YK-11: the steroid sold as a SARM, and why its evidence stops at a petri dish

YK-11, also written YK11, is a synthetic molecule built on a steroid skeleton. Its full chemical name, (17α,20E)-17,20-[(1-methoxyethylidene)bis(oxy)]-3-oxo-19-norpregna-4,20-diene-21-carboxylic acid methyl ester, describes a 19-norpregnane structure, which places it in the same broad family as nandrolone and dihydrotestosterone rather than among the non-steroidal molecules such as LGD-4033 or RAD-140 that the term SARM was coined for ^[1]. It was first characterised by Yuichiro Kanno and colleagues at Toho University in Japan, who reported in 2011 that it acted as a partial agonist of the androgen receptor in cultured cells ^[1].

So what is YK-11 in practical terms? It is a research chemical with no medical history. It has never entered a clinical trial, never been submitted to a regulator, and never been tested in a living animal in a published efficacy study. The three papers that exist all come from the same Japanese research group and all use cells grown in a dish: a 2011 androgen receptor study ^[1], a 2013 study in mouse C2C12 muscle precursor cells ^[2] and a 2018 study in mouse MC3T3-E1 bone-forming cells ^[3]. Everything else written about YK-11 results, cycles and stacks online is extrapolation from those papers or from user anecdote.

Is YK-11 a steroid? Chemically, yes. Sellers attach the YK-11 SARM label because it is marketed alongside the non-steroidal SARMs and because the 2013 and 2018 papers themselves use the phrase selective androgen receptor modulator in their titles ^[2][3]. The label matters less than the pharmacology: a compound that binds and partially activates the androgen receptor is expected to carry androgen-type effects, whatever it is called. The 2011 paper's finding that YK-11 is a partial rather than full agonist is the basis of the selectivity claim, and partial agonism in a reporter assay is not the same as demonstrated tissue selectivity in a body ^[1].

YK-11 is not a peptide. It is a small steroidal molecule, taken by mouth, with no amino acid chain. It appears on PepFinder because the research peptide stores we track sell it, usually in a section for non-peptide compounds next to MK-677, which is also not a peptide, and the SARMs. PepFinder lists it under Other compounds sold by peptide stores, and this guide follows the same format as our peptide guides so that the evidence, or its absence, can be compared like for like. If you are new to the category, what are research peptides explains the distinction.

Two mechanisms are proposed, and both come from the Toho University cell work. The first is androgen receptor activation. In the 2011 study, Kanno and colleagues tested YK-11 in cell lines carrying a reporter gene that lights up when the androgen receptor is activated. YK-11 switched the reporter on, but to a lesser degree than dihydrotestosterone, and it did not produce the full pattern of receptor behaviour that a complete agonist does. The authors concluded that YK-11 is a partial agonist of the androgen receptor ^[1]. In pharmacology, a partial agonist can act as a weak activator on its own and as a blocker in the presence of a stronger natural hormone, so the net effect in a person, who has circulating testosterone, is not obvious from the dish.

The second mechanism is the one that gave YK-11 its reputation as a YK-11 myostatin inhibitor. In the 2013 paper, the same group grew mouse C2C12 myoblasts, a standard laboratory line of muscle precursor cells, and exposed them to YK-11. The cells expressed more of the myogenic markers that indicate they are turning into muscle fibres, and this was accompanied by increased expression of follistatin, a protein that binds and neutralises myostatin, the body's own brake on muscle growth ^[2]. The authors reported that the effect on differentiation depended on follistatin: when follistatin was blocked, the effect on differentiation was lost ^[2]. The study's own title states the conclusion carefully: YK-11 regulates myogenic differentiation of C2C12 myoblasts by follistatin expression ^[2].

A third line of work, published in 2018, moved to bone. Yatsu and colleagues exposed MC3T3-E1 cells, a mouse osteoblast precursor line, to YK-11 and reported increased proliferation and differentiation, again attributed to androgen receptor signalling ^[3]. This is sometimes cited as evidence that YK-11 strengthens bone. It is evidence that it changes the behaviour of one type of mouse bone cell in culture, no more.

The gap between these findings and the marketing is wide. Follistatin induction in a mouse cell line does not establish that oral YK-11 raises follistatin in human muscle, that any rise would be large enough to matter, or that it would be selective for muscle rather than acting on every androgen-responsive tissue including the prostate, liver and the hypothalamic-pituitary axis that controls natural testosterone. Compounds that do have human data on the myostatin pathway, such as the antibody and ligand-trap drugs tested in muscle-wasting diseases, have generally produced modest changes in lean mass, which is a reason to be sceptical of large claims for a compound with no in-vivo data at all.

