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Ostarine (Enobosarm, MK-2866): Uses, Side Effects & Dangers

Written by Ana Aleksic, MSc (Pharmacy) | Last updated:
Jonathan Ritter
Puya Yazdi
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology), Puya Yazdi, MD | Written by Ana Aleksic, MSc (Pharmacy) | Last updated:

Ostarine, also known as Enobosarm, is an unapproved drug. Despite the hype among some bodybuilders, Ostarine never passed clinical trials. Learn about its uses, side effects & dangers.

Disclaimer: We highly recommend against using this drug since its effects and safety are unknown. Therefore, we are presenting the available information from the clinical and scientific literature with the aim of helping readers understand the dangers involved.

What Is Ostarine?

Steroid or Not?

Ostarine (MK-2866), also known as enobosarm, is a selective androgen receptor modulator (SARM). SARMs like Ostarine stimulate steroid hormone receptors – androgen receptors – mimicking testosterone [1].

Scientists initially hypothesized that, unlike typical steroids, SARMs would be able to target specific tissues, such as the muscles or bones. In theory, this was supposed to limit their side effects [1].

Bodybuilders tend to take this unapproved theory as “proof” that they won’t have to deal with severe testosterone suppression and increased estrogens if they take SARMs. However, this is far from true.

Still, SARMs have surfaced as a popular new class of appearance and performance enhancers. People misperceive them as safer and “cleaner” due to a basic misunderstanding of the available scientific data, which is hyped in various bodybuilding blogs.

Many bodybuilders and athletes swear by Ostarine and claim it offers significant muscle mass and performance gains with few risks.

In reality, the “SARMs selectivity theory” might be completely false. It has never been proven, and SARMs like Ostarine never passed proper clinical trials. SARMs may turn out to be much more dangerous than previously thought.

To create further confusion, some researchers believe Ostarine might be safer than steroids, though they still have no data to back up their claims. Additionally, it has not yet been approved for human use anywhere in the world [2, 3].

Ostarine is being researched for its ability to improve serious muscle wasting diseases. Interestingly enough, it is one of the few SARMs still in clinical trials while most others proved to be toxic. It may be approved for clinical use if the trials attest to its favorable benefits/risks ratio [3].

However, its use in professional sports will almost certainly remain illegal. The World Anti-Doping Agency banned Ostarine under the S1 Anabolic Agent category of the Prohibited List, along with all other SARMs back in 2008.

Ostarine is a selective androgen receptor modulator (SARM) touted to boost muscle mass. Although banned in professional sports, it’s still being medically researched.



  • Increases lean muscle mass and strength
  • Shouldn’t cause hormonal imbalance
  • May improve bone strength
  • May lower cholesterol and improve insulin resistance


  • No clinical data to back up proponents’ claims
  • Not allowed for use in professional sports
  • Not approved for human use
  • Effects and efficacy are unknown
  • Short- and long-term side effects have yet to be investigated
  • Long-term risks and safety are unknown
  • Drug interactions remain unexplored
  • Products sold online are not intended for human use
  • These products usually contain a mix of dangerous, unapproved drugs

Mechanism of Action

It’s no secret that androgens increase skeletal muscle mass – that’s the main reason why men typically have more muscles than women. The activity of androgens in the body goes well beyond muscle-building, though. When sensors for androgens are non-selectively stimulated by drugs like steroids, it usually results in a long list of side effects.

SARMs are hypothesized to target muscles and increase anabolic processes while sparing the reproductive organs. Among many synthesized SARMs, the structure of Ostarine seems to offer some potential “advantages.”

In one study on mice, Ostarine was as effective as the most influential androgen, dihydrotestosterone, at restoring the size of pelvic floor muscles. Muscles in the pelvic floor are especially rich in androgen receptors (AR) [4].

Ostarine could also activate muscle stem cells (satellite cells), which help remodel and regenerate muscles. Surprisingly, it turned out that Ostarine also stimulates cells in the connective tissue, which are important for a successful recovery from muscle injuries [4].

Ostarine increased the activity of the insulin-like growth factor 1 gene (IGF-1), which boosts muscle growth and strength [4, 5].

However, we can’t make any conclusions based on data from cellular studies. Many drugs show some “promise” at the cellular level, only to be abandoned due to a lack of efficacy or risk of toxicity in later animal or human studies.

