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SARM

Ostarine (MK-2866)

Enobosarm — (2S)-3-(4-cyanophenoxy)-N-[4-cyano-3-(trifluoromethyl)phenyl]-2-hydroxy-2-methylpropanamide·Also known as: Enobosarm, GTx-024, MK-2866, S-22, VERU-024, Ostabolic

FDARegulatory status

Not approved for any indication. FDA has issued repeated public warnings against SARM-containing OTC supplements and bodybuilding products, citing hepatotoxicity, cardiovascular risk, and reproductive toxicity signals. Phase 3 ASTRID trial in stress urinary incontinence (sponsor Veru Inc.) remained active on ClinicalTrials.gov as of April 2026.

WADARegulatory status

Prohibited at all times — Section S1.2 (Other Anabolic Agents) on the 2026 WADA Prohibited List. Ostarine has been the single most frequently detected SARM in WADA-accredited laboratory reports since its 2008 listing.

Regulatory note ·Enobosarm was developed by GTx, Inc. (originally under a Merck collaboration, hence the MK-2866 code) and was the first SARM to reach Phase 3. The POWER1 and POWER2 Phase 3 trials in non-small cell lung cancer cachexia (2013) met the lean-body-mass co-primary endpoint but failed the muscle-function co-primary endpoint (stair climb power), and FDA did not grant approval — a decision reviewed in Dubois et al. 2021. Development for cachexia and sarcopenia was discontinued. The asset was acquired by Veru Inc., which is pursuing the ASTRID Phase 3 registration trial in stress urinary incontinence and separate AR+ / AR+/ER+ metastatic breast cancer programs. No marketing authorization exists in any jurisdiction. The compound is sold in the United States only as a research chemical; retail supplement sale is illegal under the FD&C Act and has driven repeated FDA enforcement action.

§ The quick take

TL;DR · Editor’s summary

Ostarine (enobosarm / GTx-024 / MK-2866) is the most clinically-studied SARM in the world and still has no approved indication. The body-composition evidence is genuinely strong for a non-approved compound: Dalton 2011 (Phase 2, n=120 healthy elderly men and postmenopausal women, 12 weeks) reported dose-dependent, placebo-separable increases in total lean body mass at 1 mg/day and 3 mg/day, with stair-climb-power improvement of ~11% in the 3 mg arm and metabolic side-effects that looked favorable (fasting glucose down ~11%, insulin down ~17%, HOMA-IR down ~27%). The Phase 3 signal is where it falls apart.

GTx's POWER1 and POWER2 pivotal trials in non-small cell lung cancer cachexia (2013, combined n~650) met the lean-body-mass co-primary but failed the muscle-function co-primary (stair climb power) — the discordance reviewed by Dubois et al. 2021 in the Journal of Cachexia, Sarcopenia and Muscle — and FDA declined approval. Development for cachexia and sarcopenia was discontinued; the molecule was later acquired by Veru Inc., which is running the ASTRID Phase 3 trial in stress urinary incontinence and separate AR+/ER+ metastatic breast cancer programs. As of April 2026 neither program has a published pivotal efficacy readout.

What the evidence does NOT support, under any defensible reading: recreational muscle gain in healthy resistance-trained adults (zero RCTs in that population), "safer than testosterone" framing (peer-reviewed case reports including Bedi 2021 document cholestatic DILI from retail SARM products), or use as a legal supplement. Ostarine has been WADA-prohibited under S1.2 since 2008 and is consistently the most-detected SARM in anti-doping samples, frequently via supplement contamination that has produced athlete sanctions where no knowing use was admitted.

§ Grade matrix

The grade
per outcome.

One peptide can earn very different grades for different uses. Here is every outcome we’ve graded for Ostarine (MK-2866), sorted by strength of evidence.

Grade
Outcome
One-line take
Studies
Updated
B

Lean body mass in cancer cachexia and sarcopenic populations

Promising

Replicated, placebo-controlled, dose-dependent increases in total lean body mass across Dalton 2011 Phase 2 (n=120 healthy elderly men and postmenopausal women) and the POWER1/POWER2 Phase 3 NSCLC cachexia trials (n~650, 2013). Body-composition effect is one of the most-replicated SARM findings. The Phase 3 trials still failed regulatory approval because muscle-function co-primaries did not separate from placebo.

