SS-31 (elamipretide): benefits, side effects and FDA approval

SS-31 is one of the Szeto-Schiller (SS) peptides, a family of short synthetic peptides designed by Hazel Szeto and Peter Schiller. Their 2004 paper described cell-permeable peptide antioxidants built from alternating aromatic and basic amino acids, with dimethyltyrosine providing antioxidant activity; these peptides concentrated about 1,000-fold in the inner mitochondrial membrane ^[1].

SS-31 was later developed by Stealth BioTherapeutics under the name elamipretide. On 19 September 2025 the FDA granted it accelerated approval, as Forzinity, to improve muscle strength in people with Barth syndrome weighing at least 30 kg ^{[7] [10]}. It is also sold online as a research peptide under the name SS-31, which is not the approved product; for how that market works, see what research peptides are. It is sometimes discussed alongside MOTS-c, another peptide linked to mitochondria, though the two work differently.

Cardiolipin is a phospholipid found almost only in the inner mitochondrial membrane, where it shapes the folded cristae and helps organise the enzyme complexes that make ATP. A 2013 rat study showed that SS-31 binds cardiolipin with high affinity and blocks the peroxidase activity of the cytochrome c–cardiolipin complex, which otherwise damages cardiolipin under stress. Rats pretreated with SS-31 kept intact cristae during kidney ischaemia and recovered ATP more quickly ^[2].

A 2014 review by Szeto summarised this as a “cardiolipin-protective” mechanism: by binding cardiolipin, SS-31 stops cytochrome c acting as a damaging peroxidase while preserving its role in energy production. The review noted that the supporting evidence came largely from animal studies across many disease models ^[3].

The approved label describes elamipretide, whose sequence is D-Arg-2,6-dimethyl-Tyr-Lys-Phe-NH2, as a mitochondrial cardiolipin binder that localises to the inner mitochondrial membrane and improves mitochondrial structure and function ^[9].

In the original 2004 work, SS peptides reduced oxidative cell death in neuronal cells, prevented mitochondrial swelling in isolated mitochondria and improved contractile force in an isolated heart model of reperfusion injury ^[1]. Animal studies since then have covered kidney, heart, muscle and age-related conditions ^[3].

In people, the results have been mixed. In heart failure with reduced ejection fraction, a single four-hour intravenous infusion was safe in 36 patients and, at the highest dose, briefly reduced left ventricular volumes compared with placebo ^[4]. The largest trial, MMPOWER-3, randomised 218 adults with primary mitochondrial myopathy to daily elamipretide 40 mg or placebo for 24 weeks. It did not improve the six-minute walk test or fatigue scores, although the drug was well tolerated ^[5].

The approval is for Barth syndrome, an ultra-rare inherited disorder in which cardiolipin is abnormal. TAZPOWER began with a 28-week randomised, placebo-controlled phase followed by an open-label extension. Ten patients entered the extension and eight reached 168 weeks, with a cumulative improvement of 96.1 metres in six-minute walk distance from the start of the extension and improvements in some heart measurements ^[6]. The extension had no placebo group and very few patients. The US approval is an accelerated approval, a pathway under which continued approval can depend on further studies confirming clinical benefit.

Much of the interest in SS-31 and mitochondrial dysfunction comes from ageing research. In a 2013 study, old mice (27 months) given a single injection of SS-31 at 3 mg/kg into the abdominal cavity had their muscle mitochondrial energy production restored to young levels within one hour, while young mice showed no effect. Their muscle was more resistant to fatigue an hour after the dose, and eight days of treatment increased whole-body endurance ^[11]. These were mouse results with a dose that should not be converted to a human one.

A small phase 2a trial is testing daily subcutaneous elamipretide for four weeks in adults aged 65 to 80, and was recruiting when we checked in September 2026 ^[18]. No results have been published, so there is no human evidence yet that SS-31 slows or reverses ageing.

