MK-2866 Overview, Dosing & Safety | Peptide Database

MK-2866 (Ostarine)

Selective Androgen Receptor Modulator | Muscle Wasting Research

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MK-2866 (Ostarine, Enobosarm) is a non-steroidal selective androgen receptor modulator (SARM) originally developed by GTx, Inc. (now Oncternal Therapeutics) for the prevention and treatment of muscle wasting and sarcopenia. It is the most extensively studied SARM in clinical trials, having progressed through Phase I, II, and III trials for cancer-related cachexia, stress urinary incontinence, and age-related muscle loss. MK-2866 selectively binds to androgen receptors in muscle and bone tissue with high affinity while exhibiting minimal activity in prostate and sebaceous glands, which differentiates it from traditional anabolic-androgenic steroids. Despite promising clinical data demonstrating significant lean body mass gains in cancer patients and elderly subjects, the FDA has not granted approval, and GTx's New Drug Application for stress urinary incontinence was not approved in 2018. MK-2866 remains classified as an investigational compound and is prohibited by WADA in competitive sports.

MK-2866 binds to the androgen receptor (AR) with high affinity and selectivity, functioning as a partial agonist in muscle and bone tissue. Upon binding, the MK-2866-AR complex undergoes a conformational change that promotes nuclear translocation and interaction with androgen response elements (AREs) on DNA, activating transcription of genes involved in protein synthesis, nitrogen retention, and myogenic differentiation. The tissue selectivity of MK-2866 arises from its non-steroidal structure, which produces a distinct AR conformational change compared to testosterone or DHT. This results in differential coactivator and corepressor recruitment across tissues. In skeletal muscle, MK-2866 robustly activates anabolic signaling pathways including PI3K/Akt/mTOR, increasing muscle protein synthesis and reducing protein degradation via suppression of ubiquitin-proteasome and myostatin pathways. In bone, it stimulates osteoblast differentiation and mineralization. Critically, MK-2866 demonstrates reduced androgenic activity in prostate tissue and does not undergo 5-alpha reduction to a more potent metabolite (unlike testosterone), nor does it aromatize to estrogen, which accounts for its lower incidence of androgenic and estrogenic side effects.

Molecular Data

Research Indications

MK-2866 was originally developed for cancer-related muscle wasting. Phase II trials in cancer patients demonstrated statistically significant increases in lean body mass at doses of 1 mg and 3 mg daily over 16 weeks, regardless of cancer type, chemotherapy regimen, or baseline nutritional status.

Age-related loss of muscle mass and strength. MK-2866 has shown improvements in lean body mass and physical function in elderly subjects, though Phase III trial endpoints for regulatory approval were not fully met.

Potential application for preserving lean mass during prolonged immobilization or recovery from surgery, based on its anti-catabolic properties demonstrated in clinical populations.

At research doses of 12.5-25 mg daily, users report gains of 2-4 kg of lean body mass over 6-8 week cycles. Clinical trials at lower doses (1-3 mg) still showed statistically significant lean mass increases, suggesting a dose-dependent anabolic response.

MK-2866 is commonly used for simultaneous muscle preservation and fat loss during caloric restriction. Its ability to maintain nitrogen balance in a deficit makes it suitable for recomposition protocols.

While not a direct fat-burning agent, MK-2866 preserves metabolically active lean tissue during a cut, indirectly supporting a higher basal metabolic rate and improved body composition outcomes.

Preclinical and early clinical data suggest MK-2866 increases bone mineral density through direct osteoblast stimulation. This effect is consistent with its selective AR agonism in bone tissue and may benefit populations at risk for osteoporosis.

Users report modest but noticeable improvements in strength, particularly in compound lifts, during MK-2866 cycles. Clinical trials measured improved stair-climbing power as a functional endpoint.

Accelerated recovery between training sessions is commonly reported, likely mediated by improved nitrogen retention and reduced exercise-induced muscle damage.

Dosing Protocols

MK-2866 is administered exclusively via the oral route, either as a liquid solution or in capsule form. Its high oral bioavailability and long half-life of approximately 24 hours make once-daily dosing practical and effective. In clinical trials, MK-2866 was administered as oral capsules at doses of 0.1 mg to 3 mg. Research and off-label use typically involves higher doses in the 10-25 mg range.

Muscle Preservation (Cut / Deficit)

10-15 mg/day

Once daily

Oral (liquid or capsule)

Lean Mass Gain (Recomposition)

12.5-20 mg/day

Lean Mass Gain (Bulk)

20-25 mg/day

Female Protocol

5-10 mg/day

Interactions

What to Expect

Side Effects & Safety

Common Side Effects

Mild testosterone suppression (dose-dependent, typically 10-30% reduction at 25 mg)

HDL cholesterol reduction (10-20% suppression observed in clinical trials)

Headaches, particularly during the first 1-2 weeks

Mild back pain or muscle cramps

Transient fatigue toward the end of longer cycles

Slight reduction in libido at higher doses or extended cycle lengths

Stop Signs - Discontinue if:

Persistent or severe liver pain (right upper quadrant tenderness)

Yellowing of skin or eyes (jaundice)

Severe or persistent headaches not relieved by standard measures

Chest pain, heart palpitations, or shortness of breath

Signs of significant hormonal disruption: severe mood changes, persistent erectile dysfunction, or testicular pain

Unusual swelling in extremities

Contraindications

Active liver disease or significantly elevated liver enzymes

Hormone-sensitive cancers (breast, prostate) without oncologist clearance

Pregnancy or breastfeeding (potential endocrine disruption to fetus/infant)

Individuals under 21 years of age (risk of premature HPTA disruption during development)

