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ProteinLimited humanNot FDA approved

Follistatin

Follistatin

Last reviewed October 27, 2026 · 13 sources · Written for general readers · Compare with another compound →

The bottom line

Follistatin is a natural protein that blocks myostatin, a brake on muscle growth, and boosting it builds muscle in mice and monkeys. In people, only gene therapy in a few patients with muscle diseases and a related lab built drug have been tested, and that drug grew muscle without making it stronger. The follistatin 344 protein sold online has never been tested in people, is not approved and is banned in sport.

Where the evidence stands

What it is

Follistatin is a protein the body makes. It binds myostatin, a signal that limits muscle growth, and stops it from working.[1] Because it is a large protein, no simple chemical formula is listed here.

Follistatin 344 (FS344) is one natural splice form of the human follistatin gene. In research it has been given as gene therapy: an engineered virus called AAV1 carries the gene into muscle, so the muscle makes follistatin itself.[2][3]

That approach is very different from injecting follistatin protein. We found no human trials of injected follistatin protein, and it is not an approved drug.[3][4]

Name
Follistatin (Follistatin)
Other names
FST, follistatin 344, FS344
Type
Protein

How it works

Myostatin keeps muscle size in check. Follistatin grabs myostatin and blocks it from reaching its receptor, called activin receptor type IIB, which lets muscle grow larger.[1]

  • Mice built to make extra follistatin in muscle gained a large amount of muscle mass, similar to mice that lack myostatin.[1]
  • In monkeys, FS344 gene delivery into the thigh muscle produced lasting gains in muscle size and strength.[2]
  • Follistatin also blocks activin, a hormone signal involved in reproduction, and it was first found in the ovary as a protein that lowers FSH, a hormone needed for fertility.[5]
  • ACE-083, a drug built from follistatin, binds myostatin, activin A, activin B and GDF11 in the lab, and grows only the muscle it is injected into.[6]

The myostatin link is solid in animals. Whether injected protein does anything useful in people is unknown.

What the research shows

We keep animal and human evidence apart. A result in animals is a reason to study something in people, not proof that it works in people.

Animal and lab studies

Strong muscle effects in mice and monkeys

  • Mice that made extra follistatin in muscle had large gains in muscle mass.[1]
  • Macaque monkeys given FS344 gene therapy in the thigh muscle gained size and strength, and long term follistatin production caused no abnormal changes in the structure or function of key organs.[2]
  • In mice, ACE-083 injected into one muscle grew that muscle and raised its force, including in mouse models of Duchenne muscular dystrophy and Charcot-Marie-Tooth disease, without signs of effects elsewhere in the body.[6]
  • Mice engineered to make extra follistatin throughout the body had small testes and ovaries, thin uteri and blocked egg development, and several lines became infertile.[9]

Limit: These results came from genetic engineering or gene therapy, not from injecting the protein.

Human studies

Tiny gene therapy trials and a related drug that grew muscle but not strength

  • In an early phase trial, 6 men with Becker muscular dystrophy received FS344 gene therapy into both thigh muscles, with no control group. Four of the six walked farther in a six minute walk test and two did not, and no side effects were reported.[3]
  • In 6 people with sporadic inclusion body myositis, a muscle wasting disease, walk distance rose about 56 meters per year after gene therapy, while 8 matched untreated people lost about 26 meters per year. Patients were not randomly assigned.[4]
  • In a first in human trial, 58 healthy women after menopause received ACE-083, a follistatin based drug, or placebo as one or two injections into one thigh or shin muscle. Injected muscle volume rose by up to about 15 percent in the thigh and 9 percent in the shin, but strength did not change significantly. There were no serious side effects.[7]
  • In a randomized double blind trial of 58 adults with facioscapulohumeral muscular dystrophy, 6 months of ACE-083 injections into arm or shin muscles increased muscle volume by about 16 and 10 percent compared with placebo, but did not consistently improve function or how patients felt. The most common side effects were mild or moderate injection site reactions.[8]
  • We found no human trials of injected follistatin protein itself, the form sold online as follistatin 344.[3][4]

Limit: The gene therapy trials were tiny and uncontrolled, and the controlled trials tested ACE-083, a different molecule. None tested the follistatin protein sold online.

Claims check

The most common claims about this compound, and what studies actually found. Verdicts rate the evidence, not whether you should use it.

Builds muscle

Mixed results

In animals, extra follistatin builds large amounts of muscle.[1][2] In people, the closest test is ACE-083, a lab made drug based on follistatin injected into one muscle. In a placebo controlled trial in 58 healthy women after menopause, it raised the volume of the injected thigh muscle by up to about 15 percent from where it started.[7] This is a different molecule from the follistatin 344 sold online, which has never been tested in people.

