Ostarine (enobosarm, MK-2866) is the most studied SARM. In clinical trials, 1–3 mg a day added about 1–1.5 kg of lean mass over three to four months. It is often called the mildest SARM, but it still suppresses testosterone and lowers HDL cholesterol.
- Tested in more than 1,500 people across clinical trials; never approved.
- Effects are modest; side effects increase with dose.
- A common cause of positive drug tests, sometimes from contaminated supplements.
Ostarine is usually the first SARM people hear about, and it is often described as the “entry-level” option. It does have more human data behind it than any other SARM. Here is what that research shows, and what it does not.
What ostarine is
Ostarine, developed by GTx Inc. under the name enobosarm, is a non-steroidal selective androgen receptor modulator. It activates the androgen receptor mainly in muscle and bone, with less effect on the prostate and skin. It was developed to prevent muscle wasting in cancer and other conditions.
What the trials showed
- Healthy older adults (2011): 3 mg a day for 12 weeks increased lean body mass by about 1.3 kg compared with placebo and improved stair-climbing speed and power.
- Cancer patients (2013, Lancet Oncology): 1 mg and 3 mg a day for up to about four months increased lean body mass by roughly 1–1.5 kg from baseline.
- Phase 3 lung cancer trials (POWER): did not meet their combined goals for muscle and physical function, and approval was not sought.
- Later trials explored it for breast cancer and stress urinary incontinence in women.
Side effects
- Suppression: testosterone, SHBG, LH and FSH fall, in proportion to dose.
- HDL cholesterol falls; in some trials, triglycerides fell too.
- Liver: mild liver enzyme rises in some trial participants; case reports of liver injury with SARMs generally.
- Low-testosterone symptoms during or after use: low libido, fatigue, low mood.
- Headache and nausea in some users.
Because ostarine does not convert to estrogen, water retention and gynecomastia are not typical, though they have been reported, possibly from hormonal shifts or mislabelled products.
Is it really “mild”?
At trial doses, ostarine is milder than anabolic steroids: smaller gains, fewer androgenic effects, and suppression that usually recovers after stopping. The “mild” label becomes less accurate at higher amounts, over longer periods, or when combined with other compounds. See how long trials ran and recovery after SARMs.
Drug testing
Ostarine is one of the most frequently detected substances in anti-doping tests worldwide. Some positive tests have been traced to contaminated supplements. It is banned at all times by the World Anti-Doping Agency. Tested Canadian athletes should be very careful with any supplement that is not third-party certified.
Product quality
A 2017 JAMA study found that many products sold as SARMs online contained no SARM, a different one, or other drugs. With ostarine, underdosing is common; with others, substitution. Choose products with batch-specific lab results. See liquid vs capsule SARMs.
Bloodwork
- Total testosterone, LH, FSH and SHBG
- Lipid panel
- Liver enzymes
- Complete blood count
Test before starting, during and after, to confirm recovery. Your doctor can order these, or you can pay privately at LifeLabs or Dynacare. See our bloodwork guide.
Ostarine in women
Ostarine was studied in postmenopausal women, including trials for stress urinary incontinence and breast cancer, which is partly why it is promoted to women as a gentle option. At trial doses, masculinising effects were uncommon, but SARMs still act on the androgen receptor. Voice changes, acne, excess hair growth and menstrual changes are possible, especially at higher amounts. Any hoarseness or voice change is a signal to stop, since voice deepening can be permanent.
Realistic expectations
With good training and food, trial data suggests a modest lean-mass increase over a few months. Much of what people credit to ostarine in their first months comes from consistent training and protein. For comparison with other SARMs, see SARMs 101 and SARMs vs steroids.
Products and legal status
We carry Zionova Ostarine (MK-2866), with batch lab results on the product page. SARMs are not authorised by Health Canada for human use and are sold for research.
Frequently asked questions
How much muscle does ostarine build?
In clinical trials, about 1–1.5 kg of lean mass over three to four months at 1–3 mg a day.
Does ostarine suppress testosterone?
Yes. Trials showed dose-dependent drops in testosterone, LH and FSH, which usually recover after stopping.
Is ostarine legal in Canada?
It is not authorised for human use by Health Canada and is sold for research. It is banned in sport.
Photo: rawpixel (public domain).
General information about a research compound, not medical advice. Ostarine is not approved for human use in Canada.
Related on Steroids Canada
