Enobosarm — Safety, Research, Risks & Legal Status

Not medical advice. Enobosarm is discussed here for informational and harm-reduction purposes only. We do not endorse use, and any dosing context is informational, not a protocol.

Also known as	GTx-024, MK-2866, Ostarine, S-22
Category	SARM
Developer	Originally GTx, Inc.; currently Veru, Inc.
Development codes	GTx-024, MK-2866, S-22; marketed as Ostarine in research chemical market
Half-life (human)	Approximately 22 hours (oral)
Phase 2 lean mass result	+1.3 kg vs placebo at 3 mg over 12 weeks in elderly adults (p less than 0.001)
Phase 3 outcome	POWER I and II trials: lean mass improved but primary functional endpoint (stair-climb power) not met; FDA NDA rejected
FDA Fast Track	Granted for AR+/ER+/HER2- metastatic breast cancer indication
US legal status	Not FDA-approved for any indication as of 2026. Investigational drug status only. Holds FDA Fast Track designation for AR+/ER+/HER2- metastatic breast cancer. Phase 3 applications were rejected despite observed lean mass gains (POWER I and II trials failed primary functional endpoints). Not legal for human consumption outside of an approved clinical trial in the United States. Widely sold as a "research chemical" with "not for human consumption" labeling; that labeling does not confer legality for personal use under the Federal Food, Drug, and Cosmetic Act.

What is Enobosarm?

Enobosarm is a nonsteroidal, orally bioavailable selective androgen receptor modulator (SARM) developed originally by GTx, Inc. and currently in clinical development by Veru, Inc. It binds androgen receptors in a tissue-selective manner, producing anabolic effects in skeletal muscle and bone while exhibiting partial agonism or antagonism in androgenic tissues such as the prostate and seminal vesicles. It has completed Phase 2 and Phase 3 human trials across several indications but has not received marketing authorization in the United States.

How it works

Enobosarm differentially activates androgen receptors depending on tissue context. In skeletal muscle and bone it acts as a full agonist, driving anabolic signaling cascades that promote nitrogen retention and protein synthesis. In the prostate and seminal vesicles it shows partial agonism or antagonism, which is the basis for its claimed separation from androgenic side effects seen with classical anabolic steroids. This selectivity is believed to arise from ligand-induced conformational changes in the receptor that recruit different coactivator and corepressor complexes in a cell-type-specific manner. Oral bioavailability is high and plasma half-life is approximately 22 hours in humans.

Background & history

Enobosarm emerged from early SARM research at the University of Tennessee and was developed commercially by GTx, Inc. under the development codes GTx-024 and MK-2866. Phase 1 and 2 trials in the 2000s and early 2010s evaluated it in healthy elderly men and postmenopausal women, demonstrating dose-dependent lean mass gains. GTx subsequently advanced it into Phase 3 development (the POWER I and II trials, NCT01355484 / NCT01355497) targeting cancer-related muscle wasting. Despite achieving statistically significant lean mass improvements, the trials failed their primary functional endpoint (stair-climb power), and the FDA declined approval. Veru, Inc. later acquired development rights and obtained FDA Fast Track designation for the AR+/ER+/HER2- metastatic breast cancer indication, with active Phase 3 work ongoing as of 2024-2026. Veru also received regulatory clarity from FDA in 2024-2026 supporting a 3 mg dose and a 2.5 mg increment weight-loss primary endpoint for a proposed obesity-plus-GLP-1 receptor agonist combination trial.

What the research says

Human evidence exists at Phase 2 and Phase 3 levels, which is unusually advanced for a SARM. In a Phase 2 randomized controlled trial (n=120, healthy elderly men and postmenopausal women), enobosarm at 3 mg produced a mean lean body mass increase of approximately 1.3 kg versus placebo (p less than 0.001) and improved stair-climb power (p=0.013) over 12 weeks, with no serious adverse events. In the Phase 3 POWER trials (pooled n=1,027, cancer patients experiencing muscle wasting), enobosarm was well tolerated with an adverse event profile comparable to controls; however, neither trial met its primary functional endpoint, resulting in FDA rejection of the New Drug Application. A 2024 Lancet Oncology report (Study G200802) describes activity and safety in AR-positive, ER-positive, HER2-negative advanced breast cancer, representing an ongoing therapeutic focus. Phase 2 work is also underway evaluating enobosarm as an adjunct to GLP-1 receptor agonists for obesity-associated muscle preservation and in COPD with muscle weakness. Functional benefit has not been demonstrated in a completed Phase 3 program as of 2026.

