Selective Androgen Receptor Modulators (SARMs) have gained major attention in bodybuilding and performance circles for their ability to mimic the anabolic effects of steroids with fewer side effects. Among the most popular are RAD-140 (Testolone) and Ostarine (MK-2866).
Both compounds are designed to selectively activate androgen receptors in muscle and bone tissue, promoting growth and recovery without the same level of androgenic activity in the prostate or hair follicles. But despite belonging to the same class, they differ significantly in potency, side effect profile, and suitability for beginners.
In this guide, we’ll break down:
→ The history and development of RAD-140 and Ostarine
→ How each compound works in the body
→ Key benefits for muscle growth, fat loss, and performance
→ Potential side effects and safety concerns
→ Which SARM may be better suited for first-time users
What Is RAD-140 (Testolone)?
RAD-140, also known as Testolone, is one of the most potent SARMs developed to date. Originally created by Radius Health as a potential treatment for muscle wasting and breast cancer, it was designed to mimic the anabolic effects of testosterone without the same degree of androgenic side effects.
Mechanistically, RAD-140 binds selectively to androgen receptors in skeletal muscle and bone, promoting growth and density while sparing tissues like the prostate. Preclinical studies also suggest neuroprotective effects, making it unique among SARMs (Jayaram, Journal of Medicinal Chemistry).
Key Features of RAD-140
→ High anabolic potency: Considered one of the strongest SARMs available.
→ Muscle building focus: Often compared to a mild anabolic steroid cycle in effect size.
→ Potential neuroprotection: Early data suggests protective effects against neuronal degeneration.
→ Side effect potential: Because of its potency, RAD-140 is more likely than other SARMs to cause testosterone suppression and androgenic effects.
“RAD-140 was shown to increase lean body mass in preclinical models while maintaining tissue selectivity.” — Jayaram, Journal of Medicinal Chemistry
How RAD-140 Works
RAD-140 is a potent selective androgen receptor modulator (SARM) that binds strongly to androgen receptors (ARs) in skeletal muscle and bone. Its high selectivity means it mimics many of the anabolic effects of testosterone without stimulating androgen receptors in tissues like the prostate or scalp to the same degree.
Mechanism Highlights:
→ High binding affinity: RAD-140 has one of the strongest affinities of any SARM, driving pronounced anabolic activity.
→ Muscle hypertrophy: Stimulates protein synthesis and muscle fiber growth similar to anabolic steroids.
→ Neuroprotection: Preclinical studies suggest RAD-140 may protect neurons from degeneration through AR-mediated pathways (Jayaram, Journal of Medicinal Chemistry).
→ Suppression risk: Potency also means it strongly suppresses endogenous testosterone production by downregulating the hypothalamic-pituitary-gonadal (HPG) axis.
“RAD-140 exhibits tissue-selective anabolic activity, increasing lean mass while minimizing androgenic effects in non-muscle tissues.” — Jayaram, J. Med. Chem.
What Is Ostarine (MK-2866 / Enobosarm)?
Ostarine, also called MK-2866 or Enobosarm, is a non-steroidal selective androgen receptor modulator (SARM) originally developed by GTx, Inc. as a treatment for muscle wasting disorders, osteoporosis, and age-related sarcopenia. It binds to androgen receptors in muscle and bone, stimulating anabolic gene transcription that enhances protein synthesis, nitrogen retention, and lean body mass development.
Unlike testosterone or anabolic steroids, Ostarine does not aromatize into estrogen or convert into DHT, which reduces the risk of androgenic side effects such as hair loss or prostate enlargement. What makes Ostarine unique is its extensive clinical background. In Phase II and III trials, it was shown to increase lean body mass and improve physical performance in populations suffering from muscle-wasting conditions (Dalton, Journal of Cachexia, Sarcopenia and Muscle).
Despite these results, Ostarine has not been approved by the FDA for medical use and remains under investigation. In sports and bodybuilding, it is used off-label for its ability to help with muscle preservation during cutting, recomposition, and gradual lean mass gain. It is also listed as a prohibited substance by WADA (WADA Prohibited List), meaning athletes cannot use it in competition.
