SS-31 is elamipretide: what the FDA approved, and what it didn’t

SS-31 is the lab name for elamipretide, a peptide of four amino acids that the FDA approved on September 19, 2025 as Forzinity, for one use: improving muscle strength in people with Barth syndrome who weigh at least 30 kg. That is an accelerated approval that still depends on a confirmatory trial, and it covers nothing else: not aging, energy, exercise recovery or any of the “mitochondrial” uses SS-31 is marketed for online. Vials sold as SS-31 for research are not Forzinity, and the FDA has called them unapproved new drugs. This guide sets out what the FDA decided, what the trials measured and what the approval does and doesn’t mean. It doesn’t point to any seller.

What SS-31 is

SS-31 is one of the Szeto-Schiller (SS) peptides, a family known to target the inner membrane of mitochondria, the parts of cells that produce energy. According to a 2014 review in the British Journal of Pharmacology, SS-31 binds selectively to cardiolipin, a phospholipid (a type of fat) found only in the inner mitochondrial membrane (Szeto, 2014). Its drug name is elamipretide; the FDA’s own warning letters call it “SS-31 (Elamipretide)”.

Forzinity’s label describes elamipretide as a “mitochondrial cardiolipin binder” that “localizes to the inner mitochondrial membrane and improves mitochondrial morphology and function”. Chemically, it is a chain of four amino acids, D-arginine, a modified tyrosine (2,6-dimethyltyrosine), lysine and phenylalanine, with an amide at the end (Forzinity label). That short chain of amino acids is what makes it a peptide; our guide to what peptides are explains the term, and why NAD+ is not one.

What the FDA approved

Sources: Drugs@FDA, the label and the approval letter.

Barth syndrome is a rare, inherited disease of the mitochondria. The FDA describes it as one that “primarily affects males, typically starts with severe heart failure in infancy, and causes premature death”, with fatigue, poor stamina and exercise intolerance in those who survive into adolescence and adulthood (FDA). Forzinity is the first treatment approved for it. The FDA also gave Stealth a rare pediatric disease priority review voucher.

The road there was not straight. Stealth filed the application on January 29, 2024. The FDA issued an action letter on May 15, 2025, Stealth sent a complete response on August 15, 2025, and the FDA approved the drug on September 19, 2025.

What accelerated approval means here

Accelerated approval lets the FDA approve a drug for a serious condition based on a measure that is “reasonably likely to predict” a benefit, rather than the benefit itself. For Forzinity, that measure is the strength of the muscle that straightens the knee. The FDA considers it reasonably likely to predict benefits such as standing more easily or walking farther, but those benefits haven’t been shown yet.

The approval letter requires Stealth to run a randomized, double-blind, placebo-controlled trial in patients 5 years and older to verify the benefit. Stealth’s schedule in the letter has the trial starting in March 2026, completing in September 2029 and reporting in March 2030. If the trial fails to verify a benefit, the FDA may withdraw the approval.

What the trials measured

The label describes the evidence. Twelve males aged 12 to 35 with genetically confirmed Barth syndrome took part in a randomized, double-blind crossover trial: 12 weeks on Forzinity and 12 weeks on placebo, in random order. The two main measures were the distance walked in 6 minutes and a fatigue score. In the label’s words, Forzinity “was not superior to placebo on these primary endpoints”. Knee strength was a secondary measure. The label reports that it did not increase during the randomized trial, and that increases were seen during an open-label extension in which everyone took Forzinity, which 8 patients continued to week 168. That extension finding is what the accelerated approval rests on.

On safety, the label lists injection-site reactions (redness, pain, hardening, itching, bruising and hives) as the most common side effects, warns of serious allergic reactions, and notes rises in eosinophils, a type of white blood cell, that returned to baseline over time. Forzinity contains benzyl alcohol and must not be used in newborns. It has no boxed warning.

A second, larger trial tested elamipretide in a different mitochondrial disease. MMPOWER-3 randomized 218 people with primary mitochondrial myopathy to elamipretide or placebo injections for 24 weeks and measured walking distance and fatigue (Neurology, 2023). It did not meet either primary endpoint.

What the approval doesn’t mean

It isn’t an approval for mitochondria in general

SS-31 is marketed online for “mitochondrial repair”, anti-aging, cellular energy and neuroprotection. The FDA reviewed elamipretide for one rare genetic disease, in patients of at least 30 kg, on a strength measure. It hasn’t reviewed any other use. Prescribing Forzinity for anything else is off-label, with no FDA review behind it, and the one large trial in another mitochondrial disease missed its goals.

Vials sold as SS-31 aren’t Forzinity

On August 24, 2026, the FDA sent a warning letter to an online seller whose site described SS-31 (Elamipretide) as a “mitochondria-targeted tetrapeptide” and listed “Mitochondrial repair, anti-aging, neuroprotection” as uses (FDA). The FDA called the product an unapproved new drug, noting that no approved application covers it. Labels on the site saying the products were for research use only didn’t change that: the FDA found the seller’s own claims, and a “peptide calculator” alongside bacteriostatic water, showed the products were meant for people to inject.

An approved Forzinity application covers Stealth’s product, made and tested to its approved standards. It doesn’t cover powders from other sources that carry the same name. Buying those vials for personal use is unsafe and unregulated: no prescriber decides whether it suits you, no licensed pharmacy answers for what is in the vial, and no one has checked its identity, strength or sterility. Our guide on whether peptides are safe explains what to check.

Compounded SS-31 is an open question, not a green light

Elamipretide is not on any of the FDA’s 503A compounding category lists (FDA). Since September 2025 it has been a component of an FDA-approved drug, which is one of the tests a bulk ingredient can meet under section 503A (FDA). But the ingredient must come from an FDA-registered manufacturer with a valid certificate of analysis, a 503A pharmacy may not regularly compound what is essentially a copy of a drug that is on the market, and the FDA has published nothing specific about compounded elamipretide. A compounded version would not be Forzinity and would not be FDA-approved. We don’t list any provider for it.

The bottom line

SS-31 is elamipretide. The FDA approved it once, as Forzinity, for muscle strength in Barth syndrome, under an accelerated approval that still has to be confirmed by a trial due to finish in 2029, and its randomized trial missed its two main measures. That approval says nothing about energy, aging or “mitochondrial health” in anyone else, and the FDA has warned an online seller that SS-31 vials marketed that way are unapproved new drugs. For the peptides a pharmacy may lawfully make, see our peptide legal status guide and the peptides hub.