SS-31 (Elamipretide)

SS-31 is a chain of just four amino acids, written D-Arg-2,6-dimethyl-Tyr-Lys-Phe-NH2, that carries a positive electrical charge and is drawn into the inner membrane of the mitochondria, the compartments inside your cells that turn food and oxygen into usable energy. It was developed in the laboratory of Hazel Szeto and Peter Schiller, which is where the SS in the name comes from, and it has since carried three other names as it moved through drug development: MTP-131, then Bendavia, then elamipretide, and now the brand name Forzinity. That last name matters, because this is not a research-only molecule. On 19 September 2025 the US Food and Drug Administration granted accelerated approval to Forzinity, 40 mg injected under the skin once daily, to improve muscle strength in adults and children weighing at least 30 kg who have Barth syndrome, a rare inherited disease in which the mitochondrial fat cardiolipin is not assembled correctly. That is the entire approved indication. Everything else it has been tested for, including mitochondrial myopathy, heart failure, age-related macular degeneration and ordinary age-related muscle fatigue, has been studied in trials that either failed their main endpoint or were too small to settle anything, and the vials sold as research-grade SS-31 are not the approved product.

• Barth syndrome, the one approved use, where it is given to improve knee extensor muscle strength under accelerated approval
• Primary mitochondrial myopathy, an inherited muscle disease, where it went through three trials and missed the main endpoint in the largest one
• Heart failure, where two trials measured changes in heart chamber volume and pumping function with mixed and mostly null results
• Age-related muscle fatigue and low energy, based on a single-dose study in 39 adults aged 60 to 85 that measured mitochondrial energy production in the thigh
• Eye disease, including dry age-related macular degeneration and Leber hereditary optic neuropathy, studied in separate trials
• Longevity and endurance clinics also sell it for general mitochondrial support and exercise capacity, which is a marketing extension of the trial work rather than a tested indication

Picture a mitochondrion as a folded coffee filter. The folds, called cristae, are where energy actually gets made, and the folds are held in shape by a distinctive fat called cardiolipin. As tissue ages or is starved of blood, that fat gets chemically damaged, the folds flatten out and the energy line slows down. SS-31 is a small positively charged molecule that is pulled toward that fat and sits on it, which appears to keep the folds crisp and to stop one of the resident proteins from turning into a damaging oxidant. It is less a fuel and more a clip that holds the filter in shape.

1. First doseAbsorption is fast: after a subcutaneous injection, peak blood levels are reached within 30 to 60 minutes, and about 92 percent of the injected dose reaches the circulation (FDA label for Forzinity, 2025). In 39 adults aged 60 to 85, a single infusion raised the measured rate of mitochondrial energy production in thigh muscle compared with placebo on the same day, although it did not change how quickly the muscle fatigued (Roshanravan et al., 2021).
2. Week 1-2Nothing durable has been shown in this window. In the same older-adult study the single-dose improvement in mitochondrial capacity had disappeared by day 7. Expect injection site redness from the first week onwards, which occurred in every participant on drug in the Barth syndrome trial.
3. Week 12This is the point at which the two best-designed blinded comparisons read out, and both were negative on their main endpoints. In 12 men and boys with Barth syndrome, 12 weeks of 40 mg daily did not beat placebo on walking distance or fatigue score, and knee extensor strength did not increase either (Reid Thompson et al., 2021). In 71 people with heart failure, 28 days of 4 mg or 40 mg daily did not change left ventricular end-systolic volume (Butler et al., 2020).
4. Week 24The phase 3 mitochondrial myopathy trial ended here. In 218 participants, 24 weeks of 40 mg daily under the skin produced a difference in six-minute walk distance of -3.2 metres versus placebo and no improvement in fatigue (Karaa et al., 2023). A later post hoc analysis found a walking improvement confined to the subgroup whose disease was caused by nuclear DNA variants affecting mitochondrial DNA maintenance (Karaa et al., 2024).
5. Week 36-168The only evidence of sustained benefit comes from an unblinded extension with no control group. Eight to ten Barth syndrome patients who stayed on daily 40 mg showed a median rise in knee extensor strength of 34 newtons at week 12 of the extension, 68 at week 24 and 63 at week 168, and this uncontrolled trajectory is what the FDA accepted for accelerated approval. Because there was no placebo arm, natural change and the practice effect of repeated testing cannot be separated out.

