Ostarine (MK-2866):
What You NEED to Know

7 Min. February 28, 2025 By Riccardo

What is Ostarine (MK-2866)?

Ostarine, also known as MK-2866 or Enobosarm, is a selective androgen receptor modulator (SARM). This means that it binds to androgen receptors like testosterone and thereby positively affects muscle growth, regeneration, and bone density.

Since Ostarine belongs to the class of SARMs, it acts much more selectively on androgen receptors than testosterone. For example, the receptors in the prostate or liver are only minimally activated. As a result, the SARM has fewer side effects in these areas.

Currently, Phase III human studies are actively researching Ostarine. Ostarine could potentially be used as a treatment for muscle wasting and bone loss conditions (e.g., in cachexia, osteoporosis, or age-related muscle loss).

Benefits of Ostarine (MK-2866)

  • Lean Muscle Growth: Ostarine promotes muscle mass development without building body fat.
  • Improved Recovery: Taking it can reduce recovery time after training.
  • Increased Bone Density: Studies show that Ostarine can improve bone density, reducing the risk of injuries and fractures.
  • Orally Available: Ostarine can be taken orally, e.g., in tablet or liquid form.
  • Good Bioavailability: The body absorbs Ostarine efficiently, so a relatively low dose is sufficient.

How does Ostarine work?

Ostarine belongs to the class of Selective Androgen Receptor Modulators (SARMs). This means that after intake, it binds to androgen receptors in the body. These androgen receptors can be found in many areas of the body, e.g., in muscles, bones, prostate, and liver.

However, Ostarine primarily binds to androgen receptors in muscle and bone tissue and hardly at all to those in the prostate or hair follicles. This has the advantage that Ostarine works much more specifically than testosterone and therefore undesirable side effects such as hair loss and prostate enlargement hardly occur at all. The hormone testosterone affects all androgen receptors equally strongly.

The binding of Ostarine to the androgen receptor is crucial for the anabolic effect. Thus, Ostarine promotes muscle growth, tendon strength, ligament health, bone density, and collagen turnover.

No Conversion to Estrogen and DHT

A major advantage of Ostarine compared to other anabolic substances is that Ostarine does not convert to estrogen or DHT [R].

Estrogen is the female sex hormone and can cause unwanted side effects such as low libido, depression, or gynecomastia. DHT can cause side effects such as hair loss or enlarged prostate.

This makes Ostarine safer in this aspect than testosterone. However, note that Ostarine can indirectly increase the proportion of estrogen in the body compared to testosterone. Learn more about this here.

Bioavailability

Ostarine has very good bioavailability, even with oral administration. This means that Ostarine can be extremely effective even with smaller doses.

Does Ostarine Build Fat?

No, Ostarine seems to build only muscle without adding fat. In a 12-week study, Ostarine was investigated for its effects on lean body mass. The study included healthy elderly men and postmenopausal women who are in the phase after menopause when monthly menstruation permanently stops. [R]

The study was randomized, double-blind, and placebo-controlled. Participants received either Ostarine or a placebo.

figure1

Result: Participants who received Ostarine showed a significant increase in lean mass. The group that took a dose of 3 mg Ostarine per day had the highest increase in lean mass compared to the other dosage groups and the placebo group. [R]

Side Effects of Ostarine (MK-2866)

Mild, common side effects:

  • Headaches
  • Stomach pain

Severe, rare side effects:

  • Liver toxicity
  • Hypertension
  • Cholesterol

Does Ostarine Affect Cholesterol Levels?

Yes, Ostarine can, like many other SARMs, have effects on the cholesterol profile. In a 12-week Phase II study, participants received either 1mg, 3mg Ostarine, or a placebo [R]. The following dose-dependent reduction in HDL cholesterol levels was observed:

  • 1 mg per day: 17% reduction in HDL
  • 3 mg per day: 27% reduction in HDL

HDL cholesterol is also known as "good cholesterol." A lower value can increase the risk of heart problems.

Does Ostarine Suppress Testosterone?

Yes, Ostarine can suppress the body's own testosterone production. This is not because Ostarine increases testosterone levels in the body, because it doesn't. Rather, it's because Ostarine suppresses certain hormones (LH and FSH) that help the body produce testosterone [R]. The higher the dose, the lower the body's own testosterone production should be.

The Good News: Ostarine seems to have significantly less effect on hormones than classic anabolic steroids [R].

Does Ostarine Increase Estrogen?

No, Ostarine has no direct influence on estrogen levels as it does not convert to estrogen. However, Ostarine can reduce the body's own testosterone production, which can lead to the proportion of estrogen being indirectly higher compared to testosterone.

With long-term and high-dose use of Ostarine, there can even be a reduction in estrogen levels. Estrogen is created through the aromatization of testosterone – meaning testosterone is converted to estrogen. Since Ostarine can dampen the body's own testosterone production, the amount of testosterone available for this conversion is also reduced. As a result, the aromatization of testosterone to estrogen is reduced, which can lead to lower estrogen levels in the body.

Can Ostarine Cause Hair Loss?

All androgens, including SARMs, can potentially cause hair loss. This is because we have androgen receptors almost everywhere in the body – including hair follicles.

The good news: Ostarine has a very low binding affinity to androgen receptors in hair follicles, which is why long-term androgenetic hair loss is unlikely.
Ostarine does not cause direct androgenetic hair loss at usual doses.

However, temporary hair loss ("shedding") could occur, caused by hormonal fluctuations during or after intake.

Ostarine Dosage

Ostarine has been extensively researched with different dosages in clinical studies. The dosages used in most studies were between 0.1 to 18 mg/day. Currently, a Phase 3 study is running with 210 participants. They are being administered a dose of 9mg/day against breast cancer. [R]

Ostarine is often taken in higher doses in the bodybuilding scene: For example 10–30 mg/day. [R]

How to store Ostarine

Ostarine comes in liquid and solid form. Both variants should be stored in the following way:

  • Ostarine should be stored at room temperature between 15°C to 30°C (59°F to 86°F)
  • Store Ostarine in a dark place and avoid exposure to light
  • Avoid strong temperature fluctuations
  • Freezing is possible but not recommended

Frequently Asked Questions

What is the half-life of Ostarine (MK-2866)?

The half-life of Ostarine is approximately 24 hours [R].

What other names is Ostarine known by?

Other names for Ostarine are:

  • Enobosarm (official research name)
  • MK-2866 (commonly used name in the fitness and bodybuilding community)
  • GTx-024 (internal designation by GTx, the original developer)
  • S-22
  • VERU-024
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