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Ostarine (MK-2866)

The most clinically studied SARM, mild in profile, with real phase 2 data on muscle mass and function.

2 min read 5 sources

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Phase 3 · not approved

Summary

At a glance

Sources checked on 26 Oct 2026

Fat-free mass above placebo

+1.3 kg

Healthy older adults, higher level, 12 weeks¹

Fat-free mass in cancer

+1.5 kg

Median, lower level, up to 113 days, placebo 0.02 kg²

HDL cholesterol

−27 %

Higher level against placebo, 12 weeks¹

Phase 3 trials in the registry

4 trials

two completed, two stopped early, as of 26 Oct 2026⁴

Ostarine (enobosarm) is a non-steroidal modulator of the androgen receptor (SARM). It is not approved, the highest stage of development was phase 3.⁵⁴
In a phase 2 trial in healthy older adults fat-free mass rose by 1.3 kg more than on placebo within 12 weeks, stair climbing got faster and insulin resistance fell.¹
In cancer patients with weight loss median fat-free mass rose by 1.0 to 1.5 kg, on placebo hardly at all.²
In return HDL cholesterol (down to -27 percent) and the binding protein SHBG fell markedly. At higher levels in breast cancer serious side effects such as raised liver enzymes occurred.¹³
Everything here is the state of research, not a recommendation.⁴

Profile

Updated May 2026

How long does it stay in the body?

Share left after one dose, calculated with a half-life of ~24 hours

Half-life ~24 hours

After 48 hours: 25 %

Hour 24 · 50 %Hour 48 · 25 %Hour 72 · 12.5 %Hour 120 · 3.1 %

Simplified model without an absorption phase, half-life from the TPD lexicon.

Class

SARM

non-steroidal

Use

Oral

daily

Half-life

approx. 24 hours

WADA

Banned

category S1.2

Sizes

10 to 25 mg

3 sizes in the shop

Contents 14 sections

At a glance
What is Ostarine?
How it works
How to take it
Dosing
What the research shows
Research status
Side effects
Practice
Storage
Related compounds
FAQ
Glossary
Sources

What is Ostarine?

Ostarine, also called MK-2866 or Enobosarm, is probably the most clinically studied SARM. It was developed by GTx Inc as a therapeutic candidate for muscle-preservation indications, such as cancer-related muscle wasting.

In the research community Ostarine is regarded as the mildest member of the class: lower suppression, lighter side-effect load than RAD-140 or LGD-4033. Mild does not mean harmless.

Comparison

SARMs and steroids at the androgen receptor

Compounds at the androgen receptor, as stated in the TPD lexicon. Tap a card to open its entry.

SARM, tablet Ostarine (MK-2866) AR

SARM LGD-4033 (Ligandrol) AR SARM RAD-140 (Testolone) AR Steroid, tablet Anavar (Oxandrolone) AR

Simplified: all bind the androgen receptor. SARMs have no steroid skeleton and are meant to act mainly in muscle and bone, steroids act more broadly. ⁵¹

How it works

Ostarine binds the androgen receptor and activates it preferentially in muscle and bone tissue. In studies this was associated with gains in lean mass and improved physical function.

It does not aromatize and is not reduced to DHT. Suppression of the body's own axis is present but lower compared with stronger SARMs.

Where it acts

Where ostarine acted in studies

Tap a point or a card. Colour = androgen receptor, grey = observed in studies.

Skeletal muscle

Fat-free mass rose and stair climbing got faster¹

AR
Bone

Anabolic effect in bone in earlier studies, not analysed in humans¹

AR
Fat tissue

Fat mass fell by about 0.6 kg against placebo at the higher level¹

Observed in trials
Heart and blood lipids

HDL fell by 17 to 27 percent, total cholesterol and triglycerides too¹

Observed in trials
Blood sugar

Fasting blood sugar and insulin resistance fell at the higher level¹

Observed in trials
Hormone axis

SHBG fell by 61 percent in men, total testosterone fell, free testosterone did not¹

Observed in trials
Liver

Severely raised liver enzymes in 3 to 4 percent in the breast cancer trial³

Observed in trials

Targets in the ChEMBL database⁵

Compound CHEMBL1738889: one recorded mechanism, Modulator, tissue-selective.

Androgen receptor

Gene: AR
UniProt: P10275
ChEMBL: CHEMBL1871
Action: Modulator, tissue-selective

Blue molecules attaching to glass muscle fibres and to a glass honeycomb-like bone structure — Binding at the androgen receptor, mainly in muscle and bone. Symbolic image.

How to take it

Oral, once daily.

One dose per day is enough given the roughly 24-hour half-life.
With some water, with or without food.
Cycles often run over 8 weeks with a break afterwards.

