SS-31 (Elamipretide) Side Effects: What the Trials Actually Measured

SS-31 (elamipretide) side effects come from a place almost no research-market peptide can claim: a completed phase 3, a 168-week extension, two placebo-controlled eye trials — and, as of September 2025, an FDA accelerated approval as Forzinity for Barth syndrome (PMID 41335372). The short answer: the one side effect every itemizing trial put at the top is the injection-site reaction, and the serious-event ledger is short. The longer answer: every trial enrolled people with rare mitochondrial or retinal disease, and two missed their primary efficacy endpoints. Here are the numbers, cited, and where they stop applying to a vial.

In This Guide

• What Is SS-31 (Elamipretide)?
• How It Works — A Tetrapeptide That Binds Cardiolipin
• The Trial Numbers That Matter
• Adverse-Event Rates: What the Trials Actually Reported
• Injection-Site Reactions: The Side Effect That Defines This Compound
• Beyond the Injection Site: What Else the Trials Recorded
• What Users Report: Self-Reported Effects From Reddit
• When Side Effects Happen — and When They Fade
• Long-Term Safety: What 168 Weeks Can and Can’t Tell You
• Who the Trials Enrolled — and Why It’s Nothing Like a Research Buyer
• Managing and Minimizing Side Effects: What the Trials Did
• Dose and Side Effects: One Dose, Every Trial
• SS-31 Alone vs Stacked: What the Data Covers
• How SS-31 Compares to Humanin, MOTS-c, and NAD+
• Where the Trial Data Stops: The Research-Vial Reality
• Sourcing and Verification
• US Regulatory Status
• Frequently Asked Questions
• Glossary of Terms

What Is SS-31 (Elamipretide)?

SS-31 is the research-market name for elamipretide — MTP-131 and Bendavia in older papers — a four-amino-acid peptide that targets mitochondria and binds cardiolipin, a lipid of the inner mitochondrial membrane (PMID 39940712). Developed by Stealth BioTherapeutics, it has the most unusual regulatory story we cover: in September 2025 the FDA granted it accelerated approval, as Forzinity, to improve muscle strength in adult and pediatric patients with Barth syndrome weighing 30 kg or more — the first disease-specific treatment for that ultra-rare X-linked recessive disorder (PMID 41335372). It remains in phase III development for dry AMD and mitochondrial myopathies. The vial labeled SS-31 shares elamipretide’s sequence and nothing else verifiable without testing — and “FDA-approved” is exactly the phrase a listing will borrow.

How It Works — A Tetrapeptide That Binds Cardiolipin

Most research-market peptides act on a cell-surface receptor. Elamipretide doesn’t. It accumulates inside mitochondria and binds cardiolipin, stabilizing the cristae structure of the inner membrane, reducing oxidative stress, and enhancing ATP production (PMID 39940712). The bet is that diseases of mitochondrial dysfunction respond to a molecule that props up the membrane geometry they degrade; preclinical models of heart failure, neurodegeneration, and muscle atrophy showed protection. For side effects, that means no receptor to overstimulate, and nothing in the abstracts describes a mechanism-linked toxicity. The recorded adverse events are the mechanics of delivery — a daily subcutaneous injection — not the pharmacology.

The Trial Numbers That Matter

MMPOWER-3: a pivotal phase 3, placebo-controlled trial of 218 participants with genetically confirmed primary mitochondrial myopathy, randomized 1:1 (109 elamipretide, 109 placebo) to 24 weeks of 40 mg/day subcutaneously (PMID 37268435). It missed both primary endpoints: the six-minute walk difference at week 24 was -3.2 meters (p = 0.69) and the PMMSA total fatigue difference -0.07 (p = 0.37). TAZPOWER: a 28-week randomized, placebo-controlled trial in Barth syndrome, followed by a 168-week open-label extension on 40 mg daily; ten entered the extension and 8 reached week 168 (PMID 38602181). ReCLAIM-2: a phase II, placebo-controlled, double-masked trial of 176 patients aged 55 and older with dry AMD — 117 on 40 mg daily, 59 on placebo — for 48 weeks (PMID 39605874); it also missed its primary endpoints. ReCLAIM: a 24-week open-label phase 1 in 19 adults with dry AMD (PMID 36246181). Two efficacy failures and one approval in one evidence base: safety is consistent; efficacy depends on the disease.