Although Ostarine is believed to target androgen receptors in muscles while sparing reproductive organs, no data backs this hypothesis up. Clinical studies are lacking.

Potential Uses of Ostarine

This section focuses on the clinical and scientific research that has been done on Ostarine so far.

1) Muscle Mass

Ostarine is being researched for reversing muscle loss in cancer patients, people with muscular dystrophy, and the aging population.

In a clinical trial of 120 healthy elderly people, enobosarm increased lean muscle mass and fitness. Only those who received 3 mg of enobosarm daily for 12 weeks saw benefits, while lower doses did not work as well. Importantly, its side effects were similar to placebo, with no steroid-like risks [6].

In another trial, enobosarm increased lean muscle mass in 159 cancer patients. Cancer patients who already started to lose muscle mass prior to the study gained about 2 – 3 lbs after taking enobosarm for about 4 months. The lower (1 mg/day) and the higher (3 mg/day) dose worked equally well [7].

Large clinical trials of enobosarm for preventing muscle loss in cancer patients are currently underway [8].

Ostarine may increase muscle mass and prevent muscle wasting in older people and cancer patients.

2) Bone Health

A decrease in bone-protective sex hormones with age increases the risk of poor bone health and osteoporosis. Increasing muscle mass can reduce the risk of bone fractures and speed up recovery, especially in older people [9, 10].

Since Ostarine mimics testosterone in the muscles and bones, it has the potential to prevent bone diseases. Animal studies confirmed that SARMs like Ostarine could increase both muscle and bone strength. No human study has confirmed the benefits yet [9, 3].

3) Heart Health

Ostarine may also be beneficial for people with heart disease, who often suffer from muscle wasting and weight loss [11].

In a clinical trial of 120 healthy elderly people, enobosarm (1-3 mg/day) lowered triglycerides, total cholesterol, and HDL cholesterol. Healthy lipid levels help prevent heart disease. However, lowering HDL is not typically considered beneficial [12].

4) Insulin Resistance

In the above-mentioned study, enobosarm improved insulin resistance and lowered blood glucose. The effects were similar to standard anti-diabetic drugs such as metformin. With more research, Ostarine may benefit people with diabetes or metabolic syndrome [12].

According to limited and preliminary data, Ostarine may lower blood lipids and sugar and improve bone health. Far more research is needed to confirm these findings.

Advanced Ostarine Research and Dosage

Ostarine has good bioavailability. Rats completely absorbed Ostarine, transported it throughout the body, and eliminated it via feces mostly as an unchanged drug [13].

Dosage in clinical studies: 1-3 mg/day, up to 4 months [14, 6].

Ostarine and other SARMs are also being researched for breast cancer, incontinence, low testosterone in older men, and muscular dystrophy [3, 15].

There is no safe or effective Ostarine dosage since it has not passed proper clinical trials or received approval from any regulatory body.

How Ostarine Is Used & Reviews

The official scientific research about Ostarine is highly limited. It was researched only in cancer patients and elderly people. None of the conducted studies to date have determined if Ostarine works and how dangerous it can be. Based on the existing data, its effects are unknown and its potential to cause harm is high.

Here we summarized the available reviews and feedback from online communities with the attempt to additionally warn people about the dangers of taking this unapproved drug.

Note: All the information below is anecdotal and based only on the personal experiences of Ostarine users. We can’t attest to its accuracy. We also can’t say to what extent these experiences are affected by other SARMs or different drugs/supplements taken at the same time. And lastly, we are far from sure about the actual content and quality of Ostarine in numerous products users bought online.


The reported “benefits” of Ostarine appear exaggerated and overhyped across various bodybuilding forums. For example, cancer patients gained 2 – 3 lbs after taking 3 mg/day for 4 months in clinical trials, while bodybuilders report gaining 6 – 8 lbs from approximately 25 mg/day during bulking cycles.

Users that consider Ostarine a “clean drug,” saying it helps them “increase pure muscle mass and burn fat with minimal side effects,” usually promote various SARMs on their blogs. Most completely ignore the fact that Ostarine is an unapproved drug with completely unknown effects on the body.

Those who do use steroids take Ostarine as a PCT or in between cycles of steroids to maintain muscle gains. Others who stick to SARMs still take regular PCT after Ostarine cycles.