“Enobosarm treatment resulted in significant increases in lean body mass and improvements in physical performance in both cancer and elderly populations.” — Dalton, JCSM
How Ostarine Works (MK-2866 / Enobosarm)
Ostarine is a moderately potent SARM that also binds selectively to androgen receptors in muscle and bone, but with less anabolic intensity than RAD-140. Its mechanism is similar in principle but milder in effect, which is why it’s been tested in clinical settings for muscle wasting.
Mechanism Highlights:
→ Selective AR activation: Improves lean body mass and bone mineral density without converting to estrogen or DHT.
→ Muscle preservation: Particularly effective at preventing muscle breakdown during calorie restriction or illness.
→ Lower suppression: While it does reduce natural testosterone, suppression is typically less pronounced compared to RAD-140.
→ Clinical validation: Demonstrated efficacy in Phase II/III human trials for cachexia and sarcopenia (Dalton, Journal of Cachexia, Sarcopenia and Muscle).
“Enobosarm binds selectively to androgen receptors, increasing lean mass and improving muscle function in humans.” — Dalton, JCSM
Benefits of RAD-140 (Testolone)
RAD-140 is one of the most potent SARMs studied to date, designed to deliver strong anabolic activity while minimizing androgenic effects outside of muscle and bone. Its benefits are largely tied to its high binding affinity for androgen receptors (AR) and selective tissue targeting.
→ Lean Muscle Hypertrophy
RAD-140 binds tightly to AR in skeletal muscle, stimulating the transcription of anabolic genes that drive protein synthesis, myonuclear accretion, and hypertrophy. Preclinical work shows it rivals testosterone in increasing lean mass, but with greater selectivity (Jayaram, Journal of Medicinal Chemistry).
→ Strength and Performance Enhancement
By upregulating AR-dependent pathways in muscle fibers, RAD-140 increases contractile protein density and neuromuscular efficiency, leading to measurable improvements in strength output and recovery between sessions.
→ Fat Loss via Recomposition
The increase in muscle tissue translates to a higher resting metabolic rate (RMR). While RAD-140 does not directly stimulate lipolysis, it creates a metabolic environment that favors fat reduction while gaining lean mass.
→ Bone Mineral Density
RAD-140 also activates AR in osteoblasts, enhancing bone formation and potentially reducing fracture risk in high-load resistance training. This makes it relevant not just for hypertrophy but also for long-term skeletal support.
→ Neuroprotective Effects
Unique among SARMs, RAD-140 shows activity in the central nervous system, where AR activation is linked to reduced neurodegeneration. In rodent models, RAD-140 protected hippocampal neurons against amyloid toxicity, suggesting potential therapeutic crossover (Jayaram, J. Med. Chem.).
Benefits of Ostarine (MK-2866 / Enobosarm)
Ostarine was originally developed as a therapeutic agent for muscle wasting, osteoporosis, and age-related sarcopenia. Unlike RAD-140, it has been through Phase II and III human clinical trials, giving it one of the most documented safety and efficacy profiles among SARMs.
→ Preservation of Lean Body Mass
Ostarine binds selectively to androgen receptors in skeletal muscle, stimulating transcription of genes that support protein synthesis and nitrogen retention. Clinical studies confirm it can significantly increase lean body mass (LBM) in patients with muscle-wasting conditions (Dalton, Journal of Cachexia, Sarcopenia and Muscle).
→ Functional Performance Improvements
Beyond size, Ostarine has been shown to improve physical function metrics such as stair-climb speed and leg strength in human trials. These improvements highlight its application for athletic performance and rehabilitation as well as physique enhancement.
→ Muscle Protection in Caloric Deficit
Because it is less suppressive than stronger SARMs, Ostarine is often used during cutting phases. It helps preserve muscle tissue in catabolic states by reducing breakdown and supporting recovery, making it ideal for recomposition.
→ Bone and Joint Support
Androgen receptor activity extends to osteoblasts and connective tissue, where Ostarine promotes bone mineralization and joint repair. This dual effect makes it attractive not only for bodybuilders but also for clinical populations at risk for osteoporosis.