Adverse-Event Rates: What the Trials Actually Reported

Only one abstract gives a headline rate. ReCLAIM-2 reported adverse events in 86% of the elamipretide group versus 71% of the placebo group over 48 weeks, the most common being injection-site reactions (PMID 39605874). Read both columns: seven in ten placebo participants, injecting an inert solution daily for close to a year, reported an adverse event too — the attributable share is the gap, not the 86% alone. The ReCLAIM phase 1 gives a severity split instead: all 19 participants had one or more non-ocular adverse events, all either mild (73.7%) or moderate (26.3%), none serious (PMID 36246181). MMPOWER-3 said only that treatment “was well-tolerated with most adverse events being mild to moderate in severity” (PMID 37268435) — no percentages, and we won’t supply any from memory. The Barth extension likewise: no rates (PMID 38602181). What’s absent is informative too: no nausea column, no GI table, no cardiovascular signal, no laboratory abnormality named. That is consistent with a benign systemic profile — and with abstracts that summarized safety in a sentence.

Injection-Site Reactions: The Side Effect That Defines This Compound

If elamipretide has a signature adverse event, this is it; three of the four clinical abstracts name it. ReCLAIM-2 lists the most common events as injection-site reactions — pruritus, injection-site pain, bruising, and erythema (PMID 39605874). The TAZPOWER extension reports them as the most common adverse event across 168 weeks of daily injections (PMID 38602181). And the ReCLAIM phase 1 recorded the sharpest version: one of its 19 participants exited because of an intolerable injection-site reaction (PMID 36246181) — the only discontinuation for a drug-attributable adverse event in these abstracts. None reports per-participant incidence. The context that matters: 40 mg into subcutaneous tissue every day, for 24 to 168 weeks — a far heavier needle burden than a once-weekly compound carries.

Beyond the Injection Site: What Else the Trials Recorded

The serious-event ledger, as published. ReCLAIM phase 1: no serious adverse events in 19 participants over 24 weeks; two exits for adverse events — one conversion to neovascular AMD, one intolerable injection-site reaction — plus one discontinuation for self-perceived lack of efficacy and one who chose not to continue (PMID 36246181). The abstract does not attribute the neovascular conversion to the drug; neither will we. TAZPOWER’s extension: ten entered, 8 reached week 168, reasons unstated (PMID 38602181). MMPOWER-3 and ReCLAIM-2 report no serious-event counts.

Efficacy sets the risk-benefit frame, and here it is split. MMPOWER-3 provided Class I evidence that elamipretide does not improve six-minute walk distance or fatigue at 24 weeks (PMID 37268435). ReCLAIM-2 missed its primary endpoints but found a 43% reduction in progression of total ellipsoid-zone loss (nominal P = 0.0034), with 14.6% of elamipretide patients versus 2.1% on placebo gaining 10 or more letters of low-luminance acuity (nominal P = 0.0404) — secondary findings (PMID 39605874). The Barth extension recorded a cumulative 96.1 m walk improvement at week 168 (P = .003) and improved left-ventricular volumes (PMID 38602181). Cardiovascular adverse events appear in no abstract; in Barth syndrome, heart function was an efficacy measure and improved — a rare-cardiomyopathy finding, not a heart-health claim for anyone else.

What Users Report: Self-Reported Effects From Reddit

The trials on this page describe verified elamipretide at a fixed 40 mg a day in rare mitochondrial and retinal disease, where injection-site reactions led every adverse-event list and two large trials missed their primary endpoints. They say nothing about fatigued adults injecting a fraction of that from an untested vial. We searched Reddit on September 5, 2026, found 96 relevant posts from 2025 to 2026, and read the comment threads of ten, across r/cfs, r/Peptides, r/BodyHackGuide, r/Peptidesource, r/Biohacking and r/NTNPerformance. Everything below is paraphrased and linked, uses no usernames, and describes products nobody verified. Two caveats: SS-31 is almost always run as a primer for MOTS-c, usually with NAD+, and after cease-and-desist letters hit vendors in late 2025, one reportedly kept selling the same product under a new name (r/Peptidesource).