9
2026-04-21
D

Muscle strength / physical function in cancer cachexia

Weak

POWER1 and POWER2 Phase 3 NSCLC trials failed the stair-climb-power co-primary endpoint despite hitting the lean-body-mass co-primary — the exact discordance that caused FDA to decline approval (Dubois 2021). Phase 2 signal (Dalton 2011) did not survive replication at the pivotal scale. Evidence against a placebo-separable functional benefit at the tested dose range.

5
2026-04-21
Ins.

Stress urinary incontinence (postmenopausal women)

Insufficient

Phase 2 data in postmenopausal women showed pelvic-floor lean-mass increase; Phase 3 ASTRID (sponsor Veru) remains active on ClinicalTrials.gov as of April 2026 without primary-endpoint readout in the peer-reviewed literature. Mechanism is biologically grounded (ovariectomized-rat pelvic-floor data in Roch 2020 and Komrakova 2020), but the pivotal human efficacy result is not yet public.

3
2026-04-21
C

AR+ and AR+/ER+ metastatic breast cancer

Mixed

Earlier GTx-024 open-label Phase 2 data in AR+ TNBC and ER+/AR+ breast cancer showed tumor-control signals that motivated continued development by Veru. No Phase 3 efficacy readout is published in the peer-reviewed literature as of April 2026. The indication sits on open-label and single-arm evidence.

4
2026-04-21
C

Bone mineral density / fracture healing

Mixed

Preclinical bone data is consistent — Komrakova 2020 in aged-male osteoporotic rats showed improved fracture callus and mineral content; Roch 2020 reported trabecular improvements in ovariectomized rats. Human bone-endpoint data is limited to imputed BMD signals inside the cachexia trials. No dedicated osteoporosis RCT has been completed.

6
2026-04-21
C

Insulin sensitivity / glucose metabolism

Mixed

Dalton 2011 Phase 2 reported ~11% fasting-glucose reduction, ~17% insulin reduction, and ~27% HOMA-IR reduction in the 3 mg/day arm versus placebo over 12 weeks. Single Phase 2 finding, never replicated as a pre-specified endpoint in a dedicated metabolic trial. Hypothesis-generating only.

3
2026-04-21
D

Bodybuilding / recreational muscle gain in healthy adults

Weak

No randomized trial in resistance-trained healthy adults has ever been published. All recreational-use evidence is n-of-1 anecdote from bodybuilding communities, confounded by contaminated supplement products (the FDA-detected market is heterogeneous in dose and identity) and by undisclosed co-use of other anabolics. The body-composition extrapolation from cachexia populations is not a validated efficacy claim for this population.

2
2026-04-21
D

Hepatotoxicity (safety outcome, not efficacy)

Weak

Published case reports including Bedi 2021 (ACG Case Reports Journal) document cholestatic drug-induced liver injury resembling 17α-alkylated anabolic-steroid hepatotoxicity, in patients using retail SARM products. LiverTox and LIVERTOX-adjacent registries continue to accumulate SARM-DILI entries. The signal is real; population-level incidence is uncharacterized because exposure is unregulated.

6
2026-04-21

§ Why this grade

Sub-scores for this outcome.

Lean body mass in cancer cachexia and sarcopenic populations

Every grade rolls up six weighted sub-scores, each rated 1 to 5 with a written justification. Here is how the top-outcome grade was constructed.

Mechanism understood

4 / 5

AR-ligand pharmacology well-characterized (Ki ~3.8 nM, Narayanan 2018). Downstream Akt/mTOR and ubiquitin-proteasome modulation mapped in muscle (Roch 2020, Roch 2024). Non-steroidal scaffold blocks 5α-reductase and aromatase conversion. Tissue-selectivity gradient reproduced preclinically. The coregulator-recruitment map that would molecularly explain the selectivity gradient is still incomplete.

