Ostarine (enobosarm)

What it is

Ostarine is a small, orally active molecule without a steroid backbone. The company GTx developed it under the code GTx-024; the names MK-2866 and S-22 also circulate in the trade. The official name of the active substance is enobosarm. It was intended for muscle wasting in cancer and in old age, later also for urinary incontinence in women and for breast cancer.

Today the company Veru is continuing development with a new goal: people who lose weight with GLP-1 injections are meant to lose less muscle in the process. In the strength training scene, by contrast, ostarine has been known for years as supposedly the mildest SARM, an entry point for anyone wary of anabolic steroids.

How it is supposed to work

SARM stands for selective androgen receptor modulator. The idea: testosterone acts everywhere, on muscle and bone, but also on the prostate, skin and hair roots. A SARM is meant to switch the receptor fully on in muscle and bone and hardly at all in the other tissues. Because ostarine is not a steroid, it is not converted into DHT or estrogen.

In animals this looked convincing. Whether the selectivity in humans is as clean as the name promises is an open question. A critical review from 2025 argues that part of the apparent tissue selectivity simply comes from the absence of steroid metabolism, and that to this day no head-to-head comparison with classic androgens exists.

The studies that hold up

The most important paper appeared in 2011. 120 healthy men over 60 and postmenopausal women received ostarine at four dose levels or placebo for 12 weeks. At the highest level, lean mass increased by 1.3 kg more than on placebo, fat mass fell by about 0.6 kg, and body weight stayed the same. Stair-climbing performance, a genuine measure of function, improved significantly, and insulin resistance fell by 27.5 percent.

In cancer patients with incipient weight loss, the picture was repeated. In a phase 2 trial with 159 patients, 100 of whom could be evaluated for efficacy, lean mass rose from baseline at both dose levels, by a median of 1.5 and 1.0 kg, and practically not at all on placebo. In the two large phase 3 trials as well, more patients on ostarine lost no muscle mass: 41.9 versus 30.4 percent and 46.5 versus 37.9 percent.

That the androgen receptor is actually engaged in humans is shown by the hormone values and by a breast cancer study with 136 women. There, after 24 weeks, 32 and 29 percent had a clinical benefit, an indication of genuine tumor activity in receptor-positive breast cancer.

The new attempt: muscle protection during GLP-1 therapy

In the QUALITY trial, 168 people over 60 took semaglutide plus ostarine or placebo for 16 weeks. According to a company announcement, they lost 4.1 percent of their lean mass on placebo and 1.2 percent on ostarine, that is 71 percent less. A drop in stair-climbing performance of at least 10 percent occurred in 42.6 percent on placebo and 19.4 percent on ostarine.

This is an interesting finding, precisely because it measures function. So far, however, it comes only from a press release, no results are posted in the trial registry, and a peer-reviewed publication could not be found up to September 2026. The follow-up trial PLATEAU with 239 participants has body weight as its main goal and is due to be completed in 2027.

What users report

The strength training scene describes ostarine as a mild SARM, with a slow, rather dry gain in muscle and strength, often used in diet phases to preserve muscle. Just as often, users report blood tests with lowered testosterone and HDL. These are uncontrolled observations with products of unclear origin, but in direction they match the study data.

What the studies show

Dalton et al., Journal of Cachexia, Sarcopenia and Muscle 2011

Double-blind phase 2 with 120 healthy men over 60 and postmenopausal women, 12 weeks, four dose levels against placebo. The primary endpoint was lean mass by DXA: at the highest level plus 1.3 kg versus placebo, endpoint met. Stair-climbing performance and insulin resistance improved, HDL fell by up to 27 percent, and in 5 of 24 participants at the highest level the liver value ALT rose.

