When We Ask: Is Follistatin Safe for Women or Trans Individuals on Hormone Therapy?

Follistatin is a glycoprotein — a protein combined with sugar chains — naturally produced in the body. Its primary role is binding to and neutralizing proteins such as myostatin and activin, which normally act to slow muscle growth or regulate reproductive processes. By inhibiting those proteins, follistatin can promote muscle growth and can interact with reproductive hormone signalling.

The question here is whether follistatin can be used safely by women or transgender people undergoing hormone therapy — a group whose endocrine (hormonal) balance is already being medically altered. The short answer from the published literature is that nobody has studied it in that group, and follistatin has no approved human use of any kind. It exists only as gene therapy inside early-phase trials and as an unregulated "research peptide" sold outside medical supervision, with no established safe dose.

What the published trials actually studied

In a phase 1/2a trial published in Molecular Therapy in 2015, Mendell and colleagues delivered a genetic construct of follistatin (FS344) via an adeno-associated virus (AAV1) into the leg muscles of adult patients with Becker muscular dystrophy, a degenerative muscle disease. Participants were reported to walk farther in six-minute walk tests, with improved muscle histology. The report noted no adverse effects, but the trial was small, in men with one specific disease, and it measured muscle rather than hormone systems.

A primate study delivered the same AAV1-FS344 construct into the quadriceps of cynomolgus macaques and reported increased muscle size with sustained expression over many months. Hormonal measures — including estrogen, testosterone, and follicle-stimulating hormone (FSH) — were reported unchanged, which suggests reproductive hormone systems were not overtly affected in those animals. These were healthy primates, not people undergoing hormone therapy, and unchanged hormone levels in a macaque quadriceps study say nothing about someone taking estrogen or anti-androgens.

A more recent human phase 1 trial, registered as NCT06411366 and sponsored by the company Minicircle, assessed an injectable plasmid carrying the FST344 gene in adult volunteers, with scans and blood tests before and after injection. It is listed as completed on ClinicalTrials.gov. Its results have circulated only as a preprint hosted by the sponsoring company, which has not been peer reviewed or published in a journal. Findings presented that way are not independent evidence and no results from it are described here. The trial did not enrol women on hormone therapy.

Turning to women with metabolic and reproductive conditions, Díaz, de Zegher and Ibáñez published a randomized-treatment study of adolescents with polycystic ovary syndrome (PCOS) in Frontiers in Endocrinology in 2023, comparing an oral contraceptive against insulin-sensitizing treatment. The authors reported that circulating follistatin diverged sharply between the two treatments, rising on the contraceptive, and that the follistatin changes tracked with less favourable insulin and hepatic-fat measures — metabolic markers that are already relevant to people on hormone therapy.

Why We Should Be Critically Concerned

Several concerns follow from these data. First, while follistatin gene therapy has been tolerated in very specific, small populations, no study has tested it in women undergoing estrogen therapy or in transgender people on any hormone regimen. Primate data cannot substitute for human results in a hormone-altered context. Second, the PCOS findings point the wrong way: higher follistatin tracked with worse metabolic markers in young women, which matters because hormone therapy itself affects metabolism, so the two could compound.

Third, the human trials so far are short, small, and measured endpoints unrelated to reproductive or metabolic safety. There is no long-term, large-scale data in the relevant populations. There is also no efficacy data at all for follistatin in hair loss in women or transgender people — the single small phase 1 hair trial that exists enrolled men.

What targeted research would have to establish

Answering this question would require studies that enrol women and transgender people on hormone therapy, and that measure not only muscle outcomes but metabolic health (insulin sensitivity, liver fat), reproductive hormone levels, and safety over an extended period. Effects on fertility and in pregnancy are entirely unstudied. Without that data, any use of follistatin in this context is speculative.

The conclusion the published evidence supports is that this is unknown territory: the current science does not address this population. Follistatin has shown effects in muscle disorders and has been tolerated in narrow trials, but there is no evidence for safety or efficacy in women or transgender people undergoing hormone therapy. The metabolic signal from the PCOS data points to a possible downside rather than a reassurance. Until targeted research exists, follistatin should be treated as off-limits for this group.

