Ostarine (MK-2866): what the clinical trials found and UK status

People searching for "ostarine", "MK 2866" or "ostarine side effects" usually want to know three things: what the compound is, what the published research actually found, and where it stands legally in the UK and in sport. Ostarine is worth reading about carefully because, unlike most compounds sold as SARMs, ostarine has been through a full pharmaceutical development programme, with peer-reviewed phase 2 trials, a phase 3 programme and, as recently as 2024, a randomised oncology trial. That means there is real evidence to summarise, including evidence of harm. This guide does that and explains where ostarine stands under UK, EU and US law and the WADA code. It offers no guidance on use. Ostarine is sold on this site for laboratory research only.

What is ostarine?

Ostarine is a non-steroidal selective androgen receptor modulator. It has accumulated an unusual number of names: MK-2866 from its time under Merck, GTx-024 and S-22 from its originator GTx Inc, and enobosarm as its International Nonproprietary Name, the name used in clinical publications. All refer to the same aryl-propionamide molecule. "Ostarine" is the name that stuck on the grey market.

GTx developed enobosarm from the early 2000s for muscle wasting associated with cancer and ageing. After encouraging phase 2 results it ran the phase 3 POWER trials in patients with non-small cell lung cancer (Crawford et al., 2016). According to the developer's public reports, those trials did not consistently meet their co-primary endpoints, and no marketing authorisation followed. GTx then tested it for stress urinary incontinence, without success, and also tested it in androgen receptor-positive breast cancer, in a phase 2 trial run between 2015 and 2017 and published in 2024 (Palmieri et al., 2024). The rights later passed to another pharmaceutical company, which has continued clinical investigation. Ostarine has never been approved as a medicine anywhere. For background on the class, see What Are SARMs? The 2026 Research Guide.

Mechanism of action

Enobosarm binds and activates the androgen receptor. Its developers described it as having tissue-selective anabolic effects in muscle and bone while sparing androgenic tissue related to hair growth in women and the prostate in men, based on preclinical work (Dalton et al., 2011). The receptor it activates is the same one testosterone acts on, so the compound is not free of androgenic effects; the differences claimed are in degree and tissue distribution. In oestrogen receptor-positive breast cancer the androgen receptor acts as a tumour suppressor, which is why an androgen receptor agonist was tested as a cancer treatment (Palmieri et al., 2024). As a general pharmacological principle, androgen receptor agonists act on the feedback loop between the hypothalamus, pituitary and gonads, which is why suppression of endogenous testosterone is a recognised concern for the whole class.

What the research reports

Phase 2 in healthy older adults

A 12-week, double-blind, placebo-controlled trial enrolled 120 healthy men over 60 and postmenopausal women (Dalton et al., 2011). Total lean body mass by DXA rose in a dose-dependent manner and the difference from placebo was statistically significant at the highest dose tested. The authors also reported statistically significant changes in physical function (a stair-climb test) and in insulin resistance at that dose, and a similar incidence of adverse events across groups. This was a developer-funded trial in an older population, lasting 12 weeks, and it did not report on long-term outcomes.

Phase 2 in cancer cachexia

A randomised, double-blind, placebo-controlled trial enrolled 159 patients with cancer and recent weight loss, of whom 100 were evaluable for efficacy (Dobs et al., 2013). Over up to 113 days, median lean body mass rose by 1.5 kg in the lower-dose group and 1.0 kg in the higher-dose group compared with baseline, with no significant change on placebo. The most common serious adverse events were progression of the cancer, pneumonia and febrile neutropenia, none attributed to the study drug. The trial's own interpretation was that enobosarm "might" improve lean mass, and the individual results varied widely: the reported ranges ran from a loss of 2.1 kg to a gain of 12.6 kg in the lower-dose group and from a loss of 4.8 kg to a gain of 11.5 kg in the higher-dose group. The trial was funded by the developer.

