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An Ostarine cycle is a course of MK-2866 (enobosarm), a selective androgen receptor modulator (SARM) taken by mouth to build or keep lean muscle. In bodybuilding circles, a typical cycle is reported as 6 to 8 weeks at 10 to 25mg a day. Ostarine is the most studied SARM in people, but its clinical trials used 1 to 3mg a day, and it has never been approved as a medicine anywhere.
Ostarine is often sold as a safe alternative to steroids with no sexual side effects. In reality it lowers HDL cholesterol, suppresses natural testosterone at the doses users take and has been linked to liver injury in case reports.
This guide covers what the studies show, commonly reported doses and timelines, realistic results, side effects, PCT, women and how Ostarine compares with other SARMs. Nothing here is a recommendation to use it.
Key Takeaways
- Ostarine (MK-2866, enobosarm) is the most studied SARM in humans. In a 12-week trial in older adults, 3mg a day added about a kilogram of lean mass compared with placebo.
- Commonly reported cycles run 6-8 weeks at 10-25mg a day, several times the 1-3mg doses tested in trials. These figures come from user practice, not research.
- Ostarine is not free of sexual side effects: at bodybuilding doses, users commonly report suppressed testosterone and low libido.
- Trials found lower HDL cholesterol, and published case reports link SARM products, Ostarine included, to liver injury. The FDA has warned that SARMs carry serious risks.
- Women in trials took 1-3mg a day. At the higher doses women report using, androgenic effects such as a deeper voice are possible, and no safe dose has been established.
What Is Ostarine (MK-2866)?
Ostarine is a non-steroidal SARM developed by the US company GTx, Inc., which gave it the code GTx-024. Its drug name is enobosarm; MK-2866 is the code most sellers use. It was designed to treat muscle wasting, especially in people with cancer, by giving testosterone-like effects in muscle and bone with less effect on tissues such as the prostate.
It went further in clinical development than any other SARM, but it has never been approved. The rights have since changed hands, and enobosarm is still being studied for other uses, including breast cancer. What is sold online as Ostarine is marketed as a research chemical, “not for human consumption”.
How Ostarine Works
Ostarine binds to the androgen receptor, the same receptor testosterone acts on. In muscle and bone, that switches on the signals that support protein synthesis and bone strength. “Selective” means it has a different balance of effects between tissues than testosterone does. It does not mean it acts on muscle alone.
Because it is not a steroid, Ostarine is not converted to estrogen by the aromatase enzyme or to DHT by 5-alpha reductase. That removes some steroid side effects, such as estrogen-driven water retention, but Ostarine still acts on androgen receptors in the brain areas that control your own testosterone production. That is where suppression comes from.
What the Research Actually Shows
Unlike most SARMs, Ostarine has been tested in people. The trials were in older adults and patients with muscle wasting, at much lower doses than bodybuilders take:
| Research | Subjects | Dose | What It Found |
|---|---|---|---|
| 12-week phase II trial (2011) | 120 healthy older men and postmenopausal women (human data) | 0.1-3mg/day | 3mg added about a kilogram of lean mass compared with placebo and improved stair-climb power; HDL cholesterol fell |
| Phase II trial in cancer patients (2013) | 159 men and women with cancer (human data) | 1 or 3mg/day for up to about 16 weeks | Lean body mass rose from baseline at both doses |
| Two phase III trials (POWER1 and POWER2) | Patients with non-small cell lung cancer starting chemotherapy (human data) | 3mg/day | Did not meet their main goals for lean mass and physical function together; the drug was not approved |
| Trials in young, healthy lifters | None published | None | No human data on bodybuilding doses, cycle length, fat loss or recovery |
So Ostarine does build some lean mass in people, but modestly and at 1-3mg a day. The trials say nothing about 25mg a day in young lifters, or about products bought online.
Ostarine Cycle: What Users Commonly Report
The doses and timelines below come from user reports, with trial doses for comparison. They are not recommendations.
| Reported Use | Daily Dose | Cycle Length |
|---|---|---|
| Doses used in human trials | 1-3mg | 12-16 weeks |
| Lower-dose cycles | 5-10mg | 6-8 weeks |
| Most commonly reported range | 10-25mg | 8 weeks |
| Higher reported doses | 25-30mg | 8-12 weeks |
Ostarine is taken by mouth once a day, usually as a liquid or capsules, as in the trials. Users report that side effects, suppression above all, grow with dose and cycle length, which is why most reported cycles stop at 8 weeks. Time off is commonly at least as long as the cycle plus any PCT.