White tablet bottle with a black TPD label for Ostarine (MK-2866), next to it a glass of water and three white tablets — Oral as a tablet, with some water. Symbolic image.

Dosing

Dosing, titration and syringe

Loading the dosing information …

Getting started and stepping up, drawn as a staircase
Units on the U-100 syringe for every step
How long one vial lasts

What the research shows

A phase 2 study (Dalton, 2011) in healthy older men and postmenopausal women associated Ostarine with gains in lean mass and better physical function. A further phase 2 study (Dobs, 2013) tested Enobosarm against muscle wasting in cancer patients and saw improvements in lean mass and function.

That gives Ostarine the most solid human data record among SARMs, yet no approval exists.

Study results

What the trials measured

Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.

Body composition against placebo¹

120 men over 60 and postmenopausal women, difference to placebo in kg

Fat-free mass¹

Lower level +0.7 kg

Higher level +1.3 kg

Fat mass¹ given for the higher level only

Higher level -0.6 kg

Only the higher level was clearly better than placebo for fat-free mass. Body weight did not change, fat gave way to muscle. Stair climbing time fell depending on the level.

Fat-free mass in cancer patients²

Phase 2 trial, 159 participants, median change in kg

Fat-free mass²

Placebo +0.0 kg

Lower level +1.5 kg

Higher level +1.0 kg

100 participants with measurements at the start and end were analysed. The range was wide, from -5.8 to +12.6 kg. According to the trial the most common serious events were not related to the drug.

Insulin resistance (HOMA-IR)¹

Healthy older adults, change in percent

HOMA-IR¹

Placebo +2.6%

Lower level -9.3%

Higher level -27.5%

Values from the full text. Only the higher level clearly differed from placebo, fasting blood sugar also fell there.

Row of test tubes with white caps, each holding a glass muscle fibre bundle — Phase 2 trials on muscle mass and function in older adults and in cancer. Symbolic image.

Research status

Where the research stands

Enobosarm trials on ClinicalTrials.gov, without three withdrawn entries. First muscle wasting in cancer, then breast cancer and bladder weakness, most recently muscle preservation during weight loss with GLP-1 drugs. As of 26 Oct 2026. ⁴

Preclinical Tissue selectivity in muscle and bone in animal models
Phase 1 1 trial, 1 completed
Phase 2 9 trials, 6 completed, 2 stopped early
Phase 3 4 trials, 2 completed, 2 stopped early
Approval not yet

651 participants in phase 3 Not approved: ChEMBL lists enobosarm with phase 3 as the highest phase. The two phase 3 trials in lung cancer ended in 2013, two in breast cancer were stopped early.⁵

Show all 13 phase 2 and 3 trials on a timelineHide timeline 8 completed 1 running 4 stopped early 2 with results

Phase 2

Phase 3

Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026, grey = stopped early. Phase 1: 1 trial on safety and pharmacokinetics, not shown as bars.

Side effects

Suppression of the testosterone axis, milder than with stronger SARMs but present.
Falling HDL values in longer setups.
Rarely liver value changes at higher doses.

Tolerability

Serious side effects in breast cancer

Out of 100 participants in the breast cancer trial, in how many the drug caused serious side effects.

8 of 100 Breast cancer, lower level

16 of 100 Breast cancer, higher level (highest study dose, 18 mg per day)

Event	Breast cancer, lower level	Breast cancer, higher level (highest study dose, 18 mg per day)
Grade 3 or 4³	8%	16%

Most common were raised liver enzymes, too much calcium in the blood and fatigue. In the trial in healthy older adults with lower levels side effects were as frequent as on placebo, HDL fell there by 17 to 27 percent.³¹²

Practice

A good entry SARM, still start low (12.5mg).
Limit cycles, then take a break.
Banned in official competition and detectable for weeks (WADA S1.2).
Check blood values before and after the cycle.

Glass container with chicken, rice and broccoli, next to it a water bottle, two light blue resistance bands and towels on a marble counter

Storage

Store dry, cool and protected from light, out of reach of children. Keep the original container closed and mind the expiry date.

Three white tablet bottles and a folded light blue towel on a white shelf — Dry, cool and protected from light in the closed original container. Symbolic image.

Level · Ostarine (MK-2866)

How the level builds up

One dose per day: each dose adds to what is left of the previous one.

Lexicon: Oral (daily)

Peak 2×Trough 1×

Day 4 · 5. dose
1.94× of the first dose

Plateau

from Day 4

97 % of the plateau after dose 5

Peak

2×

of the first dose

Trough

1×

before the next dose

Simplified model, values relative to the first dose, no dosing information. Dashed: the first dose alone, green: the plateau.

Related compounds

Same receptors: all in the TPD lexicon.