Phase 3 and beyond

The POWER trials tested enobosarm in lung cancer patients on chemotherapy, with lean body mass and stair-climb power as co-primary endpoints (Crawford et al., 2016). The published paper describes the design; the developer later reported that the results did not support approval, and no regulator has approved the compound. The 2024 phase 2 breast cancer trial randomised 136 postmenopausal women with advanced ER-positive, HER2-negative, AR-positive disease to one of two doses (Palmieri et al., 2024). Of the 102 patients with centrally confirmed androgen receptor-positive disease, 32 per cent and 29 per cent had clinical benefit at 24 weeks. The trial was open-label, had no placebo or comparator arm and was funded by the developer. Grade 3 or 4 drug-related adverse events occurred in 8 per cent and 16 per cent of patients, most often raised liver enzymes (hepatic transaminases), hypercalcaemia and fatigue. Four deaths were judged unrelated to the drug.

Adverse findings outside trials

The clinical trials used low, pharmaceutical-grade doses in monitored patients. Outside that setting the picture changes. A 2024 review identified 20 reports published since 2020 of adverse events associated with SARMs, most of them described as drug-induced liver injury, with cholestatic or hepatocellular injury and jaundice as the main features (Leciejewska et al., 2024). The reviewers noted that data on the amounts taken and on the purity of the products were limited, and that the lack of quality control made the direct effect of SARMs on the liver hard to assess. An online survey of 343 SARM users found more than half reported side effects including mood changes, testicular shrinkage and acne (Efimenko et al., 2022). A 2017 analysis of 44 products sold online as SARMs found that only 52 per cent contained one or more SARMs (ostarine, LGD-4033 or andarine), 39 per cent contained a different unapproved drug instead, 25 per cent contained substances not on the label, and in only 41 per cent did the amount of active compound match the label (Van Wagoner et al., 2017).

None of the above is a claim about any product sold on this site. Products here are sold for laboratory research and have not been tested for any effect in people.

Regulatory status

United Kingdom

Ostarine is not a licensed medicine; the MHRA has never granted it a marketing authorisation. It is not an authorised food supplement or novel food and cannot lawfully be marketed for consumption. At the time of writing it is not a controlled drug under the Misuse of Drugs Act 1971. It is supplied in the UK as a research chemical, and Free Muscle supplies it on that basis only, to adults aged 18 and over. Brands held are compared in the Ostarine collection.

European Union

No marketing authorisation and no novel food authorisation exist. National agencies in several member states have warned the public about SARMs bought online. Buyers outside the UK are responsible for their own country's import rules.

United States

The FDA has not approved enobosarm for any indication, and has issued public warnings and warning letters about SARMs, including ostarine, in products sold as dietary supplements.

WADA and sport

Ostarine is named on the WADA Prohibited List under S1.2, other anabolic agents, prohibited at all times. It has featured in many adverse analytical findings, and a recurring defence in those cases is contamination of a supplement. A 2020 excretion study designed to test that scenario found that a single oral dose of as little as 1 microgram could be detected in urine for up to nine days by monitoring ostarine and its glucuronide, with large variation between individuals (Walpurgis et al., 2020). Detection times correlated with the amount given, and we have not found a verified reference establishing windows for larger amounts, so the accurate statement is: detectable, window not established.

Common questions

Is ostarine legal in the UK?

Ostarine is not a controlled drug in the UK at the time of writing, so it can be bought for laboratory research. It is not a licensed medicine and may not lawfully be sold for human consumption or as a food supplement. Free Muscle supplies it to adults aged 18 and over only, and it is prohibited in sport at all times under the WADA code.

Is ostarine a SARM or a peptide?

A SARM. It is a small synthetic molecule, not a peptide, and it has nothing in common with compounds such as BPC-157 or ipamorelin beyond being sold by the same shops. See SARMs vs Peptides.

MK-2866 vs MK-677

The shared "MK" prefix is a Merck code and the compounds are unrelated. MK-677 (ibutamoren) is a growth hormone secretagogue acting on the ghrelin receptor, not the androgen receptor, and it is not a SARM. It has its own trial literature, summarised in MK-677: What the Research Says and the MK-677 collection.

Ostarine vs LGD-4033 (ligandrol)

Both are non-steroidal androgen receptor agonists prohibited in sport and unlicensed everywhere. Ostarine has a far larger clinical programme including phase 3; the main published human trial of LGD-4033 lasted 21 days in healthy young men. Both appear in liver injury case reports. See the LGD-4033 collection.

