MK-2866 (Ostarine) Research Overview (also known as Ostarine, Enobosarm, GTx-024, MK2866)
MK-2866, also called ostarine, enobosarm, or GTx-024, is an investigational nonsteroidal selective androgen receptor modulator developed to promote anabolic effects in muscle and bone while producing fewer androgenic effects than testosterone. It has been studied in conditions including cancer-associated muscle wasting, sarcopenia, and osteoporosis, but it has not been approved for general medical use and products sold online may be mislabeled, contaminated, or underdosed.
What Is MK-2866?
MK-2866 is the development code commonly associated with ostarine, a nonsteroidal selective androgen receptor modulator, or SARM. The compound was developed by GTx, Inc. under the name enobosarm and GTx-024. Unlike traditional anabolic-androgenic steroids, which can activate androgen receptors throughout many tissues, a SARM is designed to produce a more tissue-selective response. Ostarine binds to androgen receptors and was investigated primarily for its ability to increase or preserve lean body mass and physical function in settings where muscle loss is clinically important.
Ostarine is often described in online fitness communities as a beginner-friendly SARM because early clinical studies used relatively low doses and reported generally modest adverse effects compared with many anabolic steroids or more potent investigational SARMs. That description should not be confused with regulatory approval or proof of recreational safety. Enobosarm was one of the furthest-along SARMs in clinical development and reached late-stage study for cancer-associated muscle wasting, including Phase 3 programs, but it did not become an approved treatment for bodybuilding, performance enhancement, or routine body recomposition. Interest in recomposition comes from its potential to support lean mass while dieting, but controlled clinical evidence does not establish that recreational users can safely achieve those outcomes.
Online products labeled MK-2866 may not contain pharmaceutical-grade enobosarm. They may be sold as research chemicals, liquids, capsules, or tablets, and independent testing has found substantial variation in identity, concentration, and contamination among unregulated performance-enhancing products. In the United States and many other countries, using or possessing an unapproved product for human consumption may carry legal and medical risks. Anyone considering use should discuss the issue with a qualified clinician rather than relying on vendor instructions, social-media protocols, or anecdotal cycle reports.
Research Indications
Lean Body Mass
Clinical lean-mass findings
Randomized phase II studies of enobosarm reported dose-related increases in lean body mass in healthy older adults and in people with cancer.
Androgen receptor selectivity
Ostarine is an investigational selective androgen receptor modulator designed to produce anabolic signaling in muscle and bone with reduced activity in prostate tissue relative to traditional androgens.
Physical Function
Stair-climbing performance
Some trial endpoints suggested improvements in stair-climbing power, but functional outcomes were not consistently confirmed across clinical programs.
Frailty and wasting research
Research interest has focused on muscle wasting and age-related loss of lean mass; it is not approved for treatment of either condition.
Bone and Tissue Effects
Preclinical bone activity
Animal studies of selective androgen receptor modulators indicate possible bone-anabolic activity, but this has not established a clinical indication for ostarine.
Tissue-selective hypothesis
Tissue selectivity is relative rather than absolute; endocrine, hepatic, and lipid effects remain relevant research safety considerations.
Body Composition Research
Fat mass outcomes
Changes in fat mass have been secondary or exploratory outcomes in available studies and should not be interpreted as evidence for weight-loss treatment.
Not an approved medicine
Enobosarm/MK-2866 is not approved by the FDA for human use and is prohibited in sport by the World Anti-Doping Agency.
Research Protocols
As reported in cited literature and research-community logs (see Research Citations below) — not a personal dosing recommendation.
| Research Application | Dose | Frequency | Route |
|---|
| Early pharmacology study | 0.1–1 mg | Once daily | Oral |
| Lean-mass clinical research | 1–3 mg | Once daily | Oral |
| Cancer wasting phase II study | 1 mg or 3 mg | Once daily | Oral |
| Duration studied | 12–16 weeks | Continuous study period | Oral |
Timing
Published trials used once-daily oral administration. Investigational use outside monitored research is not recommended; ostarine is not an approved medication, and product labeling in the supplement market may be inaccurate.
Peptide Interactions
Avoid combining with alcohol because both alcohol use and unregulated SARM products may complicate assessment of liver-related adverse effects.
Combined androgenic exposure may increase endocrine suppression and make attribution of lipid, liver, mood, and reproductive effects difficult.
Use alongside medicines with known liver injury risk requires clinician review, especially where baseline or follow-up liver testing is abnormal.
SARMs can adversely affect HDL and other lipid measures; people treated for dyslipidemia should discuss any exposure with their prescriber.
Potential effects on fetal development and reproductive biology are unknown; use is inappropriate during pregnancy or breastfeeding.
Ostarine is prohibited at all times under the World Anti-Doping Agency prohibited list and may result in an anti-doping rule violation.