Human studies (count + quality)

4 / 5

One published Phase 2 RCT at n=120 (Dalton 2011, PMC3177038), one published Phase 2 in cancer cachexia at n=159, and two Phase 3 RCTs in NSCLC cachexia (POWER1 and POWER2, combined n~650, 2013). Pivotal-scale randomized evidence exists — rare for any compound graded by Peptigrade. Offset: the pivotal trials failed their functional co-primary.

Effect vs placebo

4 / 5

Placebo-separable dose-dependent increase in total lean body mass replicated across Phase 2 (Dalton 2011) and Phase 3 (POWER1/POWER2, 2013). Body-composition endpoint is one of the most-replicated SARM effects in humans. Strength-of-evidence here is the entire basis for the B grade.

Long-term safety data

2 / 5

Controlled human exposure is bounded by the Phase 2/3 trial durations — 12 to 16 weeks at 1–3 mg/day. Long-term recreational-use exposure (higher doses, repeated cycles, common stacking) is entirely outside the controlled-safety dataset. Bedi 2021 documents cholestatic DILI in a retail user.

Side effect profile

2 / 5

In-trial tolerability at 1–3 mg/day was comparable to placebo for common AEs. Transient testosterone and SHBG suppression was observed on-cycle. Outside the trial setting, cholestatic DILI case reports (Bedi 2021, ACG Case Reports Journal), lipid-profile shifts with HDL suppression, and contamination-driven inadvertent exposures complicate the bedside safety picture.

Regulatory status

1 / 5

Not approved by any major regulator. FDA declined approval after POWER1/POWER2 failed the muscle-function co-primary. Active FDA enforcement against retail SARM products. WADA-prohibited under S1.2 since 2008. Most-detected SARM in anti-doping samples.

§ What the science says

How Ostarine (MK-2866)
works.

Plain-English explanation of the molecule and its proposed mechanism, written at an 8th-grade reading level so anyone can engage with it. Every claim is linked to a primary source below.

What it is

Ostarine (enobosarm, developmental codes GTx-024, MK-2866, and S-22) is a non-steroidal selective androgen receptor modulator (SARM) with an arylpropionamide scaffold (C19H14F3N3O3, MW 389.33, CAS 841205-47-8, PubChem CID 11326715). It binds the androgen receptor with a reported Ki of ~3.8 nM. Unlike endogenous androgens it cannot be converted to DHT by 5α-reductase or to estrogens by aromatase, which is the structural basis for its tissue-selective anabolic profile: preferential activity in skeletal muscle and bone, with lower activity in prostate and other reproductive tissues. It was developed by GTx, Inc. (Memphis, TN) — originally under an early-2000s collaboration with Merck — and was the first SARM to advance into Phase 3 trials. GTx-024 assets are now held by Veru Inc.

How it works

  1. 01

    Tissue-selective androgen receptor binding

    Ostarine is a non-steroidal AR ligand with a reported dissociation constant Ki ~3.8 nM (Narayanan 2018, Sex Med Rev). On binding, it induces an AR conformation distinct from the one induced by testosterone or DHT, producing a different coregulator-recruitment profile. The result in preclinical systems is dissociation of anabolic from androgenic activity: ostarine restores levator ani muscle mass in orchiectomized rats to near intact levels while causing only partial prostate-weight restoration — the classic dissociation signature used to triage SARM candidates.

  2. 02

    Muscle: Akt/mTOR and ubiquitin-proteasome modulation

    In skeletal muscle AR activation by ostarine drives protein synthesis via Akt/mTOR signaling and suppresses protein degradation through the ubiquitin-proteasome system. Roch 2020 (Frontiers in Endocrinology) in ovariectomized rats showed that ostarine restored soleus and gastrocnemius fiber cross-sectional area and improved histomorphometric markers of muscle quality; the Roch 2024 follow-up (Int J Mol Sci) showed advantage over raloxifene and additive effects with raloxifene in the estrogen-deficient model.

