Is SS-31 Peptide Safe? Elamipretide Won FDA Approval on a Trial of 12 Patients, and All 12 Had Injection Site Reactions
Is SS-31 peptide safe? SS-31 is the research name for elamipretide, and in randomized trials lasting up to 48 weeks it was well tolerated, with injection site reactions as the main side effect. That answer comes with limits. The FDA approved elamipretide as Forzinity on September 19, 2025, for Barth syndrome, a rare genetic disease, and the approved label lists a warning for serious allergic reactions, a dose cut for weak kidneys and a rise in a type of white blood cell. Carcinogenicity studies have not been conducted. And the safety data belong to a pharmaceutical product injected at 40 mg a day by sick patients under medical supervision, not to the vials sold online as a longevity peptide. This article walks through what the trials and the label actually report, question by question. For the efficacy side of the story, including the trials elamipretide failed, see our earlier review, SS-31 Became an Approved Drug. The Trials It Failed Are the More Useful Reading.
Key Takeaways: Is SS-31 Peptide Safe?
In MMPOWER-3, a 24-week randomized trial of 218 adults with primary mitochondrial myopathy, elamipretide 40 mg a day was well tolerated and most adverse events were mild to moderate (Karaa and colleagues, Neurology, 2023). In the 12-patient Barth syndrome crossover trial behind the FDA approval, all 12 patients had a local injection reaction on elamipretide, compared with 8 of 12 (67%) on placebo, according to the Forzinity label. The FDA label warns that hypersensitivity reactions, including serious allergic reactions requiring emergency medical intervention, have been reported, and that they can appear within minutes to months after starting treatment. Eosinophil counts rose frequently with 30 days or more of dosing, peaked around 90 days and returned to baseline after 6 to 12 months, according to the label. Elamipretide exposure rose by 125% in severe kidney impairment, and the label halves the dose in adults with an eGFR below 30 mL/min who are not on dialysis. Carcinogenicity studies of elamipretide have not been conducted, and the longest human exposure on record is 192 weeks in 3 patients.
Injection Site Reactions in the Trial Behind the Approval
Percent of patients with each reaction during 12 weeks of elamipretide 40 mg daily versus 12 weeks of placebo, crossover trial in 12 males with Barth syndrome (FDA-approved Forzinity label, Table 2, DailyMed).
- Elamipretide (%)
- Placebo (%)
What is SS-31, and is it the same drug as elamipretide?
SS-31 and elamipretide are the same tetrapeptide, a chain of four amino acids. SS-31 is the laboratory name, MTP-131 was an earlier development name, and Forzinity is the brand name of the approved injection made by Stealth BioTherapeutics. Elamipretide crosses cell membranes and settles on the inner membrane of mitochondria, where it associates with a fat called cardiolipin (Karaa and colleagues, Neurology, 2018), and the label describes it as a mitochondrial cardiolipin binder. The FDA granted elamipretide accelerated approval on September 19, 2025, according to a 2026 review by Zhao and colleagues in Drug Discoveries and Therapeutics. The approved use is narrow: to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg. The approved dose is 40 mg injected under the skin once daily, supplied as 280 mg in 3.5 mL vials (80 mg/mL). Accelerated approval means the FDA accepted a change in knee extensor strength as an intermediate endpoint, and continued approval depends on a confirmatory trial. No regulator has approved SS-31 for aging, energy, exercise performance or any use in healthy men. Every safety statement in this article describes the pharmaceutical product in the people who were studied.
How much human safety data exists for SS-31?
SS-31 has randomized, placebo-controlled human safety data from several trials, which is more than most peptides marketed for longevity can show, but most of that data covers 24 weeks or less. The largest trial, MMPOWER-3, randomized 218 adults with primary mitochondrial myopathy, 109 to elamipretide and 109 to placebo, for 24 weeks (Karaa and colleagues, Neurology, 2023). ReCLAIM-2 randomized 176 patients aged 55 or older with dry age-related macular degeneration, 117 to elamipretide and 59 to placebo, for 48 weeks (Ehlers and colleagues, Ophthalmology Science, 2025). PROGRESS-HF randomized 71 patients with heart failure with reduced ejection fraction to placebo, 4 mg or 40 mg daily for 28 consecutive days (Butler and colleagues, Journal of Cardiac Failure, 2020). Smaller trials add short exposures. MMPOWER-2 gave 30 adults 40 mg a day for 4 weeks in a crossover design (Karaa and colleagues, Journal of Cachexia, Sarcopenia and Muscle, 2020). The first MMPOWER trial gave 36 participants 2-hour intravenous infusions over 5 days (Karaa and colleagues, Neurology, 2018). EMBRACE STEMI tested a single 1 h intravenous infusion at 0.05 mg/kg/h in heart attack patients undergoing angioplasty (Gibson and colleagues, European Heart Journal, 2016). The longest exposure comes from the Barth syndrome program. Ten patients entered the open-label extension of TAZPOWER, 8 received elamipretide for 168 weeks, and 3 of those 8 continued to a total of 192 weeks, according to the label. That is the entire long-term record: a handful of young men with one rare disease.