The first mechanism studies used kidney injury in rats ^[2]. In people, a pilot phase 2a trial gave elamipretide by intravenous infusion (0.05 mg/kg per hour) before and during stenting in patients with severe narrowing of a kidney artery. Six patients received the drug and eight received placebo. Low-oxygen areas in the kidney rose by 47% 24 hours after the procedure with placebo but fell by 6% with elamipretide. Three months later, blood flow to the treated kidney had risen only in the elamipretide group, and estimated kidney function improved significantly within the elamipretide group but not within the placebo group ^[12]. With 14 patients this was a pilot, not proof of benefit.

ReCLAIM-2, a phase 2 trial, randomised 176 people aged 55 or over with dry age-related macular degeneration and geographic atrophy to daily subcutaneous elamipretide 40 mg or placebo for 48 weeks. It missed both primary endpoints, which were low-light visual acuity and growth of the atrophic area. In predefined secondary measures it slowed loss of the ellipsoid zone, a layer on retinal scans linked to photoreceptors, and more patients gained 10 or more letters of low-light vision (14.6% vs 2.1%) ^[14].

A phase 3 trial, ReNEW, has enrolled 313 people with the same condition to receive elamipretide 40 mg or placebo daily for 96 weeks; it was listed as active and not recruiting when we checked ^[16].

Before MMPOWER-3, a smaller phase 1/2 trial, MMPOWER, gave 36 adults with primary mitochondrial myopathy two-hour intravenous infusions for five days. People on the highest dose walked 64.5 metres further in six minutes, against 20.4 metres on placebo, a difference that fell just short of statistical significance (p = 0.053) ^[13]. The larger and longer MMPOWER-3 trial described above then found no benefit ^[5]. The heart failure evidence remains limited to the single-infusion study ^[4].

The answer depends on what is measured. In old mice, changes in muscle energy production were seen one hour after a single dose ^[11]. In people, the effects that matter clinically have been slow or absent. In the Barth syndrome crossover trial, 12 weeks of elamipretide did not beat placebo on walking distance or fatigue, and knee strength changed little (a median of +4 newtons, against −5 on placebo). Strength gains appeared only in the open-label extension, with a median of +34 newtons at extension week 12 and +63 newtons at week 168 in the eight patients who remained ^[9]. The eye trial ran for 48 weeks ^[14].

Forzinity is given by injection under the skin. According to the label, peak blood levels are reached 0.5 to 1 hour after injection and absolute bioavailability is about 92%, with similar exposure from the thigh or abdomen ^[9]. Early trials used intravenous infusion ^{[4] [13]}. We found no published human study of SS-31 as a nasal spray or taken by mouth, so there is no evidence on how much would be absorbed that way.

MMPOWER-3 and the TAZPOWER extension used 40 mg by subcutaneous injection once daily ^{[5] [6]}. The heart failure study used single four-hour intravenous infusions of 0.005, 0.05 or 0.25 mg/kg per hour ^[4]. Animal studies used a range of weight-based doses that should not be converted to human doses. These are reported for information only; they are not a recommendation and do not apply to unregulated research material.

The Forzinity label sets 40 mg by subcutaneous injection once daily for patients with Barth syndrome weighing at least 30 kg, reduced to 20 mg for adults with severe kidney impairment (eGFR below 30 mL/minute) who are not on dialysis ^[9]. That is prescribing information for a specific rare disease under medical supervision. It is supplied as 280 mg in 3.5 mL (80 mg/mL) vials, and the solution contains benzyl alcohol ^[9].

For vial arithmetic only: a 10 mg vial made up with 2 mL of liquid gives 5 mg/mL, and one unit on a U-100 insulin syringe (0.01 mL) then holds 0.05 mg. The peptide calculator does this conversion for other vial sizes. This is arithmetic, not dosing advice.