Reported effects

Dose-dependent increases in lean body mass observed in Phase 2 controlled human trials at 3 mg (approximately 1.3 kg vs placebo, elderly population)
Improved stair-climb power in Phase 2 elderly population (p=0.013); functional benefit was not replicated in Phase 3 POWER trials
Anabolic signaling in skeletal muscle and bone without observed androgenic effects (no virilization in women; no prostate concerns reported in trials)
Modest reductions in HDL cholesterol observed in clinical trials
Transient liver enzyme elevations reported
No major cardiovascular, vital sign, or electrocardiographic abnormalities in completed trials
Tissue selectivity spares androgenic tissues relative to classical anabolic steroids, though this advantage is derived from clinical trial data, not established clinical practice

Dosing & administration (informational)

Published Phase 2 human trial literature used doses of 1 mg and 3 mg orally once daily over 12 weeks. Phase 3 POWER trials used 3 mg orally once daily in cancer populations. Veru has confirmed with FDA that 3 mg is an acceptable dose for future trials, and a 2.5 mg increment over GLP-1 receptor agonist baseline is under consideration as a primary endpoint for an obesity indication. These figures are reported from trial protocols for informational reference only and do not constitute a dosing protocol. Consult a licensed clinician before considering any investigational agent.

This is general research/context information, not medical advice or a recommended protocol.

Safety & side effects

HDL cholesterol reductions: modest but consistently observed across trials; clinical significance over long treatment durations is unknown
Transient liver enzyme elevations reported; severe hepatotoxicity not established in enobosarm-specific trials but is a class concern for SARMs as a category
Rhabdomyolysis: two cases reported in trials of a structurally related SARM (GSK2881078), not in enobosarm-specific trials; mechanistic relevance is uncertain
No androgenic adverse effects (virilization, prostate enlargement) observed in completed human trials
Long-term safety data beyond trial durations are unavailable; no post-marketing surveillance exists given non-approval status
Use outside clinical trials involves uncontrolled dosing, unknown purity of research chemical sources, and no medical monitoring
Not evaluated in pregnancy; teratogenic risk is unknown

Drug & supplement interactions

N
o
f
o
r
m
a
l
d
r
u
g
-
d
r
u
g
i
n
t
e
r
a
c
t
i
o
n
s
t
u
d
i
e
s
h
a
v
e
b
e
e
n
p
u
b
l
i
s
h
e
d
i
n
t
h
e
o
p
e
n
l
i
t
e
r
a
t
u
r
e
f
o
r
e
n
o
b
o
s
a
r
m
s
p
e
c
i
f
i
c
a
l
l
y
.
A
s
a
n
a
n
d
r
o
g
e
n
r
e
c
e
p
t
o
r
m
o
d
u
l
a
t
o
r
,
p
h
a
r
m
a
c
o
d
y
n
a
m
i
c
i
n
t
e
r
a
c
t
i
o
n
s
w
i
t
h
o
t
h
e
r
h
o
r
m
o
n
a
l
a
g
e
n
t
s
(
e
x
o
g
e
n
o
u
s
a
n
d
r
o
g
e
n
s
,
a
n
t
i
-
a
n
d
r
o
g
e
n
s
,
a
r
o
m
a
t
a
s
e
i
n
h
i
b
i
t
o
r
s
,
G
L
P
-
1
r
e
c
e
p
t
o
r
a
g
o
n
i
s
t
s
)
a
r
e
p
l
a
u
s
i
b
l
e
a
n
d
a
r
e
u
n
d
e
r
a
c
t
i
v
e
i
n
v
e
s
t
i
g
a
t
i
o
n
i
n
c
o
m
b
i
n
a
t
i
o
n
t
r
i
a
l
s
.
E
n
o
b
o
s
a
r
m
i
s
m
e
t
a
b
o
l
i
z
e
d
h
e
p
a
t
i
c
a
l
l
y
;
i
n
t
e
r
a
c
t
i
o
n
s
w
i
t
h
C
Y
P
e
n
z
y
m
e
i
n
d
u
c
e
r
s
o
r
i
n
h
i
b
i
t
o
r
s
h
a
v
e
n
o
t
b
e
e
n
c
h
a
r
a
c
t
e
r
i
z
e
d
i
n
p
u
b
l
i
s
h
e
d
h
u
m
a
n
p
h
a
r
m
a
c
o
k
i
n
e
t
i
c
s
t
u
d
i
e
s
.
C
o
m
b
i
n
a
t
i
o
n
w
i
t
h
o
t
h
e
r
S
A
R
M
s
,
a
n
a
b
o
l
i
c
s
t
e
r
o
i
d
s
,
o
r
h
e
p
a
t
o
t
o
x
i
c
a
g
e
n
t
s
c
a
r
r
i
e
s
a
d
d
i
t
i
v
e
r
i
s
k
t
h
a
t
h
a
s
n
o
t
b
e
e
n
s
t
u
d
i
e
d
.