Talk to a doctor or pharmacist — and, for anyone on hormone therapy, the clinician who manages that therapy — before starting, stopping or combining any hair-loss treatment. Follistatin is not an approved medicine, has no established human dose, and injectable or "research peptide" products sold outside a clinical trial are unregulated, of unverified content, and of unknown safety.

User Experiences: Follistatin Use in Women and Transgender Individuals Undergoing Hormone Therapy

Community discussions on Tressless treat follistatin with curiosity and with caution, particularly for women and transgender people undergoing hormone therapy. Posters note that research describing follistatin's potential to stimulate hair growth by inhibiting myostatin and modulating activin pathways is largely preclinical, and that human safety data is thin — especially for anyone outside the male participants of the trials that exist.

Several members reference follistatin's inclusion in experimental hair regrowth preparations alongside Wnt signalling activators and growth factors. A phase 1 trial of that approach was described as well tolerated in male participants, but it did not address hormone-related contexts, pregnancy risks, or long-term endocrine effects in women or transgender people. That evidence gap is a recurring theme in the threads, with longer-standing users warning that the interaction between follistatin and hormone therapy is unknown and could in principle disturb endocrine balance.

Posters with a background in bodybuilding or peptide use describe follistatin's popularity in muscle-enhancement circles, where unregulated products are used at doses far above anything physiological. There is no established safe human dose for follistatin in any form, so these are not doses derived from evidence. Community members caution that such practices carry risks including unpredictable hormone signalling, especially in bodies already undergoing significant hormonal modulation from estrogen, testosterone, or anti-androgens. The recurring conclusion in those threads is that without targeted studies, dosing, safety, and interaction profiles are guesswork.

Threads also acknowledge that while follistatin has shown regenerative and follicle-promoting effects in mice and in one small human trial, the mechanism it works through — blocking activin — has roles beyond hair growth, including in reproductive tissue function and wound healing. For women and transgender users, community advice is that these functions may overlap with the targets of hormone therapy, which makes self-experimentation riskier and is a reason not to attempt it without medical oversight.

Overall, the community view is one of interest paired with a clear warning: follistatin is an experimental compound, not a hair-growth product. The absence of any female- or transgender-specific data means it cannot be described as safe, and the threads consistently point readers toward specialist guidance rather than self-experimentation.

References

Mendell, J. R., Sahenk, Z., Malik, V., Gomez, A. M., Flanigan, K. M., Lowes, L. P., Alfano, L. N., Berry, K., Meadows, E., Lewis, S., Braun, L., Shontz, K., Rouhana, M., Clark, K. R., Rosales, X. Q., Al-Zaidy, S., Govoni, A., Rodino-Klapac, L. R., Hogan, M. J., & Kaspar, B. K. (2015). A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy. Molecular Therapy, 23(1), 192–201. PMID: 25322757. https://pubmed.ncbi.nlm.nih.gov/25322757/

Injection of AAV1-FS344 into the quadriceps increases muscle mass in cynomolgus macaques. https://pmc.ncbi.nlm.nih.gov/articles/PMC2852878/

Díaz, M., de Zegher, F., & Ibáñez, L. (2023). Circulating follistatin concentrations in adolescent PCOS: Divergent effects of randomized treatments. Frontiers in Endocrinology, 14, 1125569. PMID: 36843579. https://pubmed.ncbi.nlm.nih.gov/36843579/

U.S. National Library of Medicine. Phase I: Safety and Efficacy of an Injectable Follistatin Plasmid Gene Therapy in Humans (sponsor: Minicircle). ClinicalTrials.gov Identifier: NCT06411366. https://clinicaltrials.gov/study/NCT06411366

Reddit user. Follistatin — thoughts about a potential treatment. https://reddit.com/r/tressless/comments/16f1m6n/follistatin_thoughts_about_a_potential_treatment/

Reddit user. Injections of follistatin and growth factors. https://reddit.com/r/tressless/comments/ud92t/injections_of_follistatin_and_growth_factors/

Reddit user. Hair regrowth following a Wnt and follistatin treatment. https://reddit.com/r/tressless/comments/27aycv/hair_regrowth_following_a_wnt_and_follistatin/