YK-11 is an experimental compound classified as a selective androgen receptor modulator, or SARM. It has never been tested in humans, is not approved for medical use in any country, and is banned in competitive sports. Despite this, it has gained attention in bodybuilding circles for its unusual dual mechanism: it activates the androgen receptor like other SARMs while also triggering a separate pathway that may limit a natural brake on muscle growth.
How YK-11 Is Classified
YK-11 sits in an unusual gray area. Its molecular formula (C25H34O6) is structurally derived from a modified steroid backbone, specifically a 19-norsteroid, which is the same family that includes the synthetic steroid nandrolone. Yet researchers classify it as a SARM because of how it behaves at the androgen receptor. Unlike traditional anabolic steroids, which fully activate the receptor and trigger a wide range of hormonal effects throughout the body, YK-11 acts as a partial agonist. It switches on some androgen receptor functions in muscle and bone tissue without producing the full activation pattern that testosterone or DHT would.
This partial activation is what makes it “selective” in the SARM sense. Research published in Biological and Pharmaceutical Bulletin showed that YK-11 activates the androgen receptor without inducing the so-called N/C interaction, a specific internal folding of the receptor that DHT strongly triggers. That distinction matters because the N/C interaction is linked to broader androgenic effects like prostate growth and hair loss. In theory, skipping that step could mean fewer of those side effects, though this has only been demonstrated in cell cultures, not in living humans.
The Follistatin Connection
What sets YK-11 apart from other SARMs is its effect on follistatin, a protein that blocks myostatin. Myostatin is your body’s built-in limiter on muscle growth. It tells muscle cells to stop growing once they reach a certain size. Follistatin counteracts that signal, essentially removing the cap.
In lab studies using C2C12 muscle cells (a standard cell line for muscle research), YK-11 increased follistatin expression in a way that DHT, the body’s most potent natural androgen, did not. When researchers blocked follistatin with an antibody, the muscle-building effects of YK-11 disappeared. The same thing happened when they blocked or knocked down the androgen receptor itself, confirming that YK-11’s follistatin boost depends on androgen receptor activation rather than being some independent effect.
This is why you’ll sometimes see YK-11 described as both a SARM and a myostatin inhibitor. Technically, it doesn’t inhibit myostatin directly. It increases follistatin production through the androgen receptor, and follistatin does the actual blocking.
Effects on Bone Tissue
Beyond muscle cells, YK-11 has shown effects on bone-forming cells in laboratory and animal research. A study using bone marrow stem cells found that YK-11 promoted their proliferation and pushed them to develop into bone-building cells (osteoblasts) at concentrations ranging from 0.25 to 4 micromolar, with stronger effects at higher concentrations. In live animals, YK-11 applied at 0.5 and 1 mg/mL promoted repair of cranial bone defects in rats.
The bone-building pathway appears to work through a signaling cascade involving bone morphogenetic protein 2, a well-known driver of bone formation. When researchers blocked the androgen receptor, both the bone cell differentiation and the skull defect repair stopped working, confirming the effect runs through the same receptor as its muscle activity. These results are promising for understanding the compound’s biology, but they remain a long way from demonstrating any clinical benefit in people.
No Human Trials Exist
Every piece of evidence on YK-11 comes from cell culture experiments or animal studies. No Phase 1, 2, or 3 human clinical trials have been conducted. The animal research that does exist raises its own concerns. A study in rats examined YK-11’s effects on the brain and found it induced oxidative stress and mitochondrial dysfunction in the hippocampus, the brain region central to memory and learning. The researchers concluded that further study is needed to assess safety in humans.
This is a critical gap. Many compounds that look effective in cell dishes or rodents fail or cause unexpected harm in people. Without human pharmacokinetic data, there is no established safe dose, no understanding of how the body metabolizes the compound over time, and no data on drug interactions. Anyone using YK-11 is essentially experimenting on themselves with no clinical roadmap.
Known and Suspected Side Effects
Because no controlled human studies exist, side effect data comes from animal experiments and self-reported experiences from users. The most consistently reported issue is testosterone suppression. YK-11 activates the androgen receptor, which signals the brain to reduce its own production of testosterone through the hormonal feedback loop. Users commonly report lowered libido during use, which typically reverses within weeks of stopping.
Liver toxicity is a theoretical concern given YK-11’s methylated steroid structure. Methylation at the 17-alpha position, a feature common to oral anabolic steroids, is a well-known driver of liver stress. While no published study has measured liver enzyme elevations specifically from YK-11 in humans, the structural similarity to hepatotoxic compounds makes this a legitimate worry, particularly at higher doses or with prolonged use.
The hippocampal oxidative stress observed in rats adds another layer of concern. Whether this translates to cognitive effects in humans is unknown, but it suggests the compound’s activity isn’t limited to muscle and bone.
Legal and Regulatory Status
The World Anti-Doping Agency lists YK-11 by name under its 2025 Prohibited List, categorized under “S1.2 Other Anabolic Agents” alongside other SARMs like ostarine, ligandrol, and RAD-140. It is banned both in and out of competition for all athletes subject to WADA testing.
In the United States, the FDA has not approved YK-11 for any medical use. It is not a legal dietary supplement. Companies that sell it typically label it “for research purposes only” to skirt regulations, but purchasing and possessing it for personal use occupies a legally ambiguous space that varies by country. In several nations, including Australia and parts of Europe, SARMs are classified as controlled or prescription-only substances.
Why People Use It Anyway
YK-11’s appeal in bodybuilding comes down to its unique profile on paper: androgen receptor activation for muscle growth, follistatin upregulation to push past genetic limits on muscle size, and bone-strengthening effects. The oral route of administration (no injections needed) adds convenience. Users typically report it in cycles lasting four to eight weeks, often stacked with other SARMs or performance-enhancing compounds.
The reality is that every claimed benefit is extrapolated from petri dishes and rats. The compound’s potency, bioavailability in humans, effective dose range, and long-term safety profile are all genuinely unknown. Products sold online also carry contamination and mislabeling risks. Independent analyses of SARM products have repeatedly found that what’s on the label doesn’t match what’s in the bottle, with some products containing no active compound at all and others containing undisclosed anabolic steroids.