The complete published evidence on YK-11 fits in one paragraph. Kanno and colleagues (2011) showed partial androgen receptor agonism in cell-based reporter assays ^[1]. Kanno and colleagues (2013) showed increased myogenic differentiation and follistatin expression in mouse C2C12 myoblasts, with the differentiation effect dependent on follistatin ^[2]. Yatsu and colleagues (2018) showed increased proliferation and differentiation of mouse MC3T3-E1 osteoblast-like cells ^[3]. All three papers are from the same laboratory group at Toho University and were published in the Biological and Pharmaceutical Bulletin, the journal of the Pharmaceutical Society of Japan ^[1][2][3].

What the research does not show is at least as important. No study has given YK-11 to a mouse, rat or any other animal and measured muscle mass, strength, bone density, body composition or survival. No study has measured how YK-11 is absorbed from the gut, how it is metabolised by the liver, how long it stays in the blood or what it is broken down into. No study has measured its effect on testosterone, luteinising hormone, liver enzymes, cholesterol or the prostate in any living organism. The pharmacokinetic and toxicology work that would normally precede a first human dose has not been published, and there is no indication it has been done.

This puts YK-11 in a weaker evidential position than most of the compounds on PepFinder. BPC-157 and TB-500 have substantial animal literatures, however limited their human data. MK-677 has completed randomised human trials. Growth hormone secretagogues such as ipamorelin and CJC-1295 have at least early-phase human pharmacology. YK-11 has three cell papers. Any claimed YK-11 benefits for muscle or bone in people are, at present, hypotheses.

Readers who want to see how PepFinder weighs this kind of evidence can read our methodology. In short, we distinguish cell studies, animal studies, small human studies and randomised trials, and we do not let sales volume or forum popularity stand in for any of them.

There are none. As of the review date of this guide, no clinical trial of YK-11 has been registered or published, no case series describes its use, and no pharmaceutical company has disclosed a development programme for it. The compound has never been through phase 1 safety testing in healthy volunteers, which is the minimum step at which a drug's basic tolerability, absorption and effect on routine blood tests are first measured.

That absence is worth dwelling on, because other SARMs are sometimes used as a proxy. LGD-4033, ostarine and RAD-140 did go through early human trials run by their developers, and the results of those trials, including dose-dependent suppression of testosterone and changes in blood lipids, are often assumed to apply to YK-11 as well. They may, but YK-11 is a different molecule with a steroid structure, a different receptor profile and no measured human pharmacology, so its effects could be smaller, larger or different in kind. Treating YK-11 as interchangeable with the trialled SARMs is a guess in both directions.

The human data that does exist for the SARM class comes from the harms side rather than the efficacy side: published case reports of liver injury in people who bought SARMs online for body building. These are discussed under safety below. None of them names YK-11 as the agent, so they describe the class and the retail market, not this compound specifically ^[4][5][6][7].

Because there are no human or animal studies, there is no YK-11 dosage from published research to report. The three cell-culture papers exposed cells to YK-11 dissolved in the culture medium at laboratory concentrations, and concentrations in a dish have no direct translation to an oral dose in a person: they say nothing about absorption, first-pass liver metabolism, distribution to muscle or clearance ^[1][2][3]. Anyone quoting a milligram dose for YK-11 is quoting a seller, a forum or their own experience, not a study.

The figures that circulate online, typically in the range of a few milligrams to around ten milligrams a day for several weeks, are user-derived and align with the capsule strengths that stores happen to sell rather than with any measured dose-response. PepFinder does not recommend any dose or protocol for any compound, and for YK-11 there is not even a trial dose to describe. Our YK-11 calculator page exists for consistency with the rest of the catalogue; it can convert between capsule strengths and liquid concentrations, but there is no research dose to plug into it.

One practical point follows from the lack of pharmacology: nobody knows the half-life of YK-11 in humans, so the once-daily or twice-daily schedules found online are convention, not evidence.

No YK-11 side effects have been recorded in any study, because no study has given it to a living organism. What follows is inference from two sources: the compound's pharmacology, and published case reports concerning other SARMs bought from the same kind of online sellers.

From the pharmacology, an androgen receptor agonist taken orally can be expected to suppress the body's own testosterone production through negative feedback on the pituitary, and the 2011 partial-agonist finding does not remove that expectation ^[1]. Androgen receptor activation in the liver is associated with changes in cholesterol fractions, and oral anabolic steroids in particular have a long record of liver stress; YK-11 is structurally a steroid and is taken by mouth, though whether it carries the specific structural feature that makes some oral steroids liver toxic is not something the published papers address.