  3. 03

    Bone: osteoblast activation and callus mineralization

    Komrakova 2020 (Calcified Tissue International) reported that enobosarm improved fracture callus mineralization, trabecular bone volume, and mechanical strength in an aged-male osteoporotic rat model. Mechanistically AR activation in osteoblasts promotes differentiation and matrix mineralization while AR in osteoclasts attenuates resorption. The bone signal is consistent across models but has not been tested as a primary endpoint in a dedicated human trial.

  4. 04

    Metabolic effects on adipocytes and glucose handling

    Macías-Villalobos 2020 (J Physiol Biochem) characterized ostarine effects on Wistar-rat adipocyte metabolism, showing modulation of lipogenic gene expression. Dalton 2011 Phase 2 observed placebo-separable reductions in fasting glucose, fasting insulin, and HOMA-IR at 3 mg/day — consistent with muscle-mediated improvement in whole-body glucose disposal. The metabolic effect has never been replicated as a pre-specified endpoint in a dedicated trial.

  5. 05

    Hepatotoxicity mechanism — not fully characterized

    Despite the non-steroidal scaffold that was supposed to reduce hepatotoxicity relative to 17α-alkylated anabolic steroids, cholestatic DILI has been reported repeatedly in retail-SARM users (Bedi 2021, ACG Case Reports Journal). The mechanism is not fully worked out in the peer-reviewed literature: AR-mediated effects on hepatic transporters, off-target effects of metabolites characterized by Sobolevsky 2024 (Drug Test Anal), and formulation-quality issues in the unregulated supplement market all plausibly contribute. The clinical pattern is indistinguishable from anabolic-steroid cholestasis at the bedside.

  6. 06

    What is NOT known about the mechanism

    The coregulator-recruitment map that would explain tissue selectivity at the molecular level is incomplete. Long-term effects on the HPG axis in humans are not well-characterized — transient testosterone and SHBG suppression have been observed on-cycle in the Phase 2/3 program, with rebound after washout, but there is no long-exposure cohort to characterize recovery in recreational users who stack or cycle. Drug-drug interactions have been mapped for itraconazole, rifampin, probenecid, celecoxib, and rosuvastatin (Nanda 2016), but co-exposure patterns in real-world SARM users (often stacked with other anabolics) are not studied.

§ Investigated uses

What it’s
been studied for.

Investigated does not mean proven. This list shows every use that appears in the published literature, regardless of evidence strength. See the grade matrix above for which ones have actually held up.

  • Cancer cachexia (NSCLC, colorectal, NHL, CLL, breast) — lean body mass endpoint

    Phase 3 LBM endpoint met (POWER1/POWER2, 2013); muscle-function endpoint failed; FDA did not approve

  • Cancer cachexia — muscle function / physical performance endpoint

    Failed Phase 3 co-primary (stair climb power)

  • Sarcopenia in healthy elderly and postmenopausal women

    Phase 2 positive (Dalton 2011); development discontinued

  • Stress urinary incontinence in postmenopausal women

    Phase 3 ASTRID active on ClinicalTrials.gov (sponsor Veru) as of April 2026; no published efficacy readout

  • AR+ triple-negative and AR+/ER+ metastatic breast cancer

    Open-label and Phase 2 program continues under Veru; no published Phase 3 efficacy readout

  • Osteoporosis / fracture healing

    Preclinical only (Komrakova 2020 aged-male rat model; Roch 2020 ovariectomized rat)

  • Insulin sensitivity / metabolic syndrome adjunct

    Phase 2 metabolic signal (Dalton 2011); no dedicated trial

  • Pharmacokinetic drug-drug interactions

    Characterized versus itraconazole, rifampin, probenecid, celecoxib, rosuvastatin (Nanda 2016)

  • Anti-doping detection and metabolite mapping

    In vitro and in vivo human metabolism characterized (Sobolevsky 2024, Drug Test Anal)

  • Recreational muscle gain in healthy resistance-trained adults

    No randomized trial in this population has ever been published

§ The honest gaps

What we don’t
know yet.

Every peptide page on this site is required to include this section. Absence of evidence is information. If we don’t flag the gaps, we’re lying by omission.

  • !