→ Favorable Tolerability Profile
Unlike RAD-140, Ostarine has been studied extensively in humans, with trials showing a predictable safety margin. While suppression of natural testosterone occurs, it is generally milder and easier to recover from, making Ostarine the most beginner-appropriate SARM available.
“Enobosarm treatment resulted in significant increases in lean body mass and improvements in physical performance in both cancer and elderly populations.” — Dalton, JCSM
RAD-140 vs. Ostarine: Benefit Comparison
Although both RAD-140 and Ostarine are SARMs that act through selective androgen receptor activation, their strength, applications, and suitability differ considerably.
→ Muscle Growth
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RAD-140: Exhibits a high anabolic/androgenic ratio in preclinical models, producing rapid hypertrophy comparable to testosterone. Its strong AR binding translates to faster and larger gains.
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Ostarine: Increases lean body mass modestly but reliably, with human trial data supporting improvements of ~1–3 kg LBM over 12 weeks. Growth is slower but clinically validated.
→ Strength and Performance
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RAD-140: Drives significant improvements in power output, contractile force, and recovery, making it favored for bulking or strength-focused phases.
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Ostarine: Provides measurable but moderate strength improvements, more aligned with improved endurance and functional capacity than raw maximal strength.
→ Fat Loss and Recomposition
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RAD-140: Indirectly supports fat loss by increasing basal metabolic rate via lean mass accrual; users often see recomposition when paired with caloric control.
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Ostarine: Especially effective at muscle preservation during caloric deficits, making it more suitable for cutting cycles.
→ Bone and Joint Health
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RAD-140: Stimulates AR activity in osteoblasts, improving bone density, but research here is mostly preclinical.
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Ostarine: Demonstrated improvements in bone density and joint recovery in clinical and preclinical studies, making it stronger in this category.
→ Safety and Beginner Suitability
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RAD-140: High potency equals high suppression; unsuitable for beginners due to more severe post-cycle recovery demands.
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Ostarine: Lower suppression and documented human tolerability make it widely considered the “entry-level” SARM.
👉 Summary:
RAD-140 is the powerful, rapid-growth option, best suited for advanced users willing to manage suppression and risk. Ostarine is the clinically validated, safer option, making it more appropriate for beginners or those prioritizing lean preservation and functional performance over sheer mass.
Side Effects of RAD-140 (Testolone)
→ Liver Toxicity
Clinical trials have shown significant rates of liver enzyme elevations in RAD-140 users. In a Phase 1 study for metastatic breast cancer, 59% of patients had elevated AST and 46% had elevated ALT (LoRusso, Clinical Breast Cancer). Case reports also describe severe cholestatic liver injury linked to recreational RAD-140 use (Vigano, Annals of Hepatology).
→ Cardiovascular Concerns
The FDA has warned that SARMs like RAD-140 may increase the risk of heart attack and stroke, in addition to causing liver toxicity (FDA Safety Communication).
→ Anti-Doping Status
RAD-140 is classified as a prohibited anabolic agent by the World Anti-Doping Agency (WADA) and is banned at all times in sport (WADA Prohibited List).
Side Effects of Ostarine (MK-2866 / Enobosarm)
→ Clinical Tolerability
In clinical trials, Ostarine demonstrated increases in lean mass and improved function with a generally good safety profile (Dalton, Journal of Cachexia, Sarcopenia and Muscle).
→ Liver Injury Cases
Despite its clinical background, Ostarine has been linked to cholestatic liver injury in case reports, showing it is not free of hepatotoxic risk (Bedi, ACG Case Reports Journal).
→ Cardiovascular Concerns
The FDA has issued consumer alerts that SARMs like Ostarine may cause liver toxicity and increased risks of heart attack and stroke (FDA Consumer Update).
→ Anti-Doping Status
Like RAD-140, Ostarine is explicitly banned for competitive athletes under the WADA Prohibited List (WADA Prohibited List).
RAD-140 vs. Ostarine: Side Effect Comparison
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Liver Toxicity: Both compounds are associated with hepatotoxicity. RAD-140 has stronger evidence from clinical trial enzyme elevations, while Ostarine has multiple published case reports.
