The Pep Test Guide 2026
The Pep Test Guide 2026U.S. vendor guide
All compounds
Anti-aging

MK-2866

Also known as: Ostarines, enobosarm

Selective androgen receptor modulator. Investigated in clinical studies for muscle wasting. Non-peptide, Orally active.

MK-2866

What is MK-2866

MK-2866 is a selective androgen receptor modulator (SARM), known by the research name Ostarine and the international non-proprietary name Enobosarm. It is chemically a non-steroidal small molecular compound, not a peptide. Strictly speaking, MK-2866 therefore does not fit into a peptide database, but is sold in the research use market parallel to peptides and is relevant in authority research.

Originally developed by GTx Inc. to treat sarcopenia and muscle wasting in cancer patients. Today continued by Veru Inc. as an oncology adjuvant. Several phase 3 clinical trials have been completed, and there is no authorisation for medicinal products.

CAS No: 841205-47-8. Molecular weight: 389.34 g/mol. Chemically an aryl-propionamide derivative.

Mechanism of action

SARMs selectively bind the androgen receptor, i.e. with different affinities in different tissues. Classic anabolic steroids such as testosterone activate AR equally in all tissues, leading to unwanted side effects in prostate, hair, skin and lipid profile.

MK-2866 shows a strong affinity for AR in muscle and bone, but lower activation in prostate and skin. This theoretically creates a more favorable benefit-risk ratio: anabolic effect without classic androgenic side effects. In practice, this selectivity is gradual, not absolute.

The binding affinity is high, the intrinsic activity at the AR is partial. This explains why MK-2866 is less potent than testosterone, but dose-effective. The inhibition of the body’s own testosterone production (suppression of the HPG axis) nevertheless occurs, especially at higher doses or longer use.

Areas of application in research

  • Cancer-related cachexia: Main indication in the original GTx studies
  • Sarcopenia in older adults: Maintaining muscle mass and strength
  • Stress fractures and bone healing: Preservation of bone density
  • AR-positive breast cancer: Veru program, clinical trials ongoing
  • Muscular dystrophy and chronic muscle wasting syndromes: Early studies
  • Post-OP convalescence: Hip fracture studies

Study situation

MK-2866 is one of the best studied SARMs. Several Phase 2 and Phase 3 studies have been published.

Important works:

  • Dalton et al. (Journal of Cachexia, Sarcopenia and Muscle, 2011): Phase 2 study in cancer patients with cachexia, significant increase in fat-free mass and stairway performance
  • Dobs et al. (Lancet Oncology, 2013): Phase 2 study in cancer cachexia, similar results
  • POWER-1 and POWER-2 (Phase 3): GTx studies in NSCLC patients. Primary endpoint fat-free mass reached, functional endpoints (stair climbing) missed. FDA then refused approval
  • Ongoing Veru studies: Enobosarm in AR-positive breast cancer (Phase 3 ARTEST)

What is clear: MK-2866 increases muscle mass measurably, in the range of 1 to 3 kg in 3 to 6 months depending on dose and patient. What remains unclear is whether this increase translates into clinically relevant functional improvements. The FDA has explicitly named this translation problem as the reason for the approval rejection.

Mixing (reconstitution)

MK-2866 is Orally active, not classical mixing as with peptides. Typical pharmaceutical forms:

  • Powder weighed over fine balance
  • Solution in MCT oil, ethanol or PEG-400
  • Prefabricated dropping solutions typically 25 mg/ml
  • Capsules 5, 10, 15, 25 mg

Storage dark, cool, tightly sealed. MK-2866 is chemically stable for months to years at room temperature, far from light.

Dosage range (research data)

From clinical studies:

  • 1 mg daily (lowest effective dose in cachexia studies)
  • 3 mg daily (frequent study dose for sarcopenia)
  • Up to 18 mg in some Phase 1 studies
  • Self-use community: typically 10 to 25 mg
  • Intake: once a day, with or without food
  • Cycle duration in studies: up to 4 months continuous

Steady-state after 4 to 5 days. Effect on Lean Mass measurable after 4 to 8 weeks.

Side effects

From clinical trials and post-marketing reports:

  • Suppression of the body’s own testosterone production (dose-dependent, 30 to 50 percent reduction at 3 mg
  • Increase in liver enzymes ALT/AST (transient in most studies, persistent in some cases)
  • Reduction of HDL cholesterol (similar to anabolic steroids, but milder)
  • Water retention
  • Lethargy, fatigue
  • Acne and hair loss in isolated cases (androgenic effect, less than in steroids)
  • Rare: cholestatic jaundice

The FDA issued a warning in 2017 over reports of severe liver damage from self-use of SARMs from online commerce. Purity of products from a non-pharmaceutical source is a central problem here.

Half-Life and Pharmacokinetics

Elimination half-life: approximately 24 hours. Oral bioavailability high (several studies show over 70 percent). Metabolism via the liver, primarily via CYP3A4.

Steady-State after 4 to 5 days of a daily dose. excretion renal and biliary as metabolites.

Detection in urine: about 30 days after discontinuation (relevant for athletes under doping control).

Frequent questions

Is MK-2866 a steroid? No. MK-2866 is a non-steroidal SARM. Although it binds the same receptor as testosterone, it has a different chemical structure and action profile.

Do I need post-cycle therapy (PCT)? Studies show suppression of testosterone production. With short cycles, the HPG axis usually recovers within 4 to 8 weeks. With longer applications or higher doses, recovery may be delayed. Formal PCT with SERMs (Clomifen, Tamoxifen) is common in the community, but clinically not evidence-based.

What is the risk of hepatotoxicity? Real, but mostly mild and reversible. At 3 mg daily over 3 months, transient ALT elevations are common. Severe liver damage in isolated cases, presumably often caused by contaminated products from non-pharmaceutical sources.

Does MK-2866 only help in building up or maintaining deficit? Clinical data show both buildup in normocaloric nutrition and better lean mass maintenance in calorie deficit. The effect size is moderate.

Is MK-2866 suitable for women? In low doses (1 to 3 mg, the virilizing effect is low. In Veru studies in mammary carcinoma patients, the tolerability is good. At higher doses, virilization may occur (voice depression, clitOral growth).

Legal status U.S.

As of 2026: MK-2866 is not approved as a medicine in Germany, Austria and Switzerland. Sales take place in the research-use-only rail. If there is a detectable therapeutic intention, the AMG is effective.

WADA status: SARMs have been on the banned list since 2008, category S1.2 (Other Anabolic Agents). For athletes under doping control clearly prohibited detectability in urine about one month after discontinuation.

Own import: §73 AMG restrictions. Customs controls are real, especially for larger quantities.

Related compounds

  • LGD-4033 (ligandrol)More anabolically effective SARM
  • RAD-140 (Testolone): Potent SARM, high suppression
  • YK-11Steroidal hybrid with SARM properties
  • GW-501516 (Cardarine)Not a SARM, but a PPARδ agonist
  • S-23Strongly suppressive SARM

Note: All information is for research information only. These peptides are not approved as medicines in Germany. The contents do not replace medical advice. Application to humans is not legally covered in Germany.