    Why the Phase 3 POWER1/POWER2 trials met the lean-body-mass endpoint but failed the stair-climb-power endpoint is still debated — whether it reflects a genuine dissociation between body composition and functional capacity at the tested dose range, a study-design issue in how function was measured in NSCLC cachexia patients, or a ceiling effect from concurrent cancer therapy. Dubois 2021 reviewed the candidate explanations without resolving them.

  • !

    The ASTRID Phase 3 SUI efficacy result is not yet public. The mechanism (AR-driven pelvic-floor muscle mass restoration) is biologically grounded, but whether it produces an incontinence-episode-rate benefit that survives blinded assessment is an open empirical question as of April 2026.

  • !

    Population-level incidence of SARM hepatotoxicity is uncharacterized because exposure occurs almost entirely through unregulated retail supplements of heterogeneous content. The denominator is unknown; the numerator is a growing LiverTox-adjacent case-report series anchored by Bedi 2021.

  • !

    Cardiovascular risk (lipid profile shifts, HDL suppression, LV remodeling) has been measured in the controlled trials at clinical doses over 12–16 weeks and looked modest, but the recreational-use population takes higher doses for longer cycles, and long-exposure cardiovascular data in that pattern does not exist.

  • !

    Long-term HPG-axis recovery in recreational users who run repeated cycles or stack ostarine with other anabolics is not documented in the peer-reviewed literature. Transient on-cycle T and SHBG suppression has been observed in the Phase 2/3 program; post-exposure recovery in the recreational pattern is undocumented.

  • !

    Efficacy in resistance-trained healthy adults — the population that actually buys retail SARMs — has never been tested in a randomized trial. All such claims extrapolate from cachexia / sarcopenia populations where baseline muscle is pathologically low.

  • !

    Product-identity risk: independent testing has repeatedly found that retail "ostarine" supplements contain variable doses, other SARMs, or unrelated anabolic agents. Athletes have been sanctioned for inadvertent exposure; the fraction of retail product that matches label is not well-characterized.

  • !

    The coregulator-recruitment map that would molecularly explain the tissue-selectivity gradient (muscle > bone > prostate > other) is incomplete.

§ On YouTube

What experts and
influencers say.

We index YouTube content discussing Ostarine (MK-2866)and tag every speaker by credential and trust level. The goal is not to summarize the internet — it’s to tell you which voices to weight.

  • SARMs and the FDA: Why Ostarine Didn't Get Approved

    Peter Attia MD·MD, longevity medicine

    Walks through the POWER1/POWER2 discordance between lean-body-mass and muscle-function endpoints and why the FDA's functional-capacity bar is defensible even when body composition improves. Explicit that SARMs are not legal supplements.

    Verified credentials

  • Ostarine Hepatotoxicity — What the Case Reports Actually Show

    Dr. Kyle Gillett·MD, family medicine

    Reviews Bedi 2021 and adjacent cholestatic-DILI case reports in retail SARM users. Distinguishes product-identity / contamination risk from intrinsic hepatotoxicity and emphasizes the absence of a denominator for population incidence.

    Verified credentials

  • I Ran an Ostarine Cycle — Blood Work Before and After

    Anonymous bodybuilding channel·Unverified

    Single-user anecdote with self-selected blood panel, unknown product identity (retail SARM supply chain is contaminated per FDA enforcement actions), and no controls. Representative of the "research log" genre; do not weight against the published trial data.

    Caution — anecdotal

§ Citations

Every claim,
linked to source.

All 15 sources informing this page, with DOI or PubMed identifiers. Click through to the primary literature.

  1. [01]

    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 JT, Barnette KG, Bohl CE, et al. · J Cachexia Sarcopenia Muscle · 2011

    RCTDOI
  2. [02]

    Should the FDA's criteria for the clinical efficacy of cachexia drugs be changed? Is ostarine safe and effective?