POWER 1 and POWER 2, results 2013

Two phase 3 trials with 321 and 330 patients with lung cancer at the start of chemotherapy. The main goals were to lose no lean mass and to improve stair-climbing performance by at least 10 percent. On mass, ostarine was ahead; on stair-climbing performance, it was not significantly ahead in either trial, in POWER 2 even numerically behind placebo at 19.5 versus 24.8 percent. The overall criteria were missed.

QUALITY, company announcement 2025

Phase 2b with 168 people over 60 on semaglutide, 16 weeks. Loss of lean mass minus 1.2 percent on ostarine versus minus 4.1 percent on placebo, and less often a marked drop in stair-climbing performance. Not published in peer-reviewed form; the safety data have also been available since May 2025 only as a company announcement.

Palmieri et al., Lancet Oncology 2024

Phase 2 with 136 women with advanced receptor-positive breast cancer, two dose levels. Clinical benefit after 24 weeks in 32 and 29 percent. Serious drug-related side effects in 8 and 16 percent, most commonly elevated liver values.

Where the data stop

Mass is not function. It is precisely at this point that ostarine failed in 2013: in both phase 3 trials, stair-climbing performance did not improve significantly. Later, a trial on urinary incontinence in women also missed its main goal. Measurement was almost always by DXA, and DXA also counts water and organ tissue. All efficacy studies concern older people, sick people or people on GLP-1 therapy who were not training specifically. For young people who train, the group that actually takes ostarine, there is no controlled study. The most positive new data from QUALITY have not been peer-reviewed. And a head-to-head comparison with classic androgens is missing; another study with the old steroid oxandrolone reached 3.0 kg of lean mass in 12 weeks, which at the very least does not prove the superiority of the SARM concept.

Status, approval and legal

Ostarine is not approved as a medicine anywhere in the world. The US Food and Drug Administration (FDA) makes clear that SARMs are not dietary supplements but unapproved drugs, even when they are marketed as supplements. In Germany, ostarine is listed by name in the annex to the Anti-Doping Act; manufacturing and trading for the purpose of doping in sports are prohibited, as are acquisition and possession of not insignificant quantities for this purpose. The World Anti-Doping Agency lists enobosarm in its 2026 list under S1.2, other anabolic agents, and prohibits it at all times, in training as in competition. Doping analysis detects ostarine in urine, hair and nails.

Safety

In the studies, HDL cholesterol fell in a dose-dependent manner by up to 27 percent, total testosterone fell in men, and LH and FSH fell in women. Rises in liver values occurred repeatedly, in the study in healthy people in 5 of 24 participants at the highest dose level; one dropped out because of this. A systematic review found ALT rises in an average of 7.1 percent of SARM-exposed people in studies. Outside studies, there are several case reports of severe cholestatic liver injury under ostarine, including a 24-year-old who needed albumin dialysis and whose bilirubin was only normal after 6 months. For SARMs, the FDA also names heart attack, stroke, infertility and miscarriage. Then there is the product risk: in a JAMA analysis, only 52 percent of products sold as SARMs contained a SARM at all, and only 41 percent contained the stated amount. Not for pregnant women, not with liver disease, not for competitive athletes.

Sources

• Dalton et al., Journal of Cachexia, Sarcopenia and Muscle 2011 — phase 2 in 120 healthy older people
• Dobs et al., Lancet Oncology 2013 — phase 2 in cancer-related muscle wasting
• Crawford et al., Current Oncology Reports 2016 — design of the POWER phase 3 trials
• ClinicalTrials.gov NCT01355484 — POWER 1, registry results
• Palmieri et al., Lancet Oncology 2024 — enobosarm in receptor-positive breast cancer
• ClinicalTrials.gov NCT06282458 — QUALITY, enobosarm during GLP-1 therapy
• Mertens et al., Zeitschrift für Gastroenterologie 2024 — liver injury after ostarine
• Vignali et al., Journal of Xenobiotics 2023 — systematic review on the safety of SARMs
• Van Wagoner et al., JAMA 2017 — contents of SARM products sold online
• FDA 2025 — warning about bodybuilding products containing SARMs