What is SS-31?

What sources report

Where SS-31 comes from

The SS-31 peptide (also written SS 31 or SS31) is the most studied of the Szeto-Schiller peptides.¹ These are short synthetic chains that alternate aromatic and positively charged amino acids.¹ Hazel H. Szeto of Weill Cornell Medical College described the series with P. W. Schiller.¹⁹ The peptides were licensed to Stealth Peptides, and Szeto is named as the inventor of SS-31 and founder of Stealth BioTherapeutics.¹⁵¹⁶ In 2026 that company renamed itself Mighty Therapeutics.¹⁷

How it is thought to work

The FDA label describes elamipretide as a mitochondrial cardiolipin binder that settles in the inner mitochondrial membrane and improves mitochondrial structure and function.⁴ Cardiolipin is a lipid of that membrane.¹¹ In lab tests, SS-31 bound it tightly and kept it from converting cytochrome c into a peroxidase, an enzyme activity that attacks cardiolipin.¹¹ The inventor's review adds that cytochrome c kept its normal electron-carrying role.¹ Earlier papers by the inventor described the SS peptides as antioxidants that scavenge hydrogen peroxide and peroxynitrite.¹⁸

FDA's 2024 review of the animal and cell data was more cautious.¹³ It found that elamipretide reaches the inner membrane only for a short time, with qualitative improvements in heart mitochondrial structure and energy production, and that how long it stays in cells in living bodies is unknown.¹³ It did not change the ratio of monolysocardiolipin to cardiolipin in Barth syndrome models.¹³ In mice with a Barth syndrome gene knockdown, it did not persuasively improve heart function or structure.¹³

What the human trials found

Elamipretide has been tested in people with Barth syndrome, primary mitochondrial myopathy, heart failure, heart attack, kidney artery disease, eye diseases and aging muscle.¹³¹⁹ In 2024, FDA reviewers wrote that benefits seen in small early trials did not hold up in later ones.¹³ Larger phase 2 and 3 trials did not demonstrate an effect on clinical outcomes.¹³ Three authors of the 168-week Barth syndrome extension report were employees of the developer.²⁰

The one approval, for Barth syndrome, rests on knee strength gains seen in an open-label extension.²¹ The 12-week randomized part before it missed its primary endpoints, walking distance and fatigue.⁴ We found no published trial of SS-31 for anti-aging, weight loss or muscle building in healthy people. In 39 older adults, one infusion briefly raised a muscle energy measure without improving fatigue resistance.¹⁴ Each finding, with its trial size and evidence level, is on SS-31 benefits and uses.

Side effects

In the 12-patient Barth syndrome trial, every patient had an injection site reaction on elamipretide, against 67% on placebo.⁴ Redness, pain, hardening and itching were the most common.⁴ The label also warns of allergic reactions, some needing emergency care.⁴ Eosinophils, a type of white blood cell, often rose for a time without causing symptoms.⁴ Cancer studies had not been done at approval, so FDA required them afterward.⁴²² Full list: SS-31 side effects.

Dosage information

The only established human dose comes from the Forzinity label, and the prescriber sets it.⁴ For Barth syndrome patients who weigh 30 kg or more, the label dose is 40 mg under the skin once a day.⁴ The label lowers this to 20 mg for adults with severe kidney impairment who are not on dialysis.⁴ Trials in other conditions used 4 to 60 mg a day under the skin, or infusions dosed by body weight, under medical supervision.²³²⁴²⁵ The label covers Barth syndrome only, so no dose is established for any other use or for research-grade SS-31.⁴ Full details: SS-31 dosage.

Reconstitution and storage

Forzinity needs no mixing. It is a ready-to-use sterile solution of 280 mg in 3.5 mL (80 mg/mL), clear and colorless to yellow.⁴ Single-patient-use vials come four to a carton.⁴ Its label says unopened vials belong in a refrigerator, between 36 and 46°F (2 and 8°C), never frozen.⁴ After the first dose, an opened vial may stay refrigerated or at 20 to 25°C (68 to 77°F).⁴ It is discarded 8 days after first opening, and FDA set an expiry of 48 months from manufacture.⁴²²

Research-grade SS-31 is a different product, made outside that approval.³ The research vial listed as SS-31 for sale holds dry powder, a form for which no label or study sets mixing or storage conditions. General lab steps for freeze-dried peptides (diluent, sterile technique, concentration math and storage) are in How to reconstitute a peptide vial. The reconstitution calculator handles the arithmetic, and How to store peptides covers storage.

Regulatory status

Approval. FDA approved Forzinity on September 19, 2025 under NDA 215244, as a new molecular entity given priority review and orphan status.³ It also received a rare pediatric disease designation, and the sponsor received a rare pediatric disease priority review voucher.² The approval is accelerated. It rests on knee extensor strength, an intermediate measure FDA judged reasonably likely to predict real gains, like easier standing or longer walks.²⁴

Confirmation. FDA requires a randomized, double-blind, placebo-controlled trial in Barth syndrome patients aged 5 and older.²² It is due to finish in September 2029, with a final report in March 2030.²² A trial built to confirm the benefit, 4TAZPower (NCT07531251), started on July 2, 2026 and plans to enroll 48 males aged 5 to 55 for 72 weeks of treatment.²⁶

The path there. FDA refused to file a first application, submitted in August 2021.¹³ The company resubmitted on January 29, 2024, and an FDA advisory committee met on October 10, 2024.²²¹³ FDA issued a complete response letter on May 15, 2025.²² The company said FDA proposed an accelerated approval path, based on knee extensor strength, that required resubmission; FDA also cited a third-party manufacturing inspection and raised no concerns about clinical safety data.²⁷ The company responded on August 15, 2025, and approval followed on September 19, 2025.²²

Younger patients and access. Safety and effectiveness are not established under 30 kg.⁴ The company has said it agreed with FDA on a 1-month drug-level study in children under 30 kg.²⁸ That study could support a 2027 application covering children as young as 5.²⁸ Treating physicians can also request elamipretide through an expanded access program for genetically confirmed rare mitochondrial diseases.²⁹

Other uses. These remain investigational. A phase 3 trial in dry age-related macular degeneration, ReNEW, enrolled 313 people, with results expected in late 2027.³⁰³¹

Research-grade SS-31. No research-grade product is approved; FDA's approval covers Forzinity under NDA 215244.³ On August 24, 2026, FDA warning letters told four online sellers that their SS-31 products were unapproved new drugs.⁸³²⁶³³ Each letter said that despite "research use only" labels, evidence from the seller's website showed the products were intended as drugs for human use.⁸³²⁶³³ All four also cited the sellers' bacteriostatic water: three for marketing it as a solvent for their products, one for selling it alongside a peptide guide and calculator.⁸⁶³³³² FDA said this provides the means to prepare an injectable drug for human use.⁸³²⁶³³ A December 2025 letter to an ingredient distributor listed elamipretide (SS-31) acetate as ineligible for human drug compounding under section 503A.³⁴

Sport. SS-31 is not named on the WADA 2026 Prohibited List.⁷ The list does name MOTS-c as a metabolic modulator and BPC-157 as an example of a non-approved substance.⁷