How Long People Have Taken Elamipretide in Trials
Planned daily dosing period in weeks: PROGRESS-HF (Journal of Cardiac Failure, 2020), MMPOWER-2 (Journal of Cachexia, Sarcopenia and Muscle, 2020), MMPOWER-3 (Neurology, 2023), ReCLAIM-2 (Ophthalmology Science, 2025), and the TAZPOWER open-label extension, where 8 patients reached 168 weeks and 3 reached 192 weeks (Forzinity label).
What are the most common side effects of SS-31?
Injection site reactions are the most common side effect of SS-31 in every subcutaneous trial. In the Barth syndrome crossover trial summarized in Table 2 of the Forzinity label, injection site redness occurred in 100% of patients on elamipretide versus 25% on placebo, pain in 75% versus 42%, hardening (induration) in 67% versus 17%, itching in 67% versus 17%, and bruising and hives in 25% each versus 0 on placebo. The pattern held in larger trials. In MMPOWER-2, injection site reactions were reported in 80% of participants on elamipretide, the majority mild, and no serious adverse events or deaths were reported. In ReCLAIM-2, adverse events were reported in 86% of patients on elamipretide and 71% on placebo over 48 weeks, and the most common events were injection site reactions such as itching, pain, bruising and redness. In the 168-week TAZPOWER extension, injection site reactions remained the most common adverse events (Thompson and colleagues, Genetics in Medicine, 2024). The placebo numbers matter. In the Barth trial, 67% of patients had a local reaction on placebo injections alone, which shows how much of the irritation comes from daily needles rather than the peptide. The label advises rotating injection sites daily between the abdomen and outer thigh, and it suggests topical corticosteroids and oral antihistamines for mild to moderate skin reactions.
Placebo Injections Cause Reactions Too
Percent of patients with any local injection reaction in the 12-patient Barth syndrome crossover (Forzinity label) and with any adverse event over 48 weeks in ReCLAIM-2, 176 patients with dry macular degeneration (Ehlers and colleagues, Ophthalmology Science, 2025).
- Elamipretide (%)
- Placebo (%)
Can SS-31 cause a serious allergic reaction?
Yes. The Forzinity label carries a warning that hypersensitivity reactions, including serious allergic reactions requiring emergency medical intervention, have been reported in patients receiving elamipretide. The reactions described include skin problems such as rash, papular lesions and eczematous dermatitis, as well as respiratory symptoms including cough. The timing is the part that surprises people. According to the label, the reactions may occur within minutes to months after treatment starts, so tolerating the first few weeks does not rule out a later reaction. The label tells prescribers to stop the drug and give emergency treatment, including epinephrine, antihistamines and corticosteroids as needed, if a serious reaction occurs. Patients who have had a serious hypersensitivity reaction should not be given elamipretide again, and serious hypersensitivity to any ingredient is the only listed contraindication. The label does not report how often serious allergic reactions occurred, so the risk cannot be expressed as a rate. What the warning establishes is that the risk is real enough for the FDA to require it on the label.
Does SS-31 affect blood counts?
SS-31 can raise eosinophils, a type of white blood cell involved in allergy and inflammation. The Forzinity label reports that increases in absolute eosinophil counts were noted frequently in studies where elamipretide was given for 30 days or longer. Counts generally peaked around 90 days after the first dose, with a mean rise of roughly 0.5 to 0.6 x 10^3 cells per microliter, and returned to baseline after 6 to 12 months of continued use or after the drug was stopped. The label presents this as a laboratory finding rather than a disease, and the counts came back down on their own. It still matters for anyone reading a blood test while on the peptide. A rising eosinophil count on a routine panel can prompt a search for allergy, parasites or other causes, and the clinician needs to know about the injection to interpret it. Whether the eosinophil rise is linked to the skin and allergic reactions described in the label has not been established in published trials.
Is SS-31 safe for people with kidney problems?
SS-31 requires a lower dose when kidney function is poor, because the kidneys clear almost all of it. According to the Forzinity label, about 100% of a dose is recovered in the urine within 48 hours as elamipretide or its two metabolites, M1 and M2, in patients with normal kidney function. When kidney function falls, exposure rises. The label reports that elamipretide exposure (AUC) increased by 39% at a creatinine clearance of 60 to 89 mL/min, 75% at 30 to 59 mL/min, and 125% below 30 mL/min in people not on dialysis. The two metabolites climbed further, by up to 280% for M1 and 640% for M2 in severe kidney impairment. The label still reports minimal accumulation of elamipretide itself with daily dosing. The approved response is a dose cut. The label reduces the once-daily dose by one half in adults with severe renal impairment (eGFR below 30 mL/min) who are not on dialysis. Kidney function is not always known without a blood test, which is one more reason a kidney panel matters before anyone takes this peptide. No hepatic metabolism was observed in laboratory testing, so liver impairment is not expected to change elamipretide levels.
