Ostarine (MK-2866)
Enobosarm / GTx-024 / S-22, Nonsteroidal SARM
Evidence grade 2 of 5, Thin. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- Not FDA approved; investigational (multiple failed and ongoing trials)
- WADA
- Prohibited at all times (S1.2 Other Anabolic Agents)
Reference only. Not medical advice.
What can go wrong
Documented and expected harms include dose-dependent suppression of endogenous testosterone, LH and FSH (HPTA suppression), reductions in HDL cholesterol and SHBG, and reports of drug-induced liver injury with SARMs as a class. Because most consumer 'ostarine' is unregulated, actual contents and doses are unknown and often mislabeled. It is banned in all WADA sport and is a frequent cause of athlete adverse analytical findings, sometimes via contaminated supplements.
This is something to take to a clinician. The Atlas can tell you what is known about ostarine (mk-2866) in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
Ostarine (enobosarm, GTx-024/S-22) is the most extensively studied selective androgen receptor modulator, developed by GTx, Inc. of Memphis. It reached Phase III for cancer cachexia (the POWER trials) and consistently increased lean body mass, but repeatedly failed to meet co-primary strength/physical-function endpoints, and it has never been FDA approved for any indication. It is sold illegally as a 'research chemical' and dietary supplement, and is one of the most common SARMs found in mislabeled and contaminated products and in athlete doping positives.
The most-studied SARM, and a case study in why 'lean mass' is not the same as approval.
What people take it for
These are the reasons people give for taking it. They are not effects it has been shown to have.
How it is supposed to work
Enobosarm binds the androgen receptor as a nonsteroidal ligand and acts as a tissue-selective agonist, favoring anabolic signaling in skeletal muscle and bone with, in preclinical models, less agonism in prostate and skin. The selectivity is relative, not absolute: like all AR agonists it feeds back on the hypothalamic-pituitary-testicular axis and suppresses endogenous LH, FSH, and testosterone.
The catch the marketing skips
Ostarine reliably adds lean body mass on a DXA scan, but the GTx Phase III POWER program showed that added lean mass did not translate into the co-primary endpoint of improved physical function (stair-climb power/muscle strength). The FDA and EMA declined the muscle-wasting indication. 'It builds muscle' and 'it makes you functionally stronger or gets approved' are different claims, and enobosarm's own pivotal trials separated them.
Safety in detail
This is an unapproved investigational drug with androgenic activity. It suppresses the hypothalamic-pituitary-testicular axis and has been associated with liver injury. Self-administration without medical oversight carries documented health risks, and consumer products are frequently mislabeled.
Replacement is not the same as a cycle
Physician-supervised trial doses were 1-3 mg/day. Bodybuilding use commonly runs far higher, and consumer products contain unknown, often mislabeled quantities, magnifying suppression, lipid, and hepatic risk with no efficacy or approval to justify it.
What it does to fertility
As an AR agonist, ostarine suppresses LH/FSH and endogenous testosterone, which can impair spermatogenesis. Trial-dose suppression was reported to recover after discontinuation, but data at consumer doses are lacking.
What has to be monitored
The bloodwork this needs, and the reason it belongs with a clinician rather than a website.
- Total/free testosterone, LH, FSH
- Lipid panel (HDL)
- Liver function tests (AST/ALT/bilirubin)
- Hematocrit
What people get wrong
Often saidOstarine is FDA-approved because it was in big trials
ActuallyIt has never been FDA approved. Its Phase III POWER trials failed the co-primary functional endpoint and the indication was abandoned.
Often saidThe label tells you what you are taking
ActuallyIn the 2017 JAMA analysis, only ~41% of SARM products contained the labeled compound at the labeled amount; ostarine is among the most commonly mislabeled.
Who profits, and who pushes back
Nobody is named here yet, because we have not done that work on this entry. Read that as unfinished. Somebody almost certainly sells this, and somebody almost certainly disputes it. Assume both until we have checked.
How we got here
Enobosarm was developed by GTx, Inc., a Memphis biotech spun out of work by James Dalton and Duane Miller at the University of Tennessee. Advanced as GTx-024 for muscle wasting and cancer cachexia, it became the flagship of the first wave of clinical SARMs. After the POWER Phase III program missed its physical-function endpoint in 2013, the muscle-wasting program was terminated. GTx later pivoted (stress urinary incontinence, breast cancer) and the compound has continued in various hands, but no approval has resulted, even as illicit consumer sales exploded.
- 2007-2011GTx Phase I/II trials show lean-mass gains
- 2013POWER Phase III trials fail functional co-primary endpoint; muscle-wasting program ended
- 2017JAMA analysis documents widespread SARM mislabeling
- 2020sStudied for muscle preservation alongside GLP-1 therapy; still unapproved
At a glance
- Developer
- GTx, Inc. (Memphis)
- Codes
- GTx-024, S-22, enobosarm
- POWER trials
- ~652 NSCLC patients (2 trials)
- FDA approvals
- None
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about Ostarine (MK-2866). A short entry is not a safe entry.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
Sources
The studies below are named and described, but not linked. You cannot open them from here and check our reading against them, which means this page is our summary of the literature rather than a document you can verify. Weigh it accordingly until the links exist.
The studies that matter
Dalton et al. (Phase II, healthy elderly/postmenopausal, J Cachexia Sarcopenia Muscle)2011
3 mg dose-dependently increased lean body mass and stair-climb power vs placebo in healthy elderly men and postmenopausal women
POWER 1 & POWER 2 (Phase III, NSCLC muscle wasting)2013
Enobosarm improved lean body mass but failed co-primary physical-function endpoint; muscle-wasting development terminated
Bhasin/USADA/USUHS chemical analysis of internet SARMs (JAMA)2017
Products marketed as SARMs, including ostarine, frequently mislabeled or containing unlisted drugs