Bulking, Recomp and Cutting
Ostarine is used in all three phases, at similar reported doses. The diet decides the outcome: a calorie surplus to gain, maintenance to recompose, or a deficit to cut, where the aim is to hold on to muscle and strength. Advice to lower the dose for cutting or raise it for bulking has no data behind it; a higher dose mainly adds side effects.
A Commonly Reported 8-Week Timeline
| Period | What Users Commonly Report |
|---|---|
| Weeks 1-2 | Little visible change; some report better recovery between sessions, others notice nothing |
| Weeks 3-5 | Strength holds or creeps up and muscles may look fuller; on a cut, strength loss is often smaller than expected |
| Weeks 6-8 | Gains slow down; suppression signs such as lower libido or flat mood appear in some users, mostly at higher doses |
| After stopping | Some fullness and scale weight drop off; low-testosterone symptoms can appear until natural production recovers |
Ostarine Results: What Is Realistic
The only controlled numbers come from the trials: about a kilogram of extra lean mass over 12 weeks at 3mg a day, in healthy older adults. Claims of 6 to 9lb of “pure muscle” from one 8-week cycle have no study behind them.
Users at bodybuilding doses commonly report a few pounds of scale weight on a gaining cycle, or a cut where strength holds up better than usual. Not all scale weight is new muscle, and results vary widely. What drives the difference:
- Diet: Gaining needs enough calories and protein, and a cut needs enough protein to protect muscle. Ostarine replaces neither.
- Training: Without hard, consistent training, very little happens.
- Product quality: If the product is underdosed or contains something else, the result reflects that, not Ostarine.
Ostarine Side Effects
Ostarine is often called the “safest SARM”. It is the best studied, and at 1-3mg a day it was generally well tolerated in trials. That does not make it safe at several times those doses.
Testosterone Suppression
Ostarine does affect sexual health. The brain senses androgen-receptor activity and turns down LH and FSH, the hormones that tell the testes to make testosterone and sperm. In trials, Ostarine lowered SHBG, the protein that carries testosterone in the blood, in a dose-dependent way. At bodybuilding doses, user blood work commonly shows lower total testosterone, LH and FSH by the end of a cycle.
Suppression usually shows up as lower libido, weaker erections, fatigue and flat mood, often from mid-cycle onward. In most user reports it is milder than with RAD-140 or LGD-4033, but it grows with dose and cycle length. It also reduces sperm production, which matters for anyone trying to have children.
Cholesterol and Heart Risk
Lower HDL (“good”) cholesterol was seen in the Ostarine trials and is a consistent finding across SARMs tested in people. Users commonly report a bigger drop at bodybuilding doses, and no one has studied what repeated cycles do to heart risk. In 2017 the FDA warned consumers that SARMs have been linked to serious safety concerns, including liver toxicity and a higher risk of heart attack and stroke.
Liver Injury
Ostarine is not a 17-alpha-alkylated steroid, so it is often called “liver-safe”. Like most oral drugs, it is still processed by the liver. Published case reports describe drug-induced liver injury, sometimes with jaundice, in people using SARM products, including Ostarine. Yellowing skin or eyes, dark urine or persistent itching are reasons to stop and see a doctor.
Other Reported Effects
Some users report headaches, nausea or stomach upset, mostly early in a cycle. Long-term effects at bodybuilding doses have never been studied.
Blood Work Before and After
The only way to know how suppressed you are, and whether you have recovered, is blood work. The markers usually checked are:
- Hormones: total and free testosterone, LH, FSH, SHBG and estradiol
- Lipids: HDL, LDL and triglycerides
- Liver: ALT, AST and bilirubin
- General: a full blood count
A baseline test before a cycle gives a reference point. A test 4 to 6 weeks after stopping shows whether levels are coming back.
PCT After an Ostarine Cycle
Post-cycle therapy (PCT) means taking a drug such as tamoxifen (Nolvadex) or clomiphene (Clomid) after a cycle to push the brain to restart LH and FSH output. Both are prescription drugs, neither is approved for this use, and no trial has tested PCT after a SARM cycle.