Weaker Kidneys, Higher Drug Levels
Increase in elamipretide exposure (AUC) compared with normal kidney function, by creatinine clearance, in subjects not on dialysis. The label halves the dose when eGFR is below 30 mL/min (Forzinity label, clinical pharmacology section, DailyMed).
What has not been tested in SS-31 safety studies?
Several standard questions about SS-31 have no human answer yet. The Forzinity label states plainly that carcinogenicity studies have not been conducted with elamipretide. Genetic toxicity tests were negative in a bacterial mutation assay, a chromosome aberration assay and a rat bone marrow test, and doses of up to 20 mg/kg/day in rats, about 5 times the clinical exposure at 40 mg, did not affect fertility. Those results lower the concern without replacing long-term human data. The safety population behind the approval is small and uniform. The label's clinical trial safety section describes 12 male patients aged 12 to 35 with genetically confirmed Barth syndrome. Older men, men on multiple medications and healthy men taking the peptide for years were not part of that group. Healthy older adults have received elamipretide only briefly. Roshanravan and colleagues gave a single 2-hour infusion to 39 adults aged 60 to 85 with poorly functioning mitochondria (PLoS One, 2021). Mitochondrial ATP capacity rose right after the infusion, the effect was gone by day 7, and fatigue resistance did not change. A one-day exposure says nothing about daily injections for months. The injection also contains benzyl alcohol as a preservative, 20 mg per mL, and the label carries a benzyl alcohol warning for newborns; the product is not approved for newborns or for intravenous use.
The Breakdown Products Pile Up Faster Than the Drug
Maximum increase in exposure in severe renal impairment (creatinine clearance under 30 mL/min, not on dialysis) compared with normal kidney function. The label reports minimal accumulation of elamipretide itself with daily dosing (Forzinity label, DailyMed).
Is SS-31 bought online as safe as the approved drug?
The safety data for SS-31 cannot be transferred to products sold online as research peptides, because those products were not the ones tested. Every trial in this article used elamipretide manufactured by its developer to pharmaceutical standards, at a defined dose, sterile, and given to patients who were monitored for adverse events. A vial labeled SS-31 from a peptide vendor carries none of those guarantees. Its identity, purity, actual dose, sterility and preservative content are not verified by any of the trials, and the injection site reaction and allergy rates in the label describe the approved formulation, not a powder reconstituted at home. The label's dose cut for kidney disease and its eosinophil findings also assume a clinician is watching the patient. The practical point is narrow. A reader can say that elamipretide, the drug, was well tolerated in controlled trials of up to 48 weeks. A reader cannot say that SS-31 from an unregulated source has been shown to be safe, because nobody has studied it.
Are longevity peptides safe in general?
Longevity peptides have to be judged one at a time, and most have far less human safety evidence than SS-31. Elamipretide is unusual: it went through placebo-controlled trials of up to 218 patients across myopathy, heart failure, heart attack and eye disease, and it has an FDA label that lists its side effects, its warnings and a kidney dose adjustment. The same reasoning applies to peptides marketed to men for testosterone, growth hormone or recovery. A peptide is a drug, and the questions are the same for each one: has it been tested against placebo in people, for how long, at what dose, in whom, and was the product used in those trials the product being sold? When the answers are thin, the honest conclusion is that safety has not been established, which is different from saying a peptide is dangerous and different from saying it is safe. For SS-31, the answers are better than average and still incomplete. That is the right frame for every longevity peptide, including epitalon, which this site covers in its own evidence review.
What should men take away about SS-31 safety?
SS-31, as the approved drug elamipretide, has a reasonably reassuring short-term safety record in sick patients: in trials of up to 48 weeks it was well tolerated, and the dominant side effect was irritation at the injection site. MMPOWER-3, the largest trial, described most adverse events as mild to moderate, and PROGRESS-HF found rates of drug-related adverse events similar across placebo, 4 mg and 40 mg. The label adds the cautions that the trial summaries tend to skip. Serious allergic reactions have been reported and can appear months in. Eosinophils rise. Kidney function changes exposure enough to cut the dose in half. Cancer risk has not been studied in animals or people, and long-term data cover 3 patients at 192 weeks. None of the trials studied healthy men using SS-31 for longevity, and none studied gray-market product. A man weighing SS-31 should treat it as a prescription drug with a specific label, discuss it with a physician who can check kidney function and blood counts, and recognize that the question the trials answered is not the one most buyers are asking.
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