Additionally, users claim the results gained with Ostarine are easier to maintain. All in all, bodybuilders claim it offers gains in all phases – from bulking cycles and cutting to body recomposition and muscle recovery.

Women seem to prefer Ostarine as well, since it supposedly doesn’t cause male-like hair growth, deepening of the voice, or other signs of high androgens.

Bodybuilders believe Ostarine will give them lasting pure muscle gains. Anecdotal information aside, we know neither how effective nor how safe Ostarine is.

Bulking Cycles

Users report:

  • Pure, clean, dry lean muscle gains (~6 – 8 lbs)
  • Milder gains than with steroids
  • Increased strength, stamina, and lifting endurance
  • No water retention
  • Great pumps during workouts
  • Increased energy and sense of well-being
  • No post-cycle crashes
  • No need for pre-cycle supplements
  • No need for liver-support supplements
  • No increases in aggression
  • Women report more obvious muscle gains and fat loss than men

Cutting & Fat Burning

Users report the following effects of Ostarine for cutting cycles:

  • The Big One: maintaining muscle mass
  • No strength loss
  • Fat loss

For all bodybuilders, the major downside to cutting cycles is muscle loss. That’s probably why so many love Ostarine – they say it prevents muscle breakdown but helps burn fat. This is probably the most talked-about aspect of Ostarine for bodybuilding.

Going back to the basics, it’s impossible to avoid being in caloric deficit if you want to burn fat and lose weight. However, your body will always interpret caloric deficit as a signal to break down all available energy stored in the body, including the proteins in your muscles.

Users report that Ostarine stalls this catabolic process, apparently stopping the body from using proteins as fuel but still helping to burn fats. Users say it helped them maintain muscles despite being on a low-calorie diet, keeping the anabolic processes going. They sometimes combine it with Cardarine to boost cardio endurance.

Ostarine is popular in bulking cycles but even more so in cutting cycles. Users say it stimulates fat burning and prevents muscle loss, but its actual effects and safety are unknown.

Body Recomposition

Used for recomping, Ostarine users reported:

  • Gaining muscles
  • Losing fat
  • Gaining some weight overall (depending on the diet and total number of calories consumed per day)


Users report:

  • Enhanced muscle recovery (at lower doses) after a week or two
  • Improved injury prevention
  • Better joint mobility
  • Healing of bone and tendon injuries

Coming off TRT

One user explained how taking only Ostarine allowed him to maintain muscle gains after coming off TRT (testosterone replacement therapy). He didn’t lose any weight and even reported increased strength. A rebound effect after TRT is common and muscle loss is one of the most feared downsides.

Bodybuilders report using Ostarine for recovery, body recomposition, and coming off TRT – but evidence supports none of these anecdotal uses.

Bodybuilding Dosage

The dosing data in clinical trials is limited and Ostarine has not been approved for human use. This drug has no “safe” or “effective” dosage and holds serious potential for harm.

Despite these dangers, the Ostarine-community seems to have established unofficial dosage guidelines based on “trial and error.” Bulking doses (3 – 8 weeks):

  • Most male bodybuilders use 25 mg/day
  • Women typically use up to 10 mg/day

The number of people who have suffered serious adverse effects is unknown.

The dose favored by most men for bulking is almost 10 times greater than the dose used in cancer patients, which is highly troubling. Megadosing an unapproved, poorly-researched chemical can be extremely dangerous. Nobody knows what the consequences may be.

Since Ostarine has a long half-life (24 h), most people take the daily amount all in one shot. Some still split the daily dose into 2-3 chunks.

The same users report cutting doses (4 – 8 weeks) or for injury prevention:

  • Men: 15 – 20 mg/day
  • Women: not mentioned, but the men dose is usually halved

Body recomposition (4-8 weeks)

  • The dose ranges between 12.5 – 25 mg/day

Some users reported taking Ostarine alone (especially for bulking cycles) while others combine it with other SARMs and slightly lower its dose.

Stacking several unapproved drugs (e.g. Ostarine and other SARMs) together is a dangerous practice. It increases the potential for harm since the effects, side effects, and interactions of these drugs have not been investigated. The long-term risks are even less certain.

Since Ostarine is an unapproved drug with unknown safety, it should not be taken at any dosage.