    Dubois V, Laurent MR, Jardi F, et al. · J Cachexia Sarcopenia Muscle · 2021

    Systematic reviewDOI
  3. [03]

    Selective androgen receptor modulators (SARMs) — current knowledge and clinical applications

    Narayanan R, Coss CC, Yepuru M, et al. · Sex Med Rev · 2018

    Systematic reviewDOI
  4. [04]

    Effect of selective androgen receptor modulator enobosarm on bone healing in a rat model for aged male osteoporosis

    Komrakova M, Nagel J, Hoffmann DB, et al. · Calcif Tissue Int · 2020

    AnimalDOI
  5. [05]

    Ostarine and ligandrol improve muscle tissue in an ovariectomized rat model

    Roch PJ, Henkies D, Carstens JC, et al. · Front Endocrinol (Lausanne) · 2020

    AnimalDOI
  6. [06]

    Advantage of ostarine over raloxifene and their combined treatments for muscle of estrogen-deficient rats

    Roch PJ, Buthe A, Carstens JC, et al. · Int J Mol Sci · 2024

    AnimalDOI
  7. [07]

    Selective androgen receptor modulators (SARMs) have specific impacts on the mouse uterus

    Overby LH, Segars JH, Hennebold JD · Reprod Toxicol · 2019

    AnimalDOI
  8. [08]

    Pharmacokinetic drug interactions of the selective androgen receptor modulator GTx-024 (enobosarm) with itraconazole, rifampin, probenecid, celecoxib and rosuvastatin

    Nanda KK, Tao Y, Mancuso J, et al. · Invest New Drugs · 2016

    RCTPMID 27105861
  9. [09]

    In vitro and in vivo human metabolism of ostarine, a selective androgen receptor modulator and doping agent

    Sobolevsky T, Dudko G, Dikunets M, et al. · Drug Test Anal · 2024

    In vitroDOI
  10. [10]

    Effect of ostarine (enobosarm/GTx-024), a selective androgen receptor modulator, on adipocyte metabolism in Wistar rats

    Macías-Villalobos JA, López-Gamboa KL, González-Moreno S, et al. · J Physiol Biochem · 2020

    AnimalPMID 31642815
  11. [11]

    Drug-induced liver injury from enobosarm (ostarine), a selective androgen receptor modulator

    Bedi H, Hammond C, Sanders D, et al. · ACG Case Rep J · 2021

    Case seriesDOI
  12. [12]

    Selective androgen receptor modulators in preclinical and clinical development

    Gao W, Dalton JT · Expert Opin Investig Drugs · 2007

    Systematic reviewDOI
  13. [13]

    ASTRID — a Phase 3 study of enobosarm for stress urinary incontinence in postmenopausal women

    Veru Inc. · ClinicalTrials.gov · 2021

    RegistrationTrial
  14. [14]

    WADA 2026 Prohibited List (in force January 1, 2026) — Section S1.2 Other Anabolic Agents

    World Anti-Doping Agency · WADA · 2026

    RegulatoryLink
  15. [15]

    FDA in brief: FDA warns against using SARMs in body-building products

    U.S. Food and Drug Administration · FDA · 2023

    RegulatoryLink

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alongside this.

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RAD-140 (Testolone)

Second-most-prominent SARM in recreational use. Higher reported AR selectivity in preclinical models than ostarine but a thinner human dataset — no Phase 3 program. Same hepatotoxicity-signal and WADA-S1 category.

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MK-677 (Ibutamoren)

Commonly conflated with SARMs in bodybuilding forums but is actually an orally-active ghrelin-receptor agonist / GH secretagogue — not an AR ligand. Useful cross-reference for a reader trying to understand what is and is not a SARM.

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IGF-1

Downstream mediator of the anabolic signal that ostarine's AR activation converges on. Reader wanting the endogenous-growth-axis comparator should read that page alongside this one.

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Tesamorelin

An actual FDA-approved muscle-and-body-composition-adjacent peptide (approved for HIV-associated lipodystrophy). Useful contrast for readers who assume "investigational muscle-wasting therapy" means "unapproved" — tesamorelin cleared the bar that ostarine did not.

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Where to research further

Looking for Ostarine (MK-2866)
for laboratory research?

Peptigrade does not sell peptides. RiboCore is one supplier we track that publishes batch-level certificates of analysis (mass spec, HPLC purity) for research-grade material. We have no commercial relationship with them — listing here is editorial.

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