Ostarine (MK-2866)
Also known as: Ostarine, MK-2866, Enobosarm, GTx-024, Ostarin, MK2866
The mildest and best-studied SARM — and the one that shows most clearly that 'selective' does not mean 'without suppression'.
Substance family
Non-steroidal compounds binding the same androgen receptor as testosterone, but favouring muscle and bone over prostate and hair. The selectivity is real; the suppression of the body's own production is not avoided by it. None is approved anywhere.
Effects on
Which outcomes this substance acts on — descriptive, not a recommendation.
Strain profile
Where this substance is most likely to hit — a rough orientation, individually dependent on dose, duration and predisposition.
This substance causes
Dosing & protocols by goal
Rough orientation ranges as reported in harm-reduction literature — not an individual treatment plan. Doses are individual and open-ended; frequency depends on the ester or half-life.
Clinical trials (dose range studied)
MedicalDose
1–3 mg / day
Frequency
1× daily
Cycle length
12–16 weeks in trials
Investigated for muscle wasting in cancer patients. Reached phase 3; development discontinued. There is no approved dose.
Muscle gain (grey market practice)
IntermediateDose
10–25 mg / day
Frequency
1× daily
Cycle length
8 weeks
Several times the doses used in trials. SuppressionSuppressionThe body shutting down its own testosterone production because an external source is present. scales with the dose and is reliably present at this level.
Higher doses
AdvancedDose
> 25 mg / day
Frequency
1× daily
Cycle length
—
SuppressionSuppressionThe body shutting down its own testosterone production because an external source is present. approaches that of a mild steroid cycle, and liver values rise more often. The 'mild' reputation stops applying here.
Side effect & countermeasure
Typical problems and what you can do to reduce harm — at a glance. Does not replace medical monitoring.
Problem
Suppression despite the 'no PCT needed' reputation
Countermeasure
Ostarine suppresses LH and testosterone dose-dependently — at 20 mg over 8 weeks, reliably. The claim that SARMsSARMsA non-steroidal compound that binds the androgen receptor with claimed tissue selectivity. need no recovery plan comes from marketing, not from data. Measure LH, FSH and testosterone before and after; if suppressedsuppressedThe body shutting down its own testosterone production because an external source is present., a recovery plan applies exactly as it would after a steroid cycle.
Problem
Elevated liver values
Countermeasure
Documented in trials and in case reports, occasionally severe. Check ALT, AST and GGT before and during. Do not combine with orals or alcohol; jaundice or dark urine means stop and see a doctor.
Problem
Lowered HDL
Countermeasure
Milder than with oral steroids but present. Cardio, omega-3, possibly citrus bergamot. Measure a lipid baseline before starting.
Problem
Contaminated and mislabelled product
Countermeasure
Analyses of products sold as SARMsSARMsA non-steroidal compound that binds the androgen receptor with claimed tissue selectivity. repeatedly find different compounds, wrong doses or nothing at all. This is also why ostarine is a leading cause of positive doping tests among athletes who never intended to take it. Only sources with an independent HPLC/MS analysis.
Problem
Assumption that non-steroidal means harmless
Countermeasure
Ostarine acts on the same receptor as testosterone. It is more selective for muscle and bone, which reduces prostate and hair effects — it does not remove the hormonal consequences.
Overview
Ostarine is the most studied SARM and reached phase 3 trials for muscle wasting in cancer patients before development was discontinued. That makes it the compound in this class with the most actual human data — which also means the least room for speculation about what it does.
It is widely described as the mild entry point into SARMs, and relative to the others that is accurate. The problem is what "mild" gets taken to mean.
Mechanism of Action
SARMs bind the androgen receptor, the same receptor testosterone binds. The difference is tissue selectivity: their structure favours receptors in muscle and bone over those in prostate, skin and hair follicles.
That selectivity is real and it is the reason SARMs exist — the original goal was treating muscle wasting without the prostate and virilisation effects of testosterone.
What selectivity does not change is the feedback loop. The hypothalamus registers androgen receptor activation regardless of which tissue is involved. It concludes that enough androgen is present and reduces LH — and with it the body's own testosterone production. In trials, ostarine suppressed testosterone dose-dependently even at three milligrams.
This is the point where the marketing and the data diverge most sharply. "No PCT needed" is a claim about SARMs that the trial data does not support, and the doses used in practice are five to ten times those trials used.
Typical Context
Grey-market use runs at ten to twenty-five milligrams daily over eight weeks — against one to three milligrams in the clinical trials.
Ostarine is often chosen as a first compound by people who want to avoid injections and believe they are avoiding the hormonal consequences. The first part is true. The second is not: at these doses, suppression is reliable, and recovery follows the same logic it would after a mild steroid cycle.
Two practical issues matter more than the pharmacology. Product quality is genuinely poor across this market — analyses repeatedly find other compounds, wrong doses, or nothing. And ostarine is one of the most common causes of positive doping tests in athletes who took a contaminated supplement rather than a SARM. If you compete under testing, this substance is a risk even when you do not take it deliberately.
Side Effects & Risks
- Suppression of LH and the body's own testosterone production
- Elevated liver values, occasionally severe
- Lowered HDL
- Fatigue and mood changes during suppression
- Headache
- Frequently contaminated or mislabelled product
- Long detection window in doping tests
Blood Work & Monitoring
- LH, FSH and total testosterone — before and after; the values that show whether recovery is needed
- Liver values (ALT, AST, GGT) — before and during
- Lipid profile (LDL, HDL, triglycerides)
- Estradiol (sensitive) — falls along with testosterone under suppression
Harm-Reduction Notes
- "No PCT needed" is not supported by the trial data — measure LH and testosterone before and after
- Keep the duration to 8 weeks; suppression and liver values both scale with time
- Do not combine with oral steroids; the liver burden adds
- Only sources with an independent analysis — this market is unusually unreliable
- Unsuitable for anyone subject to doping controls, including via contaminated supplements
- Selectivity reduces prostate and hair effects; it does not remove hormonal suppression
Legal status by region
Regulation differs considerably between jurisdictions and changes over time. This is a rough orientation, not legal advice — check the rules that apply where you are.
| Region | Regulatory status |
|---|---|
| United States | Not approved; FDA warning issuedThe FDA has published explicit warnings about SARMs sold as supplements. |
| EU / UK | No marketing authorisationNeither a medicine nor a supplement — no legal route for human use. |
| Most other jurisdictions | Unregulated grey areaPossession is legal in many places because no framework covers it. That is not an indication of safety. |
| Competitive sport | Prohibited at all timesListed by WADA under S1.2. One of the most frequent causes of positive tests from contaminated supplements. |