Ostarine (MK-2866)

LGD-4033 (Ligandrol) Another SARM

RAD-140 (Testolone) Another SARM

Andarine (S4) Another SARM

S-23 (SARM) Another SARM

Anavar (Oxandrolone) Oral steroid, also studied against muscle wasting

FAQ

Common questions about Ostarine (MK-2866)

Short answers, each backed by the sources below.

What is ostarine?

Ostarine, also enobosarm or MK-2866, is a non-steroidal modulator of the androgen receptor (SARM). It was developed against muscle wasting and is meant to act mainly in muscle and bone. It is not approved, the highest stage was phase 3.⁵¹

What did the study in healthy older adults show?

120 men over 60 and postmenopausal women received placebo or one of four levels for 12 weeks. At the higher level fat-free mass rose by 1.3 kg more than on placebo, fat fell by about 0.6 kg and weight stayed the same. Stair climbing got faster and insulin resistance fell by 27.5 percent. Side effects were as frequent as on placebo.¹

And in cancer patients?

In a phase 2 trial with 159 cancer patients with weight loss median fat-free mass rose by 1.5 kg (lower level) and 1.0 kg (higher level), on placebo by 0.02 kg. According to the trial the most common serious events were not related to the drug.²

What happens to blood lipids and hormones?

HDL cholesterol fell by 17 and 27 percent. The binding protein SHBG fell by 61 percent in men and 80 percent in women. Total testosterone fell in men as well. Free testosterone did not change significantly.¹

What did the highest study dose show?

A trial in 136 women with advanced breast cancer had two levels, the higher at 18 mg per day. After 24 weeks 29 to 32 percent had a clinical benefit (response or stable disease). Serious side effects caused by the drug occurred in 8 and 16 percent, mainly raised liver enzymes, too much calcium in the blood and fatigue.³

Are trials still running?

Yes. As of 26 October 2026 a phase 2 trial with 239 participants is running that tests weight and body function on semaglutide. A first trial on this with 168 participants is completed. Two phase 3 trials in lung cancer ended in 2013.⁴

Glossary

Terms explained briefly

Dotted words in the text open these explanations directly.

Androgen receptor: Protein inside the cell that testosterone and its derivatives bind to. Genes are then read, for example for muscle building.
Aromatisation: Conversion of an androgen into oestrogen by the enzyme aromatase.
DHT: Dihydrotestosterone, a stronger androgen made from testosterone. Ostarine is not converted into it.
Fat-free mass: Body weight without fat: muscle, bone, organs and water.
HDL cholesterol: The so-called good cholesterol. If it falls, the risk of vessel plaque rises.
Muscle wasting (cachexia): Unintended loss of muscle mass in serious illness, for example cancer.
Phase 2 study: Study in dozens to hundreds of participants that tests effect and suitable levels.
Placebo: Dummy preparation without active substance. Comparing against it shows what is due to the substance itself.
SARM: Selective androgen receptor modulator: binds the androgen receptor but has no steroid skeleton and is meant to act mainly in muscle and bone.
SHBG: Sex hormone-binding globulin, a blood protein that binds testosterone. If it falls, measured total testosterone falls.
Suppression: Suppression of the body's own hormone axis: the body makes less of its own testosterone while an androgen signal comes from outside.

Evidence

Sources

Show all 5 sourcesHide sources 3 Studies1 Trial registry1 Database checked 26 Oct 2026

Studies

Phase 2 trial in healthy older adults: fat-free mass and function
Dalton et al., Journal of Cachexia, Sarcopenia and Muscle, 2011

DOI 10.1007/s13539-011-0034-6 PMID 22031847

Checked 26 Oct 2026 via doi.org, Crossref, PubMed, PMC-Volltext
Phase 2 trial in cancer patients with muscle wasting
Dobs et al., The Lancet Oncology, 2013

DOI 10.1016/S1470-2045(13)70055-X PMID 23499390 NCT00467844

Checked 26 Oct 2026 via doi.org, Crossref, PubMed
Phase 2 trial in advanced breast cancer, two levels
Palmieri et al., The Lancet Oncology, 2024

DOI 10.1016/S1470-2045(24)00004-4 PMID 38342115 NCT02463032

Checked 26 Oct 2026 via doi.org, Crossref, PubMed

Trial registry

Enobosarm trials on ClinicalTrials.gov
ClinicalTrials.gov, U.S. National Library of Medicine

Checked 26 Oct 2026 via ClinicalTrials.gov API v2

Database

Enobosarm in ChEMBL (mechanism and development stage)
ChEMBL, EMBL-EBI (CHEMBL1738889)

Checked 26 Oct 2026 via ChEMBL

In the shop

Ostarine (MK-2866) in the catalog

Sizes, prices and shipping times are on the product page.

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