The dominant report is a fast, clean lift in energy and mental clarity, often at doses far below the trials. A 27-year-old man housebound with severe ME/CFS found 500 mcg subcutaneous more stimulating than caffeine and regained several hours a week of cognitive work; the gains held five months on, though physically he was unchanged (r/cfs). A poster a week into 5 mg a day said brain fog and a years-long afternoon crash had gone (r/Peptides). Under a post summarising the trials, at least eight commenters in their forties and fifties described smooth background energy within days to weeks on 500 mcg to 5 mg (r/NTNPerformance).

The mirror image is nearly as common: fatigue, crashes and flat mood, especially early. One poster’s first 500 mcg dose gave energy until mid-afternoon, then a major crash with light-headedness and irritability; a reply had seen the same, fading after the first dose (r/Peptidesource). A poster two weeks into roughly 500 mcg a day was “shattered” every day (r/Biohacking), and a couple in their forties crashed midday for ten days after adding SS-31 to MOTS-c (r/Biohacking). The most acute report is a first-day 25 mg dose followed by a racing heart, light-headedness and near-blackout in a poster with no underlying conditions who mentioned no medical visit (r/NTNPerformance).

A large minority felt nothing, and the placebo argument runs through every thread. At least eleven posters noticed no effect: a 38-year-old after eight weeks at 5 mg a day (r/Biohacking); one a week into 20 mg a day, told to check his source (thread); several running 2 to 5 mg for weeks as a MOTS-c primer (r/BodyHackGuide); and one who pushed a 50 mg vial in one injection and called it a non-event (thread). Believers say only damaged mitochondria feel it, which explains away every null result.

One of the most-upvoted SS-31 threads of 2026 is about eyesight, and it split the room. A lifelong glasses-wearer on 4 mg five days a week said the smudge in his uncorrected vision had gone and his glasses now felt too strong; at least seven commenters said the same, including one whose gains reversed on stopping. The top reply had finished eight weeks at 5 mg and still woke up unable to see; others credited a GLP-1, called it coincidence, or argued such doses cannot reach the retina (r/Biohacking). ReCLAIM-2 above missed both primary endpoints at 40 mg a day.

Injection-site and histamine-type reactions are the one place the anecdotes and the trials agree. In the ME/CFS thread one commenter broke out in hives after a few doses and stopped, a reply described shortness of breath and a severe migraine after a second dose, and the original poster’s mast-cell symptoms, insomnia and constipation flared, then cleared on a break (r/cfs). Others describe a faint weekly itch (r/BodyHackGuide) or a tender, bruised site (thread). Doses described run from 100 mcg to 50 mg a day (thread).

When Side Effects Happen — and When They Fade

No abstract characterizes the time course of elamipretide’s adverse events; no week-by-week calendar exists. What the designs tell you is structural: injection-site reactions were the most common event in a 48-week trial (PMID 39605874) and still the most common event in a 168-week extension (PMID 38602181). A local reaction to a daily needle doesn’t “fade” the way a gut-hormone peptide’s early GI effects do, because the stimulus recurs every day. The phase 1’s intolerable injection-site reaction fell somewhere inside a 24-week window; the abstract doesn’t say when. Efficacy timing is better documented: no separation from placebo at week 24 in MMPOWER-3; walk gains at every time point through week 168 in Barth syndrome.

Long-Term Safety: What 168 Weeks Can and Can’t Tell You

Elamipretide has the longest human exposure of any compound in this batch: TAZPOWER’s 168-week open-label extension, with safety and tolerability as its primary endpoints, concluded “sustained long-term tolerability” (PMID 38602181). That is a long run of daily injections. It is also 8 people at the final visit, with no placebo arm, in an ultra-rare X-linked disorder (PMID 41335372). The larger trials are shorter: 48 weeks in 176 AMD patients, 24 weeks in 218 myopathy patients. So: long duration in very few people, moderate duration in a few hundred, and an accelerated — provisional — approval in one indication. Anyone describing SS-31 over years of use in healthy adults is describing something no trial has done.

Who the Trials Enrolled — and Why It’s Nothing Like a Research Buyer

This matters more for SS-31 than for almost any compound we cover. MMPOWER-3 enrolled participants with genetically confirmed primary mitochondrial myopathy — 74% with mitochondrial DNA alterations — whose most frequent bothersome symptom was tiredness during activities (28.9%) and who walked a mean 336.7 meters in six minutes at baseline (PMID 37268435). TAZPOWER enrolled Barth syndrome patients (PMID 38602181). ReCLAIM and ReCLAIM-2 enrolled adults aged 55 and older with dry AMD (PMID 36246181, PMID 39605874). Every rate on this page was generated in people with a confirmed mitochondrial or retinal disease, under specialist care. The research buyer — healthy, curious about “mitochondrial energy,” reading our peptides for energy roundup — appears in none of these abstracts, and no healthy-volunteer safety data is among the sources we can cite. A disease population can mask, magnify, or simply differ from what a healthy body reports.