Concurrent use of hepatotoxic medications without liver function monitoring

Known hypersensitivity to MK-2866 or any formulation excipients

Competitive athletes subject to WADA or USADA anti-doping testing

Quality Checklist

Good Signs

Third-party lab testing (HPLC/MS) confirming compound identity and purity above 98%

Clear labeling with compound name, concentration, batch number, and expiration date

Certificate of analysis (CoA) available with matching batch number

Liquid solution is clear with no visible particulates, cloudiness, or discoloration

Capsules are uniformly filled with consistent weight across the batch

Vendor has an established reputation with verified independent lab testing

Warning Signs

No third-party lab testing or certificate of analysis available

Unusually low price compared to market average (potential underdosing or substitution)

Vague or incomplete labeling missing concentration, batch number, or expiration

Vendor makes explicit therapeutic or curative claims

Product marketed as a dietary supplement rather than a research chemical

Bad Signs

Lab testing reveals wrong compound, contamination, or significant underdosing

Product contains undisclosed active ingredients (prohormones, methylated steroids)

Liquid is discolored, cloudy, or contains visible particles or precipitate

No labeling or labeling does not match contents

Vendor provides no verifiable contact information or has a history of failed third-party testing

Product seized or flagged by regulatory authorities for adulteration

Frequently Asked Questions

Do I need PCT after a MK-2866 cycle?

MK-2866 is only mildly suppressive compared to anabolic steroids, so full PCT is usually not necessary at doses below 20 mg/day. However, bloodwork at the end of your cycle will determine if your testosterone recovered naturally within 2-4 weeks. If you ran 25 mg/day or combined MK-2866 with other SARMs, a mini-PCT with enclomiphene or tamoxifen may accelerate recovery.

Should I stack MK-2866 with RAD-140?

Stacking MK-2866 with RAD-140 compounds the HPTA suppression significantly beyond what either produces alone, requiring bloodwork monitoring and potentially full PCT. Most experienced users find RAD-140 alone sufficient for their goals and don't see meaningful additional benefit from adding MK-2866, making it an unnecessary increase in suppression.

How long before I see lean mass gains on MK-2866?

Noticeable strength improvements typically appear in weeks 2-3, measurable lean mass gains (1-2 kg) by week 4-5, and the full 2-4 kg of gains by weeks 6-8 depending on dose and diet. At lower doses (10-15 mg), muscle preservation during a cut is noticed faster than new growth on a bulk.

Does MK-2866 cause water retention?

No, MK-2866 does not aromatize to estrogen and does not cause the water retention seen with testosterone. Any weight gain is predominantly lean mass and intramuscular water (inside the muscle cell), not subcutaneous or extracellular bloating, which is why users on MK-2866 maintain clear muscle definition.

References

Phase II randomized, double-blind, placebo-controlled trial of enobosarm (GTx-024/MK-2866) in 120 healthy elderly men and postmenopausal women. Doses of 1 mg and 3 mg daily for 12 weeks produced statistically significant, dose-dependent increases in total lean body mass and improvements in physical function (stair-climbing power). The compound was well tolerated with no serious adverse events.

Phase II trial of enobosarm in 159 cancer patients with muscle wasting. Both 1 mg and 3 mg doses produced significant increases in lean body mass compared to placebo at 16 weeks, regardless of cancer type or concurrent chemotherapy. Median lean body mass increased by 1.5 kg (3 mg group) versus a loss of 0.02 kg in placebo. The drug was well tolerated with a safety profile comparable to placebo.

Two pivotal Phase III trials (POWER 1 and POWER 2) evaluating enobosarm 3 mg daily in patients with non-small cell lung cancer cachexia. Both trials met the lean body mass co-primary endpoint, demonstrating significantly greater muscle mass versus placebo. The stair-climbing power co-primary endpoint showed a positive trend but did not reach statistical significance in the responder analysis, contributing to the FDA's decision not to approve the drug for this indication.

Comprehensive review of SARM development with focus on enobosarm as the most clinically advanced SARM. Describes the tissue-selective mechanism of action, clinical trial results, and potential therapeutic applications for muscle wasting, sarcopenia, breast cancer, and stress urinary incontinence.

Detailed pharmacological characterization of aryl propionamide SARMs including MK-2866. Describes structure-activity relationships, androgen receptor binding kinetics, and the molecular basis for tissue selectivity. Provides the foundational medicinal chemistry context for understanding how MK-2866 achieves selective anabolic activity without significant androgenic effects in non-target tissues.

Related Peptides

MK-677 (Ibutamoren) stimulates growth hormone secretion through ghrelin receptor agonism, while MK-2866 provides direct androgen receptor activation in muscle. The combination supports lean mass accrual through complementary mechanisms without additive HPTA suppression, as MK-677 does not affect the androgen axis. Commonly stacked for body recomposition and lean gain goals.

Enclomiphene is frequently used as a mini-PCT or on-cycle support with MK-2866 to mitigate testosterone suppression. Enclomiphene stimulates endogenous LH and FSH release, counteracting the mild HPTA suppression caused by MK-2866. This combination can help maintain natural testosterone levels during and after a SARM cycle.

Cardarine (GW-501516) is a PPAR-delta agonist that enhances fatty acid oxidation and endurance without interacting with the androgen receptor. It does not contribute to HPTA suppression, making it a common addition to MK-2866 cycles targeting body recomposition or cutting goals.

No known negative interactions. BPC-157 supports tendon and connective tissue healing through distinct mechanisms unrelated to androgen receptor signaling. Can be used concurrently to support joint and soft tissue health during training.

Disclaimer

This information is for educational and research purposes only. Consult a healthcare professional before use.