From the case reports, four papers published in 2024 describe drug-induced liver injury in people who had taken SARMs bought online for body building. Labban and colleagues reported a case linked to LGD-4033 and discussed the clinical implications of off-label SARM use ^[4]. Demangone and colleagues reported a case of liver injury attributed to SARM use ^[5]. Perananthan and colleagues, writing in Australian Prescriber, reported severe liver injury following RAD-140 ^[6]. Mertens and colleagues reported a case after SARM intake and reviewed the literature on similar cases ^[7]. None of these four reports concerns YK-11: they concern LGD-4033, RAD-140 and SARMs generally. We cite them because they are the best available human evidence that products sold in the SARM category can cause serious harm, not because they show YK-11 does.

The FDA's 2023 consumer warning on SARMs makes the same class-level point from a regulator's position. It states that SARMs are not approved for any use, that they are being sold illegally in products marketed to teenagers and young adults, and that the agency has received reports of adverse events including liver toxicity, and it warns against products described as SARMs or containing SARM ingredients ^[8]. YK-11 products fall within the scope of that warning by the way they are marketed.

Two further risks belong to the retail market rather than the molecule. First, the actual content of a YK-11 capsule or dropper bottle is not verified by any regulator, and analyses of SARM products sold online have repeatedly found under-dosed, over-dosed or substituted products; a certificate of analysis helps only if it is genuine and batch-specific, and our guide to reading a peptide COA covers what to look for. Second, because there is no human safety data, there is no list of warning signs to watch for. Anyone who has taken a SARM product and develops jaundice, dark urine, pale stools, abdominal pain or unusual fatigue should seek medical attention promptly, as those are the presenting features in the published liver injury cases ^[4][5][6][7].

YK-11 is not an approved medicine anywhere in the world. It has no marketing authorisation from the MHRA, the FDA, the EMA, the Therapeutic Goods Administration or Health Canada, and it is not on the pathway to one. The question is YK-11 legal therefore has different answers depending on what is being done with it and where. The summary below is general information, not legal advice; see our legal status overview for the framework we use across the site.

United Kingdom. YK-11 is not a licensed medicine. A product sold for human consumption that claims to build muscle is making a medicinal claim, and supplying such a product without a licence falls outside medicines law; stores label YK-11 as a research chemical or not for human consumption to sit outside that framework. Separately, the Misuse of Drugs Act controls anabolic steroids as Class C drugs, including a generic definition covering compounds structurally derived from certain steroid skeletons; whether a 19-norpregnane derivative such as YK-11 falls inside that definition is not something PepFinder can determine, and buyers should not assume it does not. Possession of Class C anabolic steroids for personal use is not an offence in the UK, but supply and import with intent to supply are.

United States. SARMs, including products sold as YK-11, are not approved by the FDA for any use and the agency has stated that they are being sold illegally as dietary supplements or as research chemicals ^[8]. Legislation to schedule SARMs as controlled substances has been introduced in Congress more than once and, to our knowledge, has not been enacted as of this review. The FDA has issued warning letters to sellers and pursued enforcement against companies marketing SARM products as supplements.

European Union. There is no EMA authorisation for YK-11 and no EU member state licenses it as a medicine. National rules on sale and possession vary; several member states control anabolic steroids under doping or narcotics legislation, and YK-11's steroid structure means it may be caught by such laws in some countries.

Australia. SARMs are listed in Schedule 4 of the Poisons Standard, which makes them prescription-only medicines. Since no doctor can prescribe an unapproved product for a non-existent indication, this effectively makes import, supply and possession without a prescription unlawful, and the Australian Border Force intercepts SARM shipments. The Australian Prescriber case report on RAD-140 liver injury was published in that context ^[6].

Canada. YK-11 is not authorised by Health Canada as a drug or a natural health product, and Health Canada has publicly warned about unauthorised SARM products sold online. We are not aware of any authorised route to buy YK-11 in Canada.

Sport. The World Anti-Doping Agency's Prohibited List bans selective androgen receptor modulators under section S1.2, other anabolic agents, and the S1 class is prohibited at all times, in and out of competition ^[9]. YK-11 is detectable in anti-doping analysis and adverse analytical findings for it have been reported. Any athlete subject to testing who takes a YK-11 product, whether or not it actually contains YK-11, is taking on a strict-liability risk of a multi-year ban.

YK-11 is a small organic molecule, not a lyophilised peptide, so most of the storage advice on PepFinder does not apply to it. It does not need reconstitution, bacteriostatic water is irrelevant, and it is not refrigerated as a matter of course. Capsules and tablets are stored as any oral product would be: in the original container, closed, at room temperature, away from direct light, heat and moisture. Liquid preparations sold as drops are typically YK-11 dissolved in a carrier such as polyethylene glycol or an alcohol-based solution; they should be kept closed, out of light, at the temperature the label states, and discarded if they change colour, cloud or develop sediment.