Reported Research Timeline
01Days 1–7 (reported in cited studies): no reliable visible body-composition change should be expected; adverse effects, product-quality problems, or changes in appetite, mood, or sleep may occur before any measurable benefit.
02Weeks 2–4 (reported in cited studies): clinical research generally evaluates laboratory and body-composition measures rather than short-term subjective effects; lipid or hormone changes can emerge during this period.
03Weeks 4–8 (reported in cited studies): in trials, changes in lean mass were assessed with standardized methods such as DXA; individual gym performance changes are not a validated substitute for clinical endpoints.
04Weeks 8–12 (reported in cited studies): longer exposure may increase the relevance of monitoring liver enzymes, fasting lipids, blood pressure, and reproductive hormone markers under medical supervision.
05Weeks 12–16 (reported in cited studies): this corresponds to the duration used in several clinical programs; long-term safety beyond study settings remains insufficiently characterized.
06After discontinuation (reported in cited studies): recovery of endocrine and lipid measures is variable and may warrant clinical assessment, particularly if symptoms or abnormal laboratory results persist.
Safety Notes
Included for harm-reduction awareness only, in the event this compound is encountered outside its labeled research use. Inclusion here does not imply RUO Codes endorses, recommends, or instructs human use.
Ostarine/enobosarm is not FDA-approved for human use; products marketed as supplements may be adulterated, mislabeled, or contain different quantities than stated.
Potential concerns include reduced HDL cholesterol, altered liver enzymes, endocrine suppression, acne, hair changes, mood changes, and effects on fertility.
Avoid use with known liver disease, uncontrolled cardiovascular disease, hormone-sensitive cancer, pregnancy, or breastfeeding unless specifically evaluated by a qualified clinician.
Baseline and follow-up medical evaluation should include discussion of medications, cardiovascular risk, liver history, lipid status, and reproductive health.
Competitive athletes should note that ostarine is prohibited by WADA; inadvertent exposure through contaminated products is still a potential anti-doping violation.
Seek Medical Attention If:
You develop yellowing of the skin or eyes, dark urine, severe abdominal pain, persistent nausea, or unusual fatigue.
You experience chest pain, shortness of breath, fainting, a sustained rapid heartbeat, or new one-sided weakness.
You develop severe mood changes, agitation, depression, suicidal thoughts, or symptoms of an allergic reaction.
You have persistent testicular pain, marked changes in libido, or other concerning endocrine or reproductive symptoms.
Quality Indicators
Verified Marker
Lot-specific certificate of analysis
A current COA should identify the product lot, test date, laboratory, analytical method, and measured result rather than providing a generic certificate.
Verified Marker
Identity confirmation by LC-MS or equivalent
Identity testing should specifically confirm enobosarm/ostarine rather than relying only on a supplier statement or unlabeled chromatogram.
Verified Marker
Quantitative assay with stated uncertainty
For oral products, the reported milligrams per unit should be supported by a quantitative assay and a clearly stated acceptable variance.
Acceptable Range
HPLC purity result
A reported HPLC purity value is useful only when accompanied by chromatographic conditions, lot traceability, and testing for relevant impurities.
Quality Concern
No independent screening for contaminants
Missing third-party testing for undeclared pharmaceuticals, residual solvents, heavy metals, and microbiological contaminants is a significant concern for unregulated oral products.
Research Citations
- 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
Dalton, J. T., Barnette, K. G., Bohl, C. E., et al., 2011, Journal of Cachexia, Sarcopenia and Muscle - Effect of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised controlled phase 2 trial
Dobs, A. S., Boccia, R. V., Croot, C. C., et al., 2013, The Lancet Oncology - Selective androgen receptor modulators: a novel approach to androgen therapy
Basaria, S., 2010, The Journal of Clinical Endocrinology & Metabolism - Selective androgen receptor modulators: the future of androgen therapy
Mohler, M. L., Bohl, C. E., Jones, A., et al., 2009, Endocrine Reviews - Selective androgen receptor modulators: current status and future prospects
Narayanan, R., Mohler, M. L., Bohl, C. E., et al., 2015, Drugs
Research Focus
muscle wasting, osteoporosis, cancer cachexia, lean mass preservation, breast cancer
Frequently Asked Questions
What should researchers watch for with MK-2866 (Ostarine)?
Included for harm-reduction awareness only, in the event this compound is encountered outside its labeled research use. Inclusion here does not imply RUO Codes endorses, recommends, or instructs human use.
What should researchers expect over time with MK-2866 (Ostarine)?
Days 1–7 (reported in cited studies): no reliable visible body-composition change should be expected; adverse effects, product-quality problems, or changes in appetite, mood, or sleep may occur before any measurable benefit.
How is MK-2866 (Ostarine) typically administered in research?
As reported in cited literature and research-community logs (see Research Citations below) — not a personal dosing recommendation.
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