SS-31 and MOTS-c are sold side by side as "mitochondrial peptides". The label is accurate as far as it goes — both are associated with mitochondria — and misleading beyond that. On the record, they are about as far apart as two compounds on this site get.
Side by side
| SS-31 (elamipretide) | MOTS-c | |
|---|---|---|
| Origin | Synthetic (designed tetrapeptide) | Natural — encoded in mitochondrial DNA (12S rRNA) |
| Sequence | D-Arg-2,6-dimethyl-Tyr-Lys-Phe-NH2 | MRWQEMGYIFYPRKLR |
| Length / weight | 4 residues / 639.8 g/mol | 16 residues / 2,174.6 g/mol |
| Described mechanism | Binds cardiolipin in the inner mitochondrial membrane (label) | Metabolic signalling via AMPK (research literature) |
| Approval | FDA, 19 Sep 2025 — FORZINITY, Barth syndrome (accelerated) | None |
| Human safety data | Label adverse-reaction table (12 patients) | None by any route (FDA, 2026) |
| ClinicalTrials.gov (3 Oct 2026) | 21 studies | 9, mostly measuring the body's own MOTS-c |
| PubMed (3 Oct 2026) | 468 records, 18 tagged RCTs | 257 records |
Two unrelated mechanisms
FORZINITY's label calls elamipretide "a mitochondrial cardiolipin binder that localizes to the inner mitochondrial membrane and improves mitochondrial morphology and function." Cardiolipin is a lipid; Barth syndrome is a genetic defect in how the body builds it, which is why that disease was where the drug found an approval.
MOTS-c is a different kind of thing: a peptide the mitochondrial genome itself encodes, studied as a metabolic signal. The 2015 paper that named it (Lee et al., Cell Metabolism) reported that its cellular actions inhibit the folate cycle and purine synthesis, "leading to AMPK activation", with skeletal muscle as the apparent main target — in cells and mice. Most human papers measure how much MOTS-c people already carry — after exercise, with age, in diabetes — rather than giving it. Our MOTS-c dosage page sets out that literature and the one registered dosing study.
Nothing about one mechanism predicts the other. Grouping them is a marketing decision.
The regulatory record
SS-31. FDA approved FORZINITY (NDA 215244) on 19 September 2025. The approval is narrow in three ways: one rare disease; patients of at least 30 kg; and accelerated approval on an intermediate endpoint. The randomised crossover trial in 12 patients did not beat placebo on its primary endpoints — six-minute walk distance and total fatigue score. Knee-extensor strength did not improve in the randomised phase either; the gains that supported approval appeared in the open-label extension (median +34 to +68 newtons from a baseline of 124, in 8 to 10 patients). Continued approval depends on a confirmatory trial. The labelled dose and kidney adjustments are on our SS-31 dosage page.
MOTS-c. FDA's evaluation for its July 2026 compounding advisory committee found no clinical studies or human exposure data by any route and called potential risks in humans "unknown". One registered phase 2a study administers MOTS-c, from a sponsor whose eight near-simultaneous registrations raise questions set out on the dosage page; it had posted no results when last checked.
Adverse reactions: a table for one, an absence for the other
FORZINITY's label reports this from its placebo-controlled crossover study:
| Reaction | Elamipretide (n = 12) | Placebo (n = 12) |
|---|---|---|
| Any local administration reaction | 12 (100%) | 8 (67%) |
| Injection-site erythema | 12 (100%) | 3 (25%) |
| Injection-site pain | 9 (75%) | 5 (42%) |
| Injection-site induration | 8 (67%) | 2 (17%) |
| Injection-site pruritus | 8 (67%) | 2 (17%) |
| Injection-site bruising | 3 (25%) | 0 |
| Injection-site urticaria | 3 (25%) | 0 |
The label adds warnings for hypersensitivity, including serious reactions needing emergency treatment, that "may occur within minutes to months after treatment initiation"; frequent rises in eosinophil counts with use of 30 days or more; and benzyl alcohol (20 mg/mL in the solution), with a warning against use in neonates. Twelve patients is a very small safety population, all male, eleven of them Caucasian — rare reactions would not show up.
For MOTS-c there is no table to put beside it. The nearest record is CB4211, a modified MOTS-c analogue, whose trial was paused in 2018 over "persistent painless bumps" at injection sites; our MOTS-c side-effects page covers it.
What the amounts compare to
The doses circulating for the two differ in kind, so a molar comparison is the only common unit:
| Mass | Molar amount | Per week | |
|---|---|---|---|
| FORZINITY label: 40 mg once daily | 40 mg | ≈62.5 µmol | ≈438 µmol |
| MOTS-c protocol-page figure: 5 mg, 2–3 times a week | 5 mg | ≈2.3 µmol | ≈4.6–6.9 µmol |
That is roughly 60 to 95 times fewer molecules of MOTS-c a week. It is arithmetic from the labelled and quoted figures, not a measure of potency — the two act on different targets — but it shows that nothing about one dose can be read across to the other. The MOTS-c figure has no regulator-reviewed origin; its source pages state it has not been validated in a human trial.
What is not established
No study has compared SS-31 and MOTS-c, given them together, or tested either in healthy people for longevity, endurance or body composition — the uses they are marketed for. Elamipretide's safety is characterised only in small disease populations; MOTS-c's not at all. The performance framing of both is covered on MuscleLedger's SS-31 and MOTS-c pages, and the molecules themselves in Peptide Lexicon's SS-31 and MOTS-c entries. Decisions about using either belong with a clinician.
Sources and dates
- FORZINITY (elamipretide) injection, Stealth BioTherapeutics, NDA 215244, label effective 2025-12-10 via openFDA: indications, warnings, adverse reactions (Table 2), clinical studies (Table 3), mechanism, pharmacokinetics, description, read 2026-10-03.
- Drugs@FDA via openFDA, NDA 215244, original approval date 2025-09-19, read 2026-10-03.
- PubChem CID 11764719 (elamipretide) and CID 146675088 (MOTS-c); UniProt A0A0C5B5G6 (MOTS-c sequence), read 2026-10-03.
- PubMed E-utilities counts (elamipretide/SS-31/MTP-131/Bendavia; MOTS-c; with the randomised-controlled-trial publication type) and ClinicalTrials.gov v2 API counts, read 2026-10-03.
- Lee C et al. The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance. Cell Metab 2015;21(3):443–54 (PMID 25738459), abstract read 2026-10-03.
- FDA briefing for the July 2026 Pharmacy Compounding Advisory Committee, MOTS-c evaluation, and the CB4211 record, as read for this site's MOTS-c side-effects page (2026-09-24).
Corrections go to the contact page.