Managing and Minimizing Side Effects: What the Trials Did

The trials’ management was minimal, because the profile gave them little to manage: one dose, daily, with the injection site as the recurring issue. The only discontinuation for a drug-attributable adverse event across these abstracts is the single intolerable injection-site reaction among ReCLAIM’s 19 participants (PMID 36246181). No abstract describes premedication, dose reduction, or titration — every arm started at and stayed on 40 mg daily. We report that as trial design, not as a protocol.

Dose and Side Effects: One Dose, Every Trial

SS-31 is the mirror image of the weight-loss peptides: there is no dose–side-effect curve, because there was only one dose. MMPOWER-3 gave 40 mg/d (PMID 37268435); TAZPOWER’s extension, ReCLAIM-2, and ReCLAIM all gave 40 mg daily (PMID 38602181, PMID 39605874, PMID 36246181). Subcutaneous, every time. Excellent for consistency — every rate on this page describes the same exposure — and useless for anyone wondering whether a lower dose would spare the skin, or a higher one would do anything. No published human data answers either question.

SS-31 Alone vs Stacked: What the Data Covers

Every published rate on this page comes from elamipretide alone. No combination trial appears in the abstracts. Research forums pair SS-31 with the so-called mitochondrial stack — humanin, MOTS-c, NAD+ — and none of those pairings has any human safety data. The monotherapy numbers cannot be borrowed: the 86% describes people injecting one compound under observation, not three unobserved. Humanin, most often listed beside SS-31, has a far thinner human record — our humanin side effects page walks through what exists. MOTS-c is covered in our MOTS-c guide, NAD+ in our NAD hub. All are sold for non-human research use; their interactions have never been measured in a person.

How SS-31 Compares to Humanin, MOTS-c, and NAD+

No head-to-head trial exists, so this compares evidence bases, not rates — the populations differ so completely that no number transfers. Elamipretide’s base is unique in the mitochondrial category: a phase 3 (PMID 37268435), a 168-week extension (PMID 38602181), two placebo-controlled eye trials, and an FDA approval (PMID 41335372). Humanin and MOTS-c are mitochondrial-derived peptides whose human record doesn’t approach this. NAD+ is a metabolite rather than a peptide; our NAD injections guide covers it. The comparison that matters: elamipretide is the only one of these with a published, placebo-controlled adverse-event rate. The values don’t transfer, but the existence of a denominator does. For the wider category, start with our peptides for aging guide.

Where the Trial Data Stops: The Research-Vial Reality

Everything above describes pharmaceutical elamipretide from Stealth BioTherapeutics — the substance the FDA reviewed. A lyophilized vial labeled SS-31 shares its sequence and nothing else that’s verifiable without testing. The gap shows up in three places: identity and purity (a batch-matched COA is the only evidence the vial contains dose-accurate elamipretide), concentration arithmetic (reconstitution with bacteriostatic water is on the buyer — our peptide calculator handles the math and deliberately nothing else), and no medical monitoring — the 86% and the 71% were counted by clinicians, and nobody counts anything for a research buyer. A fourth gap is specific to this compound: Forzinity is an FDA-approved product (PMID 41335372); a research vial is not Forzinity and is sold strictly for non-human research use. A listing that borrows the approval is borrowing the one thing it cannot have.

Sourcing and Verification

SS-31 has been on research-market shelves longer than most compounds in this batch — more vendors, more variation in what “SS-31” turns out to contain. The checklist doesn’t change: a batch-matched COA naming the compound and quantity, independent third-party testing with a verifiable report number — see our lab testing guide — and hard skepticism toward any listing quoting “FDA-approved.” (Disclosure: some links on this site are affiliate links; they don’t change what we report.) Our Amino Club review shows what passing documentation looks like, our vendor directory lists who we’ve tested, and our guide library covers verification in depth. We don’t yet publish live SS-31 prices.