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Cardiovascular Risk: Both fall under FDA warnings for potential increased risks of heart attack and stroke.
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Doping Status: Both are banned under WADA regulations, with strict penalties for athletes who test positive.
RAD-140 (Testolone) Cycling
RAD-140 is considered one of the strongest SARMs available, which means its cycles are typically kept shorter and more conservative.
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Typical Length: 6–8 weeks (non-medical use). Extending beyond this increases risks of liver toxicity and prolonged HPG axis suppression.
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Dosing Rationale: Preclinical data suggest RAD-140 has a high binding affinity to androgen receptors, producing powerful anabolic effects even at relatively low exposures. Because of this, escalating duration or dose does not proportionally increase benefits but does amplify side effects.
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Cycle Goals: Primarily used for bulking and recomposition, where the goal is rapid muscle gain and strength progression.
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Risks with Cycling: Even short cycles cause marked suppression of natural testosterone, requiring careful planning for recovery post-cycle.
Ostarine (MK-2866 / Enobosarm) Cycling
Ostarine is a milder SARM with the most human trial data, which supports its longer, steadier use compared to RAD-140.
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Typical Length: 8–12 weeks (non-medical use). Clinical studies ran for up to 12 weeks with documented increases in lean body mass.
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Dosing Rationale: Because Ostarine is less potent, slightly longer cycles are often used to maximize lean mass accrual and muscle preservation.
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Cycle Goals: Often chosen for cutting and recomposition, where the focus is maintaining or slightly building lean mass while in a calorie deficit.
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Risks with Cycling: While suppression is milder than RAD-140, longer cycles still result in measurable reductions in testosterone, meaning extended use is not risk-free.
RAD-140 (Testolone) Dosing
RAD-140 is one of the most potent SARMs, and even low exposures trigger measurable anabolic activity.
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Preclinical Data: Animal studies demonstrated significant increases in lean mass at 0.1–30 mg/kg, showing strong tissue selectivity at the lower end (Jayaram, Journal of Medicinal Chemistry).
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Human Data: In the first-in-human breast cancer trial, patients received 50–150 mg/day, which led to dose-limiting liver enzyme elevations in many participants (LoRusso, Clinical Breast Cancer).
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Non-Medical Reports: Bodybuilding communities often reference 10–20 mg daily for 6–8 weeks. However, these regimens are experimental and carry risks of hepatotoxicity and suppression.
Key Point: RAD-140’s potency means that low doses still cause strong anabolic and suppressive effects. Escalating dose increases side effect risk without proportional benefit.
Ostarine (MK-2866 / Enobosarm) Dosing
Ostarine has the most clinical trial evidence of any SARM, making its dosing profile better understood.
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Clinical Trials: In Phase II and III cachexia studies, subjects received 1–3 mg/day for up to 12 weeks, resulting in significant increases in lean body mass and improved function (Dalton, Journal of Cachexia, Sarcopenia and Muscle).
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Research Context: Other studies tested doses up to 9 mg/day, showing a clear dose-response effect on lean mass accrual.
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Non-Medical Reports: In bodybuilding circles, Ostarine is often reported at 10–25 mg daily for 8–12 weeks, though this exceeds studied doses and increases the risk of suppression and hepatotoxicity.
Key Point: Ostarine is less potent than RAD-140, but longer cycles and slightly higher exposures are often used outside clinical settings to compensate for its milder anabolic effect.
RAD-140 vs. Ostarine: Cycling & Dosing Comparison
Category
RAD-140 (Testolone)
Ostarine (MK-2866 / Enobosarm)
Potency
Very high — strong anabolic activity even at low doses
Moderate — clinically validated but weaker anabolic effect
Typical Cycle Length
6–8 weeks (non-medical reports); kept short due to high suppression and toxicity risk
8–12 weeks (non-medical reports); supported by clinical studies up to 12 weeks
Preclinical Range
Effective at 0.1–30 mg/kg in animal models (Jayaram, J. Med. Chem.)