Raw powder, where it is sold, should be treated as a research chemical of unknown purity: handled with gloves, kept dry and sealed, and weighed on a scale accurate to a fraction of a milligram, since a 5 mg quantity is far below what a kitchen scale can resolve. Our peptide storage guide is written for freeze-dried peptides and vials; the general points about light, heat and labelling carry over, the reconstitution advice does not.

The shelf life of YK-11 in solution has not been published. Steroidal esters can hydrolyse over time in the presence of water, and the methoxyethylidene ketal group in YK-11's structure is the kind of group that is sensitive to acid, so a liquid product that has been stored for a long time or in poor conditions may not contain what its label states even if it did when bottled.

YK-11 is usually bought by people looking for muscle growth, so the natural comparison is with the other compounds in that category. Our peptides for muscle growth page lists them; the short version is that none of them has strong human evidence for building muscle in healthy people, but several have far more evidence than YK-11.

MK-677 is the closest neighbour on a store shelf: also oral, also not a peptide, also sold in capsules and liquid. Unlike YK-11 it has completed randomised placebo-controlled human trials, which measured increases in growth hormone and IGF-1, modest changes in fat-free mass, and side effects including increased appetite, water retention and raised blood glucose. It acts on the growth hormone axis rather than the androgen receptor, so it is not an anabolic agent in the WADA sense, though it is banned under a different section. IGF-1 LR3 is an injectable analogue of insulin-like growth factor with animal data and no human trials for muscle; it works downstream of growth hormone rather than through androgen signalling.

The growth hormone secretagogue peptides, ipamorelin, CJC-1295 and their common pairing described on our CJC-1295 and ipamorelin page, are injectables with early-phase human pharmacology showing they raise growth hormone; their effect on muscle mass in healthy adults is not established. HGH itself is a licensed medicine with decades of trial data in deficiency states, and trials in healthy older adults show gains in lean mass that do not reliably translate into strength.

BPC-157 and TB-500 are not muscle-building compounds at all; they are bought for tendon and soft-tissue recovery and are covered on our healing and recovery page. They come up in YK-11 discussions because forum stacks often combine an anabolic with a recovery peptide.

Against all of these, YK-11's distinguishing features are that it is a steroid derivative acting on the androgen receptor, that it is the only one in this list with no in-vivo data of any kind, and that its regulatory position, as a SARM prohibited in sport and warned about by the FDA, is among the least favourable. The follistatin mechanism is interesting and is the reason the compound is discussed at all, but interesting in a dish is where the evidence currently stops.

YK-11 for sale from the stores PepFinder tracks appears in three main forms. Capsules or tablets, usually 5 mg or 10 mg each, sold in bottles of 30 to 90. Liquid drops, most often at 10 mg/mL in a 30 mL dropper bottle, so a full bottle holds around 300 mg. And, less commonly, raw powder in gram quantities aimed at buyers who intend to make their own solution. A typical listing shows the form, the strength per capsule or per millilitre, the count or volume, a price, and sometimes a certificate of analysis; a research-use-only disclaimer is almost universal.

PepFinder shows YK-11 capsule and liquid listings on the YK-11 price page, but it does not rank them against each other on a per-milligram basis, and this is deliberate. Per-mg comparison works for a lyophilised peptide vial because the vial contains a stated mass of a single substance in a standard form. It works badly for YK-11 because a capsule, a drop and a powder are different products: capsules are dosed in fixed steps, liquids depend on the accuracy of a dropper, and a powder requires the buyer to weigh and dissolve it. It works worse still because independent testing of SARM products has so often found that the stated content is wrong, so a cheaper price per stated milligram may simply be a cheaper price for less of the compound. Our guide to peptide prices explained sets out when unit pricing is and is not meaningful.

If you do decide to buy YK-11, the checks are the same as for any research chemical and they matter more here, because the retail market is the only source of information about what is actually in the product. Ask for a batch-specific certificate of analysis from a named laboratory, ideally by HPLC or mass spectrometry, and read it with our guide to third-party peptide testing at hand. Be wary of stores whose YK-11 certificate is undated, unsigned, or identical across every product; our guide to spotting a fake peptide supplier lists the common patterns. Our supplier directory records which stores publish testing and which do not.

Finally, be clear about what buying YK-11 does and does not tell you. A store that sells it is not vouching for it, and the research-use label is a legal position, not a description of how buyers use it. The compound is a partial androgen receptor agonist with a follistatin effect in mouse cells, no human data, no animal data, a class-level record of liver injury from online products, and a ban in sport. Those are the facts a listing does not show, and they are the ones to weigh before the price.