Many users skip PCT after low-dose Ostarine cycles and wait for natural recovery; others run a short course, especially after higher doses or longer cycles. The most commonly reported approaches:
| Approach | Commonly Reported Protocol | Duration |
|---|---|---|
| Tamoxifen (Nolvadex) | 10-20mg/day | 3-4 weeks |
| Clomiphene (Clomid) | 25mg/day, sometimes up to 50mg | 3-4 weeks |
| No PCT | Wait for natural recovery, checked with blood work | Weeks to a few months |
Without blood work, none of these choices is an informed one. Clomiphene can cause mood changes and visual disturbances, and vision changes are a reason to stop it. A doctor can test hormones and, where appropriate, treat persistent low testosterone properly. Our guide to PCT for SARMs covers timing and what the evidence shows.
Ostarine for Women
Ostarine is often described as the best SARM for women because it is mild. It is one of the few SARMs that has been tested in women, but those were postmenopausal women in clinical trials, taking 1-3mg a day under medical monitoring.
Women who use it for physique goals commonly report 5-10mg a day, above anything tested. Ostarine is still an androgen-receptor activator, and androgens can masculinize women. Commonly reported signs:
- A deeper or hoarse voice
- Facial and body hair growth
- Acne and oily skin
- Clitoral enlargement
- Irregular or missed periods
Voice changes and clitoral enlargement may not reverse after stopping. Periods that do not return to normal after a cycle need a doctor’s assessment. Our guide to the best SARMs for women compares the options and their risks.
Ostarine Stacks
Ostarine is the SARM most often stacked, because it is seen as the mildest. Common pairings:
- Cardarine (GW501516): Used on cuts. It is not a SARM but a PPAR-delta agonist, and its development was stopped after it caused cancers in long-term rodent studies. Our Ostarine and Cardarine stack guide covers that combination.
- MK-677 (ibutamoren): A growth hormone secretagogue, not a SARM; commonly reported to raise appetite and water retention, and it can raise blood sugar.
- A second SARM (LGD-4033 or RAD-140): Adds to the suppression and to the drop in HDL cholesterol.
Bigger stacks, such as Ostarine with S4, Cardarine and SR9009, multiply the unknowns: none has been studied, and if something goes wrong there is no way to tell which compound caused it.
Ostarine vs RAD-140, LGD-4033 and S4
Ostarine is usually the first SARM people try. This is how it compares with the common next steps:
| Compound | Human Data | Reported Strength | Notable Concerns |
|---|---|---|---|
| Ostarine (MK-2866) | Phase II and III trials at 1-3mg/day | Mildest of the four; least suppressive in user reports | Lower HDL; liver injury case reports |
| LGD-4033 (ligandrol) | Short trials in healthy men at up to 1mg/day | Stronger for size and strength | Lowered testosterone, SHBG and HDL within 21 days at 1mg/day; liver injury case reports |
| RAD-140 (testolone) | A phase 1 trial in women with advanced breast cancer only | Stronger and more androgenic | Liver injury case reports; hair loss and aggression commonly reported |
| S4 (andarine) | No published efficacy trials in people | Used on cuts for a harder, drier look | Vision changes such as a yellow tint, commonly reported |
Stronger SARMs bring more reported muscle and more side effects, not one without the other. The common advice to start with Ostarine reflects that it has the most human data, not that it is risk-free.
Legal Status, Drug Testing and Product Quality
Ostarine is not approved for human use in the US or anywhere else. In the US it cannot legally be sold as a dietary supplement, which is why vendors label it “for research purposes only”. Rules differ by country; see are SARMs legal for a country list.
In sport, Ostarine is banned by WADA at all times, in and out of competition, and it is one of the SARMs most often behind positive tests. Anti-doping rules hold athletes responsible for whatever is in their sample, even when a contaminated supplement is to blame.
Product quality is the other unknown. In a 2017 study published in JAMA, researchers bought 44 products sold online as SARMs and analyzed them. Only about half contained a SARM at all, and many contained other unapproved drugs or amounts that did not match the label. Every reported dose assumes the product holds what it claims.
If you buy for research, choose a supplier that publishes batch-matched, third-party certificates of analysis (COAs). A COA confirms identity and purity, not safety.
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The Bottom Line
Ostarine is the best-studied SARM, but the evidence shows modest muscle gains at 1-3mg a day, not a side-effect-free shortcut at the doses bodybuilders take.
- Commonly reported cycles run 6-8 weeks at 10-25mg a day. These doses come from users, not trials, and product quality is unreliable.
- Ostarine lowers HDL cholesterol and, at the doses users take, suppresses natural testosterone. Blood work before and after a cycle is the only way to know where you stand.
- Women in trials took 1-3mg a day; the higher doses women report carry a risk of androgenic effects that may be permanent.
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