Who should avoid it

Combination with hepatotoxic agents (alcohol, acetaminophen at high doses, other hepatotoxic research chemicals) given observed liver enzyme elevations in trials
Combination with other SARMs or anabolic steroids; additive suppression and toxicity risk is uncharacterized
Use in women who are or may become pregnant; androgenic and anti-androgenic effects on fetal development are unknown
Use with lipid-lowering medications without monitoring; HDL reductions may interact with cardiovascular risk management
Sourcing from unverified research chemical vendors given documented adulteration and mislabeling in the SARM market

How it is commonly combined

Enobosarm is under formal clinical investigation in combination with abemaciclib (a CDK4/6 inhibitor) for ER+/AR+/HER2- metastatic breast cancer (NCT05065411) and in combination with GLP-1 receptor agonists for obesity-related muscle preservation. These combinations are being evaluated within controlled trials. No evidence-based stacking guidance exists for non-clinical use. The practice of combining multiple research chemicals or SARMs compounds unknown risks and has no established safety or efficacy basis.

Legal & regulatory. Not FDA-approved for any indication as of 2026. Investigational drug status only. Holds FDA Fast Track designation for AR+/ER+/HER2- metastatic breast cancer. Phase 3 applications were rejected despite observed lean mass gains (POWER I and II trials failed primary functional endpoints). Not legal for human consumption outside of an approved clinical trial in the United States. Widely sold as a "research chemical" with "not for human consumption" labeling; that labeling does not confer legality for personal use under the Federal Food, Drug, and Cosmetic Act.

Quality & harm reduction

Safer, legal alternative we recommend

Creatine Supreme. For supporting muscle mass and strength, creatine monohydrate is the most evidence-backed legal option, with decades of safety data - ours ships from the Peptropix line.

See our recommended pick

Find Enobosarm →

Know your numbers before you research

Order your own bloodwork online — hormone, metabolic and inflammation panels, no doctor visit needed. Know your baselines before and during any protocol. Independent bloodwork is the cheapest insurance there is.

Get tested with Ulta Lab Tests →

Frequently asked questions

What dose should I use for muscle building?

This reference does not provide dosing protocols. Clinical trials used 1 mg and 3 mg daily under medical supervision. Enobosarm is not approved for human use outside clinical trials. Consult a licensed clinician before considering any investigational compound.

Did enobosarm pass FDA approval?

No. Despite demonstrating lean mass gains in Phase 3 (POWER I and II trials, pooled n=1,027), the trials failed their primary functional endpoint of stair-climb power improvement. The FDA declined to approve the New Drug Application. Development continues for breast cancer and obesity-related indications.

Is Ostarine the same as enobosarm?

Yes. Ostarine is a common market name for enobosarm (also known as GTx-024 and MK-2866). Research chemical suppliers use this name; however, independent laboratory analyses of commercial Ostarine products have frequently found mislabeling, incorrect dosing, and adulteration.

Does enobosarm cause testosterone suppression?

Published Phase 2 and Phase 3 trial reports do not prominently report HPG axis suppression as a primary safety finding at the 1-3 mg doses studied. However, SARM-class agents are capable of suppressing endogenous testosterone at sufficient doses, and long-term endocrine effects at doses used in the research chemical community (often far exceeding trial doses) are not established.

Is enobosarm legal to buy in the United States?

Enobosarm is not FDA-approved and is classified as an unapproved investigational drug. It is widely sold as a research chemical with 'not for human consumption' labeling, but that label does not confer legal protection for personal use. The FDA has issued warning letters to SARM distributors. Regulatory risk exists for both sellers and buyers.

Are there human trials showing it works?

Yes, at the Phase 2 level. A 12-week Phase 2 RCT (n=120) showed statistically significant lean mass gains and improved stair-climb power in elderly adults. Phase 3 POWER trials (cancer patients, n=1,027 pooled) confirmed tolerability and lean mass gains but failed the functional primary endpoint, preventing approval. Evidence in breast cancer and obesity is still accumulating in ongoing trials as of 2026.

References & further reading

PubMed: enobosarm GTx-024 MK-2866 Phase 2 lean body mass elderly postmenopausal (see PMC3177038)
PubMed: selective androgen receptor modulators SARM clinical applications review (PMID 30503797)
PubMed: POWER trials enobosarm study design cancer cachexia Phase 3 (PMID 27138015)
ClinicalTrials.gov: NCT05065411 enobosarm abemaciclib ER-positive HER2-negative metastatic breast cancer
The Lancet Oncology 2024: activity safety enobosarm androgen receptor-positive oestrogen receptor-positive HER2-negative advanced breast cancer Study G200802