Ostarine and cardarine

Cardarine (GW-501516) is not a SARM at all but a PPAR-delta agonist whose development was stopped over rodent carcinogenicity findings. We do not publish guidance on combining compounds. See the Cardarine collection and our Cardarine research guide.

Ostarine dosage, before and after, results, benefits

We do not publish dosing, cycle or outcome guidance for ostarine and we make no claims about what it does in people. The trials summarised above describe what researchers measured in specific patient groups under medical supervision. Products here are sold for research.

Ostarine and professional athletes

Ostarine has been named in anti-doping cases across a range of sports. Whether each finding was deliberate or the result of contamination is decided case by case by the relevant authorities; the excretion data above show how little of the compound is needed to trigger a finding.

Tablets, capsules or liquid?

The form changes handling in a laboratory, not the molecule. Capsules and tablets have a fixed fill that a certificate of analysis can confirm; liquids rely on accurate concentration and solvent stability. In every form, the question is whether that batch has been independently tested.

How to verify a batch

The Van Wagoner analysis is the reason to insist on batch-level testing. A certificate of analysis for the specific batch should show:

  1. Identity confirmed by HPLC or LC-MS against a reference standard, proving the substance is ostarine and not another SARM or an unapproved drug.
  2. Purity as a percentage, alongside identity. Purity alone can describe a pure sample of the wrong compound.
  3. Content per unit compared with the stated strength.
  4. Batch number, test date and laboratory name, with the batch number matching the pack you receive.

Every Free Muscle-brand batch is assayed by Janoshik Analytical, and the certificate for your batch is sent on request by email. For other brands on Muscle Market, a certificate is shown only where the manufacturer supplies one for the batch held. A generic PDF with no batch number is not a batch certificate. Our guides How to Read a Certificate of Analysis and What Is Janoshik Analytical? cover the detail, and the lab tests page explains our policy.

Related reading

Ostarine is sold for laboratory research use only. It is not a licensed medicine, has not been evaluated by the MHRA, EMA or FDA, is not a food supplement and is not for human consumption. Products are supplied to adults aged 18 and over. Nothing on this page is medical advice or dosing, cycle or usage guidance. Summaries of published studies describe what researchers reported in their study populations and are not claims about any product.

References

  1. Dalton JT, et al. The selective androgen receptor modulator GTx-024 (enobosarm) improves lean body mass and physical function in healthy elderly men and postmenopausal women: results of a double-blind, placebo-controlled phase II trial. J Cachexia Sarcopenia Muscle. 2011;2(3):153-161. PMID: 22031847.
  2. Dobs AS, et al. Effects of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised controlled phase 2 trial. Lancet Oncol. 2013;14(4):335-345. PMID: 23499390.
  3. Crawford J, et al. Study Design and Rationale for the Phase 3 Clinical Development Program of Enobosarm, a Selective Androgen Receptor Modulator, for the Prevention and Treatment of Muscle Wasting in Cancer Patients (POWER Trials). Curr Oncol Rep. 2016;18(6):37. PMID: 27138015.
  4. Palmieri C, et al. Activity and safety of enobosarm, a novel, oral, selective androgen receptor modulator, in androgen receptor-positive, oestrogen receptor-positive, and HER2-negative advanced breast cancer (Study G200802): a randomised, open-label, multicentre, multinational, parallel design, phase 2 trial. Lancet Oncol. 2024;25(3):317-325. PMID: 38342115.
  5. Leciejewska N, et al. Selective androgen receptor modulator use and related adverse events including drug-induced liver injury: Analysis of suspected cases. Eur J Clin Pharmacol. 2024;80(2):185-202. PMID: 38059982.
  6. Efimenko IV, et al. Adverse effects and potential benefits among selective androgen receptor modulators users: a cross-sectional survey. Int J Impot Res. 2022;34(8):757-761. PMID: 34471228.
  7. Van Wagoner RM, et al. Chemical Composition and Labeling of Substances Marketed as Selective Androgen Receptor Modulators and Sold via the Internet. JAMA. 2017;318(20):2004-2010. PMID: 29183075.
  8. Walpurgis K, et al. Elimination profiles of microdosed ostarine mimicking contaminated products ingestion. Drug Test Anal. 2020;12(11-12):1570-1580. PMID: 32959982.