SS-31 side effects

Very common: injection site reactions

In the Barth syndrome crossover trial, the label reports these reactions in 12 patients on each treatment:²

Other trials showed the same pattern. In a 30-adult mitochondrial myopathy trial, 80% had injection site reactions on elamipretide, mostly mild.⁷ In a 176-adult macular degeneration trial, 86% on elamipretide and 71% on placebo had an adverse event.⁸ Injection site itching, pain, bruising and redness were the most common.⁸ The label says such reactions can be treated with antihistamines or topical corticosteroids.²

Reactions after injection made 2 of 10 patients in the Barth syndrome extension stop for good.⁶ One had moderate hives at the injection site. The other had redness and hives on both sides of the abdomen, plus a rash on the opposite side that FDA reviewers thought was a whole-body (systemic) reaction; both events cleared the same day.⁶

Serious: allergic reactions

• The label reports hypersensitivity reactions, including serious allergic reactions that needed emergency treatment.²
• Signs have included rash, raised bumps, eczema-like skin inflammation and cough.²⁹
• They can begin within minutes of starting treatment or only months later.²
• The label rules out use in anyone with a known serious allergy to elamipretide or any ingredient.²
• After a serious reaction, the label says to stop the drug, give emergency care and never restart it.²

Lab changes: eosinophils

Counts of eosinophils, one kind of white blood cell, often rose in studies lasting 30 days or more.² They peaked around day 90, with a mean rise of roughly 500 to 600 cells per microliter.² Counts returned to baseline after 6 to 12 months of continued use or after stopping.² The rise was not linked to symptoms or to changes in other lab values.²

Serious adverse events in trials

• Barth syndrome extension: five serious adverse events in 3 patients, all related to infection or inflammation and judged unlikely to be caused by the drug.⁶
• Mitochondrial myopathy: no serious adverse events or deaths in the 30-adult crossover trial.⁷ In the 218-adult phase 3 trial, most adverse events were mild to moderate.³
• Heart failure: a single infusion in 36 adults caused no serious adverse events.¹⁰ In a 71-patient trial, drug-related adverse event rates were similar across groups.¹¹
• Eye drops: in 12 adults with Leber hereditary optic neuropathy, there were no serious adverse events.¹²
• Across trials: a 2022 systematic review found no serious treatment-emergent adverse events with mitochondria-targeted antioxidants.¹³ Injected elamipretide did add mild to moderate injection site events.¹³

Label warnings and unknowns

• Benzyl alcohol: Forzinity contains 20 mg per mL as a preservative.² It is not approved for newborns, because benzyl alcohol can cause fatal toxicity in premature and low-birth-weight infants.²
• Drug interactions: elamipretide blocks the kidney transporter MATE1 in lab tests, so FDA required a study of its effect on metformin.²⁴ It did not change the effects of aspirin or clopidogrel.²
• Heart rhythm: there was no clinically significant QT prolongation at 3 times the peak level of the top recommended dose.²
• Cancer: risk is unknown, and FDA required animal carcinogenicity studies after approval.⁴
• Pregnancy and breastfeeding: there are no human data, because Barth syndrome is not likely to affect females.² Animal reproduction and fertility studies found no harm at the doses tested.²
• Age and size: safety is not established under 30 kg, and the Barth syndrome studies enrolled no one aged 65 or older.²
• Overdose: none was reported in the trials, and the label expects histamine-type signs such as low blood pressure and feeling faint.²

Kidney function changes the label dose; details are on SS-31 dosage.

Research-grade SS-31

These findings come from clinical trials of elamipretide and from the label of the approved product.⁵ Research-grade SS-31 is not that product. FDA warns that injectable products go straight into the body, sometimes into the bloodstream, and bypass some of its defenses against toxins and microbes.¹⁴ FDA's August 2026 warning letters said online sellers' SS-31 was an unapproved new drug even with "research use only" labels.¹⁴ General safety points are in Peptide safety basics. Back to What is SS-31?, or see SS-31 benefits and uses.