US Regulatory Status

Here SS-31 parts ways with every other page in this batch. Elamipretide is an FDA-approved drug: in September 2025 it received accelerated approval as Forzinity to improve muscle strength in adult and pediatric patients with Barth syndrome weighing 30 kg or more (PMID 41335372). That approval covers one ultra-rare indication, one manufacturer’s product, and one route to a patient — a prescription. It does not cover primary mitochondrial myopathy, where the phase 3 failed (PMID 37268435), or dry AMD, still in phase III. And it does not cover research-market SS-31, which is not Forzinity and is sold for non-human research use. The FDA’s broader posture toward research peptides — see our coverage of the PCAC peptide vote — is the frame that applies to the vial.

Frequently Asked Questions

What are the most common SS-31 (elamipretide) side effects?

Injection-site reactions — pruritus, pain, bruising, and erythema. In the 48-week ReCLAIM-2 trial, adverse events were reported in 86% of participants on elamipretide versus 71% on placebo, with injection-site reactions the most common. No abstract reports a nausea, GI, or cardiovascular rate.

Is SS-31 FDA-approved?

Elamipretide is: in September 2025 it received accelerated approval as Forzinity to improve muscle strength in Barth syndrome patients weighing 30 kg or more. Research-market SS-31 is not Forzinity, has not been reviewed by any regulator, and is sold for non-human research use only.

Did SS-31 work in its clinical trials?

It depends on the disease. The 218-person MMPOWER-3 phase 3 in primary mitochondrial myopathy missed its primary endpoints (six-minute walk difference -3.2 meters, p = 0.69). ReCLAIM-2 in dry AMD also missed its primary endpoints but showed a 43% reduction in ellipsoid-zone loss progression (nominal P = 0.0034). The Barth syndrome extension recorded a cumulative 96.1 m walk improvement at week 168 — the indication that was approved.

Is SS-31 safe long-term?

The longest published exposure is TAZPOWER’s 168-week open-label extension, which concluded “sustained long-term tolerability” with injection-site reactions the most common adverse event. That is 8 patients at the final visit out of ten who entered, in an ultra-rare disease, with no placebo arm. No long-term data in healthy adults exists.

Does SS-31 cause nausea or other systemic side effects?

No abstract names a GI, cardiovascular, or laboratory adverse event. In the 19-person ReCLAIM phase 1, all adverse events were mild (73.7%) or moderate (26.3%), none serious. MMPOWER-3 reported most adverse events as mild to moderate without itemizing them. Absence from an abstract is not proof of absence.

Can you stack SS-31 with humanin, MOTS-c, or NAD+?

No published human trial has tested elamipretide in any combination. Every safety number on this page comes from elamipretide alone in supervised rare-disease patients. Any stack’s side-effect profile is unmeasured.

Glossary of Terms

• Cardiolipin: a lipid of the inner mitochondrial membrane that elamipretide binds; stabilizing it is the compound’s entire mechanism.
• SS-31 / MTP-131 / Bendavia: earlier and research-market names for elamipretide, the drug approved as Forzinity.
• Barth syndrome: an ultra-rare X-linked recessive disorder of mitochondrial function; the only approved indication for elamipretide.
• Accelerated approval: the FDA pathway used for elamipretide — provisional, with further evidence expected.
• Primary mitochondrial myopathy (PMM): genetic disorders impairing mitochondrial oxidative phosphorylation in muscle; the MMPOWER-3 population.
• Ellipsoid zone (EZ): a retinal layer on OCT imaging whose loss tracks photoreceptor damage; ReCLAIM-2’s secondary finding.
• Open-label extension (OLE): a follow-on study in which everyone knowingly receives the drug and there is no placebo arm — TAZPOWER’s 168 weeks.

References

• Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial — Neurology, 2023 (PubMed)
• Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER — Genet Med, 2024 (PubMed)
• ReCLAIM-2: A Randomized Phase II Clinical Trial Evaluating Elamipretide in Age-related Macular Degeneration — Ophthalmol Sci, 2024 (PubMed)
• Phase 1 Clinical Trial of Elamipretide in Dry Age-Related Macular Degeneration and Noncentral Geographic Atrophy: ReCLAIM NCGA Study — Ophthalmol Sci, 2021 (PubMed)
• Elamipretide: First Approval — Drugs, 2026 (PubMed)
• Elamipretide: A Review of Its Structure, Mechanism of Action, and Therapeutic Potential — Int J Mol Sci, 2025 (PubMed)