Short answer: In the label's 12-patient placebo-controlled crossover trial, the side effects were injection site reactions: redness in 12 of 12 patients (100%) on Forzinity vs 3 (25%) on placebo, pain 75% vs 42%, hardening 67% vs 17% and itching 67% vs 17%. The label also warns about allergic reactions and benzyl alcohol risk in newborns. There is no boxed warning.
What Forzinity is approved for: Forzinity is approved to improve muscle strength in adults and children with Barth syndrome who weigh at least 30 kg. It has accelerated approval, based on improved knee extensor muscle strength; continued approval may depend on a confirmatory trial showing clinical benefit (section 1).
Source
FORZINITY (elamipretide) injection, prescribing information and Instructions for Use. Stealth BioTherapeutics. DailyMed, version published December 12, 2025. DailyMed setid 146bf34c-76f2-48db-ac07-fb29cce2cd75
No. The current Forzinity label has no boxed warning.
These are the label's own numbers from the Adverse Reactions section. Where the label compares with placebo (a dummy treatment), both columns are shown, so you can see how much more often a reaction happened on Forzinity.
| Adverse reaction | Elamipretide N=12 n (%) | Placebo N=12 n (%) |
|---|---|---|
| Any local administration reaction | 12 (100) | 8 (67) |
| Injection site erythema | 12 (100) | 3 (25) |
| Injection site induration | 8 (67) | 2 (17) |
| Injection site pruritus | 8 (67) | 2 (17) |
| Injection site pain | 9 (75) | 5 (42) |
| Injection site bruising | 3 (25) | 0 |
| Injection site urticaria | 3 (25) | 0 |
| Injection site hemorrhage | 0 | 1 (8) |
12 male patients aged 12 to 35 with genetically confirmed Barth syndrome took 40 mg daily. Each had 12 weeks of Forzinity and 12 weeks of placebo, in random order, with a 4-week washout between. Numbers are patients (percent) across both periods combined.
Ten of the 12 patients finished the randomized trial and continued on open-label Forzinity once daily. Eight took it for 168 weeks, and three of them for a total of 192 weeks.
Eosinophil counts (a type of white blood cell) often rose in studies lasting 30 days or longer, peaking around 90 days after the first dose (mean increase from baseline about 0.5 to 0.6 x 10^3/uL). They returned to baseline after 6 to 12 months of continued use or after stopping, and were not linked to symptoms or other lab changes.
Forzinity contains 20 mg of benzyl alcohol per mL. Serious and fatal reactions have been reported in low-birth-weight newborns (under 2,500 grams) and preterm newborns (under 34 weeks) given benzyl alcohol-containing drugs into a vein.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Benzyl alcohol risk in newborns. Forzinity is not approved for newborns or for use into a vein. Its preservative, benzyl alcohol, has caused serious and fatal reactions, including gasping syndrome, in premature and low-birth-weight newborns given benzyl alcohol-containing drugs intravenously. The smallest amount that can cause this is not known.
Allergic (hypersensitivity) reactions. Allergic reactions, including serious ones needing emergency care, have been reported. They have included rash, raised bumps, eczema-like skin inflammation and cough, and can start within minutes to months after treatment begins. After a serious reaction, no further doses are given and the person is not restarted on Forzinity.
Skin reactions. For mild to moderate skin reactions, the label says to consider steroid creams and antihistamine tablets. Stopping Forzinity is considered if skin reactions are persistent or severe.
Who should not use it. People who have had a serious allergic reaction to elamipretide or any of the ingredients in Forzinity.
Injection site reactions. The label tells patients that injection site reactions such as redness, pain, hardening, itching, bruising or hives can be treated with antihistamines or topical corticosteroids.
From the Patient Counseling Information (section 17) and Warnings (section 5.2); the label has Instructions for Use but no Medication Guide or Patient Information that comes with Forzinity:
Dose Track has a side-effect log. Each entry records the symptom, a severity (none, mild, moderate, severe or life-threatening), the date and an optional note, and can be tied to a specific dose or logged on its own. The history screen shows what you logged over time, which makes it easier to tell your prescriber when a reaction started and whether it is easing. The app records what you enter; it does not judge whether a symptom is serious. For anything on the label's call-your-doctor list, call your doctor.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.