fb29cce2cd75, published 2025-12-12 — Indications and Usage.
  • ClinicalTrials.gov NCT02245620 results section (ATPmax and P/O ratio outcome measures), NCT05162768, NCT06373731, NCT07275424, all retrieved 2026-09-08.
  • PubMed record counts for elamipretide, SS-31 and MTP-131, retrieved 2026-09-08.

Frequently asked questions

Is SS-31 an FDA-approved drug?

Yes, as of 19 September 2025, under the brand name FORZINITY and the generic name elamipretide. That makes it a genuine outlier in this market — nearly every compound sold alongside it is an unapproved drug. The approval is narrow in three ways at once: it covers one ultra-rare genetic disease, it is an accelerated approval granted on an intermediate endpoint rather than a demonstrated clinical benefit, and the label states that continued approval may depend on a confirmatory trial.

Does the approval mean SS-31 builds muscle?

It means a regulator accepted evidence that it improves knee extensor strength in people with Barth syndrome, a mitochondrial disease that impairs muscle function through a specific defect in cardiolipin. Restoring function that a genetic defect has taken away is a different proposition from adding function to a healthy trained adult, and the label makes no claim about the second. The trial that would test the second has been registered only recently and is small.

What did the trial in older adults find?

Not much separation. A 41-participant phase 2 study measured maximal ATP synthetic rate in skeletal muscle — a direct measure of mitochondrial capacity, and the right endpoint for the mechanism. The change on elamipretide was 0.17 mM/sec against 0.12 on placebo two hours after infusion and 0.09 against 0.06 at day 7, with standard deviations two to three times the size of the differences. On the secondary measure of mitochondrial coupling, placebo moved further than drug at day 7. This was a short exposure in a small sample, so it is not a definitive negative — but it is what exists.

How does SS-31 differ from the peptides it is sold beside?

Mechanically, entirely. Ipamorelin, CJC-1295 and the rest act on the growth hormone axis — they make the pituitary release GH. Elamipretide does nothing to that axis. It is a small peptide that concentrates in the inner mitochondrial membrane and binds cardiolipin, the phospholipid that organises the respiratory chain, with the aim of restoring energy production in cells whose mitochondria are failing. It is an energetics drug, not a hormone drug.

Is there a trial of SS-31 in healthy people?

One is running. NCT07275424, sponsored by an academic investigator rather than the manufacturer, began recruiting in November 2025 to study healthy ageing and physical function in 30 participants, with safety and tolerability of four weeks of daily injections as the primary endpoint. A 30-participant safety study is the beginning of an answer, not an answer, and it is not powered to detect a performance effect.

What is the strongest argument for taking SS-31 seriously?

That it is the only compound in this market with a real, completed drug-development programme behind it — 21 registrations, 16 randomised trials, a phase 3, a marketing approval, and a manufacturer that took it through the FDA rather than to a research-chemical website. The strongest argument against extrapolating from that is contained in the same record: that programme also produced failures, and the population it succeeded in is defined by a genetic defect that the buyers of research-grade SS-31 do not have.