Improved lean mass at 1–3 mg/day in humans (Dalton, JCSM)
Human Data
Phase 1 cancer trial used 50–150 mg/day, with high rates of liver enzyme elevation (LoRusso, Clin Breast Cancer)
Clinical trials tested 1–9 mg/day with good tolerability; higher lean mass response at ≥3 mg/day
Non-Medical Reports
10–20 mg/day for 6–8 weeks; high suppression risk
10–25 mg/day for 8–12 weeks; suppression occurs but usually milder than RAD-140
Cycle Goal
Bulking, rapid recomposition, advanced users
Cutting, muscle preservation, entry-level SARM option
Suppression Risk
Severe — requires structured recovery
Moderate — requires monitoring, but recovery is typically smoother
Post-Cycle Therapy: RAD-140 vs. Ostarine
RAD-140 (Testolone)
RAD-140 is one of the most suppressive SARMs, with clinical and anecdotal evidence showing marked shutdown of the HPG axis. Recovery without proper intervention is difficult.
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SERM Protocols: Stronger PCT is typically required, often involving Clomid (clomiphene) or Nolvadex (tamoxifen) for 4–6 weeks to restore LH and FSH.
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Supplement Support:
→ ZMT™ (6 capsules nightly) can help lower cortisol, improve sleep, and support natural testosterone.
→ DHEA (100 mg daily) provides a precursor hormone to stabilize androgen levels while natural production recovers. -
Risks if Ignored: Without adequate PCT, users risk extended hypogonadism, muscle loss, reduced libido, and fatigue.
Ostarine (MK-2866 / Enobosarm)
Ostarine is less suppressive than RAD-140, but testosterone suppression is still documented, especially at higher doses or longer cycles.
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SERM Protocols: Many users can recover with shorter or lighter PCT protocols (e.g., 3–4 weeks of Nolvadex), though monitoring is still necessary.
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Supplement Support:
→ ZMT™ assists recovery by optimizing sleep and reducing stress-related hormonal disruption.
→ DHEA (100 mg daily) helps provide hormonal balance during the transition off-cycle. -
Risks if Ignored: Without even a light PCT, suppressed testosterone can still result in lethargy, mood swings, and difficulty maintaining gains.
Head-to-Head: RAD-140 vs. Ostarine in PCT
Category
RAD-140
Ostarine
Suppression Severity
High — significant shutdown, strong PCT mandatory
Moderate — lighter SERM PCT usually sufficient
PCT Length/Intensity
4–6 weeks, aggressive (Clomid/Nolvadex standard)
3–4 weeks, lighter protocols often effective
Supplement Support
ZMT™ + DHEA critical for recovery, alongside SERMs
ZMT™ + DHEA valuable for smoother rebound and hormonal support
Risk if Skipped
Severe hypogonadism, muscle loss, prolonged recovery
Slower rebound, loss of progress, reduced well-being if completely ignored
Conclusion: RAD-140 vs. Ostarine — Which SARM Fits Beginners?
Both RAD-140 and Ostarine act through selective androgen receptor activation, but their applications differ significantly. RAD-140 (Testolone) delivers rapid, dramatic muscle and strength gains, yet carries a higher burden of suppression and liver risk, making it more suitable only for advanced users who understand aggressive PCT. Ostarine (MK-2866/Enobosarm) provides steadier lean mass accrual, stronger clinical validation, and milder suppression, which makes it the more realistic option for first-time or cautious users.
Regardless of potency, both compounds are not FDA-approved for bodybuilding, both carry documented hepatotoxicity risks, and both are banned in competitive sports by WADA. Any cycle requires careful consideration of risks and an appropriate post-cycle therapy protocol, where supplements like ZMT™ and DHEA can play supportive roles in restoring natural testosterone, sleep quality, and recovery.
Bottom line:
→ RAD-140 = fast results, higher risks
→ Ostarine = slower progress, safer entry point
⚠️ Disclaimer
This content is for educational purposes only and does not constitute medical advice. RAD-140 and Ostarine are not approved for human use outside research. Always consult a qualified healthcare professional before starting, stopping, or combining any performance-enhancing compound or supplement.