Increases strength

No benefit in trials so far

In the 58 healthy women, ACE-083 grew the injected muscle, but average strength did not change significantly.[7] In a randomized placebo controlled trial of 58 adults with facioscapulohumeral muscular dystrophy, 6 months of injections grew the treated muscles by about 10 to 16 percent, but function and patient reported outcomes did not consistently improve.[8]

Helps people with muscle diseases walk better

Mixed results

In 6 men with Becker muscular dystrophy given follistatin gene therapy into the thighs, 4 walked farther in a six minute walk test and 2 did not.[3] In 6 people with inclusion body myositis, walking distance rose about 56 meters per year while 8 untreated people declined.[4] Neither study was randomized or placebo controlled.

Burns fat

Not tested

We found no human study of follistatin for fat loss. In one trial of ACE-083 in muscular dystrophy, the fat share inside one treated leg muscle fell, but whole body fat was not the target.[8]

Slows aging

Not tested

We found no study that tested follistatin protein or gene therapy for aging in people. The human studies were in muscle diseases or tested a related drug for muscle size.[3][7]

Known risks and side effects

  • Unknown safety of injected protein. Human studies used gene therapy or a different, lab built drug, so the safety of injecting follistatin protein is unknown.[3][4] In trials of the related drug ACE-083, the most common side effects were mild or moderate injection site reactions.[8]
  • Possible effects on fertility hormones. Follistatin lowers FSH, a hormone needed for fertility, and researchers have flagged reproduction as a safety concern.[5] Mice that made extra follistatin throughout the body had shrunken testes and ovaries, and many became infertile.[9]
  • Fake or contaminated products. Falsified protein drugs can contain the wrong ingredient, the wrong amount, nothing active, or toxic contaminants.[10] In a study of 1,190 doping products seized at the Swiss border, 12 percent were labeled as peptide hormones or growth factors, most products were of poor quality and fewer than 20 percent held the claimed amount.[11]
  • Banned in sport. WADA bans follistatin and other agents that block activin receptor IIB signaling.[12]

Regulatory status

Today: Follistatin is not approved by FDA for any use, and it does not appear on FDA's 503A compounding nomination list.[13] WADA bans it.[12]

  1. The 2026 WADA Prohibited List took effect and names follistatin as a prohibited agent.[12]

Sources

  1. Lee SJ, McPherron AC. Regulation of myostatin activity and muscle growth. Proc Natl Acad Sci U S A. 2001;98(16):9306-9311. PMID 11459935
  2. Kota J, Handy CR, Haidet AM, et al. Follistatin gene delivery enhances muscle growth and strength in nonhuman primates. Sci Transl Med. 2009;1(6):6ra15. PMID 20368179
  3. Mendell JR, Sahenk Z, Malik V, et al. A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy. Mol Ther. 2015;23(1):192-201. PMID 25322757
  4. Mendell JR, Sahenk Z, Al-Zaidy S, et al. Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes. Mol Ther. 2017;25(4):870-879. PMID 28279643
  5. Rodino-Klapac LR, Haidet AM, Kota J, Handy C, Kaspar BK, Mendell JR. Inhibition of myostatin with emphasis on follistatin as a therapy for muscle disease. Muscle Nerve. 2009;39(3):283-296. PMID 19208403
  6. Pearsall RS, Davies MV, Cannell M, Li J, Widrick J, Mulivor AW, et al. Follistatin-based ligand trap ACE-083 induces localized hypertrophy of skeletal muscle with functional improvement in models of neuromuscular disease. Sci Rep. 2019;9(1):11392. PMID 31388039
  7. Glasser CE, Gartner MR, Wilson D, Miller B, Sherman ML, Attie KM. Locally acting ACE-083 increases muscle volume in healthy volunteers. Muscle Nerve. 2018;57(6):921-926. PMID 29486514
  8. Statland JM, Campbell C, Desai U, Karam C, Díaz-Manera J, Guptill JT, et al. Randomized phase 2 study of ACE-083, a muscle-promoting agent, in facioscapulohumeral muscular dystrophy. Muscle Nerve. 2022;66(1):50-62. PMID 35428982
  9. Guo Q, Kumar TR, Woodruff T, Hadsell LA, DeMayo FJ, Matzuk MM. Overexpression of mouse follistatin causes reproductive defects in transgenic mice. Mol Endocrinol. 1998;12(1):96-106. PMID 9440814
  10. Janvier S, De Spiegeleer B, Vanhee C, et al. Falsification of biotechnology drugs: current dangers and/or future disasters? J Pharm Biomed Anal. 2018;161:175-191. PMID 30165334
  11. Weber C, Krug O, Kamber M, et al. Qualitative and Semiquantitative Analysis of Doping Products Seized at the Swiss Border. Subst Use Misuse. 2017;52(6):742-753. PMID 28156209
  12. World Anti-Doping Agency. World Anti-Doping Code: 2026 Prohibited List, section S2 (peptide hormones, growth factors, related substances and mimetics). In effect January 1, 2026. WADA 2026
  13. U.S. Food and Drug Administration. Bulk Drug Substances Nominated for Use in Compounding Under Section 503A of the Federal Food, Drug, and Cosmetic Act. Updated May 14, 2026. FDA 503A categories
Medical disclaimer

This page is educational information only. It is not medical advice and does not replace care from a licensed medical provider. Do not use this page to decide whether to take any substance. Talk to a licensed medical provider first.

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