Cycles & Time Off

Ostarine is commonly cycled for up to 12 weeks. Users who stopped cycles at the 8-week mark experienced fewer side effects. While some users don’t experience any side effects even with cycles lasting over 12 weeks, the chances undoubtedly increase.

As a general rule among experienced bodybuilders, time off should equal the duration of the cycle. As an example, this would mean taking a 12-week break from ostarine after cycling it for 12 weeks and using PCT for 3-4 weeks. The time off is usually counted after PCT.

Some think that the strict time on=time off rule should be abandoned for more personalized strategies. Proponents of shortening the time off recommend ordering detailed lab tests after the cycle + PCT to assess overall hormone levels and any potential problems.

If everything is alright, some restart Ostarine cycles soon after (1-week breaks). However, if the labs point to health problems, delaying Ostarine or any other bodybuilding supplements is a must for all sensible users.

Ostarine is a dangerous drug. Speak with your doctor about safer alternatives.

We advise speaking to a doctor before taking any drug, especially an unscheduled drug with limited long-term safety data in humans.

Ostarine Side Effects and Cautions

Clinical Data

As mentioned, the World Anti-Doping Agency banned SARMs as they can increase physical fitness and stamina.

In limited studies, Ostarine was well tolerated and posed an insignificant risk of drug interactions with commonly used medication (antibiotics, antifungals, statins, and Celebrex). However, the existing studies did not focus on Ostarine’s side effect profile. Larger, better-designed studies are needed [16, 17].

Side effects from clinical studies include headaches and back pain. The long-term effects of Ostarine are unknown [18].

Beyond the Clinical Data

The truth is that so little information is out there about the side effects of Ostarine. Being a banned substance with clinical research done only in cancer patients, the available scientific literature is extremely limited.

To get a better idea of what side effects people report, we decided to take a closer look at user reviews. Ostarine has been reported to raise blood estradiol and suppress testosterone.

User Reports

The side effects users reported include the following:

  • Decreased sense of wellness after the first 9 days
  • Testicle Pain
  • Joint pain
  • Reduced strength
  • Testosterone suppression 4 weeks after stopping it (didn’t do PCT)
  • Erection difficulties
  • Disrupted sleep
  • Low energy
  • Depression
  • Increased estrogens
  • Menstrual cycle imbalances
  • Acne and skin changes
  • Headaches
  • Increased hunger
  • Nausea
  • Baldness
  • Although uncommon, some users do report gyno flare-ups

These reports imply that Ostarine might cause the same side effects as testosterone. But unlike testosterone, Ostarine didn’t pass proper clinical trials. Yet people undermine the risks of taking it.


With the goal of minimizing estrogens and the detrimental effects on testosterone short- and long-term, most bodybuilders start PCT right after an Ostarine cycle. PCT (Post-Cycle Therapy) consists of anti-estrogens (like Clomid, Nolvadex), which are usually taken for 3-4 weeks.

Users who are more sensitive to estrogen or those using high doses (35 mg/day) combine it with aromatase inhibitors (Anastrozole/Arimidex), which prevent the conversion of androgens to estrogen.

The opinions are mixed when it comes to PCT. Some don’t feel the need for it after Ostarine, especially with lower doses (approximately 10 mg/day). Others consider both anti-estrogens and aromatase inhibitors a must after Ostarine cycles. Users who check their labs and see increased estrogens start aromatase inhibitors during a cycle.


Ostarine is a dangerous drug that bodybuilders use, despite the lack of safety and efficacy data from clinical research. Large studies are needed to determine how Ostarine works and whether it has any potential for use in humans. It is currently not approved anywhere in the world. Bodybuilders often claim that Ostarine is “safer” because it’s a SARM. However, the whole theory about SARMs being “selective” and “causing fewer side effects” has never been proven. It may turn out that SARMs like Ostarine are as dangerous–or even more dangerous–than steroids or other illegal bodybuilding substances. In conclusion, we highly recommend against using this drug. Speak with your physician about lifestyle changes or approved therapies to achieve your goals instead.

About the Author

Ana Aleksic

Ana Aleksic

MSc (Pharmacy)
Ana received her MS in Pharmacy from the University of Belgrade.
Ana has many years of experience in clinical research and health advising. She loves communicating science and empowering people to achieve their optimal health. Ana spent years working with patients who suffer from various mental health issues and chronic health problems. She is a strong advocate of integrating scientific knowledge and holistic medicine.


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