Ethics of Myostatin Inhibitors in Bodybuilding

Why fairness comes up first

Most debates about drugs in bodybuilding open with fairness. The World Anti-Doping Code itself lists "fair competition" and "a level playing field" among the reasons for anti-doping programs, next to athlete health.

That framing has serious critics in the bioethics literature. In a 2004 paper in the British Journal of Sports Medicine titled "Why we should allow performance enhancing drugs in sport," Julian Savulescu and colleagues argued against blanket bans. A later reply described their position as treating risk to athletes' welfare as the only legitimate ground for restricting these drugs. Bengt Kayser and colleagues went further in BMC Medical Ethics (2007). They wrote that the case for anti-doping rests on "largely unsubstantiated assumptions about fairness in sports" and proposed enhancement under medical supervision with a harm-reduction approach.

The opposing view has its own defenders. In that 2011 reply, also in the British Journal of Sports Medicine, J. W. Devine argued that doping is objectionable for reasons beyond health: it disrupts the "balance of excellences" a sporting contest is meant to measure.

For myostatin inhibitors, the fairness question sits on top of a more basic gap. Very little is known about what these drugs do in healthy, trained people (the myostatin inhibitors overview covers the biology), so much of the debate shifts toward consent and health.

Autonomy and informed consent

The autonomy argument says adults may accept risks to their own bodies, as they do in climbing or combat sports. But informed consent depends on information. Here the record is thin.

As of October 2026, the only approved myostatin-targeted drug is apitegromab (Isembyld). The FDA approved it in September 2026 for spinal muscular atrophy (SMA) in people aged 2 and older who already receive an SMN2-targeted treatment. According to the FDA's announcement, the main evidence comes from a 52-week trial in 188 young people with SMA who couldn't walk independently, and the agency notes an increased risk of fractures, including serious ones. None of that tells us much about a healthy lifter.

The earlier history is less reassuring. ACE-031, a decoy receptor that mops up myostatin and related proteins, was stopped in a Duchenne muscular dystrophy trial in 2011 after nosebleeds, gum bleeding and small dilated skin blood vessels (telangiectasias). The program was dropped by 2013. Our ACE-031 overview covers what that trial found.

The products bodybuilders can actually buy are a separate problem. "Follistatin" and "ACE-031" sold as research chemicals have no human data, and there's no independent evidence on what those vials contain. Kayser and colleagues pair their autonomy argument with medical supervision, which assumes a doctor can monitor known risks. For research-market myostatin products, the known risks are borrowed from disease trials of different molecules.

Terminology matters here too. Myostatin signals through activin type II receptors, mainly ActRIIB (the gene is ACVR2B). The receptor itself isn't an inhibitor. Drugs that block it, such as the antibody bimagrumab, or that act as decoys, such as ACE-031, also interfere with other proteins that use the same receptor, including activin A.

Health of the sport and younger lifters

The collective-responsibility argument starts from the fact that elite physiques set expectations. If the top of the sport depends on drugs that aren't disclosed, younger competitors may chase a look that was never achieved naturally, and take risks they don't understand to get there.

The mortality data give that concern some weight, although they aren't specific to myostatin inhibitors. A 2025 study in the European Heart Journal followed 20,286 male competitors from 730 IFBB events held between 2005 and 2020. Researchers recorded 121 deaths, 46 of them classed as sudden cardiac deaths. Professionals died at a much higher rate than amateurs over follow-up (2.8% vs 0.4%), and available autopsies of the sudden cardiac deaths showed enlarged hearts. The authors argued that the sport lacks robust anti-doping testing and called for health screening.

Tested and untested federations

Bodybuilding is unusual because tested and untested competition exist side by side. According to a 2022 Washington Post investigation, the IFBB Pro League, the largest professional federation in the United States, doesn't routinely drug test athletes, and there's no widespread testing at NPC shows either. A subset of shows are branded "natural" and use urine tests or polygraphs.

NPC Worldwide's natural contest rules (last updated August 2026) set the minimum. Testing must cover at least anabolic agents (WADA class S1) and diuretics and masking agents (S5). At a natural regional show, only the overall winner in each open division must be tested; at a natural pro qualifier, the top two. The rules also set a three-year clean period for hormone and metabolic modulators. Myostatin inhibitors sit in WADA class S4, so a show testing only the required categories may not screen for them. We couldn't find any federation that says it routinely screens for myostatin agents.

The World Natural Bodybuilding Federation (WNBF) says its testing follows the World Anti-Doping Code and Prohibited List. It requires a polygraph for all competitors and asks for 10 years free of peptides and growth hormone before a first contest. Since January 2022 it has imposed lifetime bans for failed tests involving peptides and other performance-enhancing drugs.

Part of the ethical debate here is about honesty: untested federations make no drug-free claim, while tested ones make a claim that may not be checked for every drug class. Kayser and colleagues' harm-reduction proposal, which pairs enhancement with medical supervision, speaks most directly to untested contests, where drug use is open but no medical oversight is required.

How WADA decides what to ban

Under Article 4.3 of the 2021 World Anti-Doping Code, WADA may add a substance to the Prohibited List if it meets any two of three criteria. These are evidence that it enhances or has the potential to enhance performance, evidence of an actual or potential health risk, and WADA's judgment that it violates the spirit of sport. Because the criteria include potential effects, WADA can list a drug before anyone has data on its use in athletes.

As of October 2026, WADA's 2026 Prohibited List places these agents in section S4.3, "Agents preventing activin receptor IIB activation," prohibited at all times. Examples named on the list include ACE-031, bimagrumab, follistatin, myostatin propeptide, and myostatin antibodies such as apitegromab. The 2027 List, which takes effect on January 1, 2027, uses the same S4.3 wording.

Athletes with a genuine medical need can apply for a therapeutic use exemption under Article 4.4 of the Code, although NPC Worldwide's natural rules don't allow exemptions at all. The legal status page for myostatin blockers in sport covers sanctions in more detail.

Does the ban block research?

An earlier version of this page claimed that WADA's ban kills research investment and guarantees that the only human data will come from underground use. That isn't what has happened. The Prohibited List governs athletes and others bound by the Code rather than drug developers. Apitegromab reached approval in 2026 while named on the list.

The BELIEVE Phase 2 trial was sponsored by Eli Lilly's subsidiary Versanis and published in Nature Medicine in 2026. It randomized 507 adults with obesity across nine groups comparing bimagrumab and semaglutide, alone and together, against placebo. At 48 weeks, people on the combination lost 0.8% to 2.3% of their lean mass, compared with 4.7% to 6.9% on semaglutide alone. In Regeneron's Phase 2 COURAGE trial, reported on October 1, 2026, people given trevogrumab (an anti-myostatin antibody) with semaglutide lost 5.8% or 4.2% of lean mass at 52 weeks, depending on dose. The semaglutide-only group lost 7.3%.

A narrower critique still holds: these trials study disease and obesity, not muscle building in trained people. Kayser and colleagues' point that anti-doping pushes use underground, away from medical supervision, applies to the research-chemical market whatever happens in clinical trials. Our broader page on myostatin-inhibitor ethics looks at how the questions change as trial data accumulate.

What we still don't know

The obesity trials haven't yet shown clearly whether preserving lean mass translates into more strength or better function, and no trial has tested these drugs for muscle building in healthy, trained adults. It isn't known whether the fracture risk seen with apitegromab in SMA applies to other groups. It's also unclear how often, if ever, bodybuilding federations test for S4.3 agents, which leaves the "tested" label hard to interpret for this drug class.