Across the published trials elamipretide was generally well tolerated. In MMPOWER-3 most adverse events were mild to moderate ^[5], and in the Barth syndrome extension the most common adverse events were injection-site reactions ^[6]. No serious adverse events occurred in the single-infusion heart failure study ^[4]. A 2026 review of peptides used in sports medicine, which included SS-31, cautioned that human safety data for peptides obtained outside the approved route are scarce ^[8].

The label gives the clearest picture. In the 12-patient Barth syndrome crossover study, every patient had a local injection reaction on elamipretide, against 67% on placebo. Injection site redness occurred in 100% vs 25%, pain in 75% vs 42%, and hardening and itching in 67% each vs 17% ^[9]. In the dry macular degeneration trial, adverse events were reported in 86% on elamipretide and 71% on placebo, most commonly injection site reactions ^[14].

The label also reports raised eosinophils, a type of white blood cell, in studies lasting 30 days or more. Counts generally peaked around 90 days and returned to baseline after 6 to 12 months of treatment or after stopping, without symptoms ^[9]. It warns about hypersensitivity reactions and about benzyl alcohol, which can cause serious reactions in newborns, and notes that carcinogenicity studies have not been done ^[9]. None of this safety information covers research-grade SS-31, whose content and purity are unknown.

Both are discussed as mitochondrial peptides, but they are very different. SS-31 is a synthetic four-amino-acid peptide that binds cardiolipin inside mitochondria ^{[1] [2]}. MOTS-c is a 16-amino-acid peptide that occurs naturally, is encoded in mitochondrial DNA, and in mouse studies acted mainly on muscle metabolism and insulin sensitivity through the enzyme AMPK ^[15]. SS-31 has an approved drug and several human trials; MOTS-c has no approved form and its evidence is mainly from animals. Our MOTS-c guide covers that evidence.

SS-31 and MOTS-c are often sold together or in stacks. We found no published study, animal or human, that tested SS-31 and MOTS-c in combination, so there is no evidence on whether the combination does anything different from either alone, or on its safety.

Elamipretide (Forzinity) has US accelerated approval for Barth syndrome in patients weighing at least 30 kg, granted on 19 September 2025, and was described as the first disease-specific treatment for the condition. It remains in development for other conditions, including dry age-related macular degeneration and mitochondrial myopathies ^[7]. We could not confirm an approval in the UK, EU, Canada, Australia or New Zealand as of September 2026.

SS-31 bought as a research chemical is not Forzinity and is not covered by that approval. Its legal position varies by country; see our legal status pages.

The elamipretide FDA approval rests on knee extensor muscle strength, which the FDA treats as an intermediate endpoint. The approval letter requires a randomised, double-blind, placebo-controlled trial in Barth syndrome patients aged 5 and over to confirm clinical benefit, with study completion due in September 2029 and a final report in March 2030 ^[10]. A phase 3b/4 Barth syndrome trial began recruiting in July 2026 ^[17]. The company also received a rare pediatric disease priority review voucher ^[10].

Enforcement has followed the research market. In August 2026 the FDA sent a warning letter to Royal Peptides LLC, a US seller whose products included SS-31 ^[19]; see peptide rules in the US. In April 2026 Health Canada listed SS-31 among unauthorised peptide products seized in Canada and warned against injecting peptides bought online ^[20]; see Canada.

The label does not state a half-life. It reports peak levels 0.5 to 1 hour after injection, low protein binding (about 39%), breakdown into inactive fragments, and excretion in urine ^[9]. The approved vials are stored refrigerated at 2°C to 8°C and discarded 8 days after first opening ^[9].

Research SS-31 is usually supplied as a freeze-dried powder, which is generally stored cold, dry and away from light, and refrigerated once reconstituted. See how to store peptides and bacteriostatic water.

PepFinder does not sell SS-31. You can compare prices, learn how to read a COA, and see which vendors publish third-party tested results.

Country pages list SS-31 UK prices and US prices. Suppliers that post batch results are listed under published COAs; our guides to third-party peptide testing and spotting a fake supplier explain what to check, and our methodology explains how listings are scored.