• Injection site reactions are the dominant and near-universal effect. In the placebo-controlled Barth syndrome crossover study, 12 of 12 patients (100 percent) had injection site erythema on elamipretide versus 3 of 12 (25 percent) on placebo, with induration and itching in 67 percent, pain in 75 percent, and bruising and hives each in 25 percent.
• A rise in blood eosinophils, a type of white cell, was seen frequently when dosing continued for 30 days or longer. Counts peaked around day 90 with a mean increase of roughly 0.5 to 0.6 thousand per microlitre, then returned to baseline after 6 to 12 months of continued use or after stopping. The FDA label states this was not accompanied by symptoms or other laboratory changes.
• Immediate hypersensitivity reactions are recognised on the label, and serious hypersensitivity to elamipretide or the formulation is the only listed contraindication. Signs of an immediate reaction are an instruction to stop and seek care.
• Symptoms of overdose are described as histamine-related, meaning a drop in blood pressure and near-fainting. That flags the general mechanism behind the local reactions as well.
• Across the mitochondrial myopathy and heart failure trials, investigators reported the drug as generally tolerated with adverse events mostly mild to moderate; the rate of drug-related events in the 71-person heart failure trial was similar across placebo, 4 mg and 40 mg.
• Clinically significant prolongation of the QTc interval on the electrocardiogram was not observed at three times the peak concentration of the approved dose.

• You are pregnant, trying to conceive, or breastfeeding. Because Barth syndrome is X-linked and essentially does not occur in females, there are no human pregnancy or lactation data at all; the only reassurance is rat and rabbit reproductive studies, and the approved formulation contains benzyl alcohol.
• The patient is a neonate. The approved product carries a warning about benzyl alcohol toxicity, which has caused fatal metabolic acidosis and gasping syndrome in low-birth-weight and preterm infants, and it is not approved for neonates or for intravenous use.
• You have severe kidney impairment. Elamipretide and its two metabolites are cleared almost entirely in urine; exposure rose 125 percent with a creatinine clearance below 30 mL/min, and metabolite exposure rose up to 640 percent, which is why the label halves the dose in that group and gives no recommendation for people on dialysis.
• You have had a serious hypersensitivity reaction to elamipretide or to any component of the injection.
• You have a history of cancer. No carcinogenicity data were located in this research, and the compound concentrates in mitochondria across all tissues.
• You are an athlete subject to drug testing, until you have checked the current status with your anti-doping organisation, since a newly approved peptide drug used off-label sits in an area this research could not settle from the published Prohibited List.
• You are not under a physician who knows you are injecting this and can check kidney function and blood counts.

• Assuming the cheatsheet dose matches the tested dose. Every completed trial used 40 mg subcutaneously once daily, or an intravenous infusion, and the approved dose is 40 mg daily; the 10 to 20 mg figure on the cheatsheet is a compounding-market convention that no published trial evaluated.
• Cycling it 4 to 8 weeks on and 4 weeks off because that is what peptide protocols do. The only signal of benefit anywhere in this literature came from continuous daily dosing for 168 weeks in the Barth syndrome extension, and every blinded comparison of 12 to 24 weeks was negative. Stopping and restarting has never been studied.
• Reading the FDA approval as an endorsement of general mitochondrial use. It is an accelerated approval for one ultra-rare genetic disease, based on knee extensor strength as an intermediate endpoint in an uncontrolled extension of a 12-patient trial, and continued approval depends on a confirmatory trial.
• Ignoring injection site care. Reactions occurred in 100 percent of treated patients, and the label's own advice is to rotate sites daily, inject into the abdomen at least two inches from the navel or the outer thigh, avoid tender, bruised or hardened skin, and treat reactions with antihistamines or topical corticosteroids.
• Storing it like a room-temperature supplement. The approved solution is refrigerated at 2 to 8 degrees Celsius, must not be frozen, and is discarded 8 days after first opening; lyophilised research powder reconstituted at home has no comparable stability data.