Ostarine, also called enobosarm, GTx-024 or MK-2866, is an oral, nonsteroidal selective androgen receptor modulator (SARM). SARMs were designed to activate the androgen receptor in muscle and bone while having less effect on tissues such as the prostate, skin and hair. GTx, Inc. developed ostarine as a treatment for muscle wasting in cancer and aging, and later tested it in breast cancer and urinary incontinence.
Ostarine is not approved as a medicine anywhere in the US. Two phase 3 trials in lung cancer patients did not show a consistent benefit on physical function, and the FDA states that SARMs cannot legally be sold as dietary supplements. Despite this, ostarine is widely sold online, is frequently found in doping tests, and has been linked to case reports of liver injury.
This page summarizes what the clinical trials, trial registries and regulators report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Drug class | Nonsteroidal selective androgen receptor modulator (SARM) |
| Other names | Enobosarm (generic name), GTx-024, MK-2866 |
| Developer | GTx, Inc.; recent trials sponsored by Veru Inc. |
| Route | Oral, once daily in trials |
| Doses studied in trials | 0.1–3 mg/day (muscle trials); 9 and 18 mg/day (breast cancer trial) |
| Half-life | No peer-reviewed human value located; in rats 0.6 h (males) and 16.4 h (females) |
| Converts to DHT or estrogen | No; nonsteroidal SARMs are not aromatized or 5α-reduced |
| US status | Not FDA-approved for any use; cannot legally be marketed as a dietary supplement |
| WADA status | Prohibited at all times (class S1.2, other anabolic agents) |
What it is
Ostarine’s chemical name is (2S)-3-(4-cyanophenoxy)-N-[4-cyano-3-(trifluoromethyl)phenyl]-2-hydroxy-2-methylpropanamide: formula C₁₉H₁₄F₃N₃O₃, molecular weight 389.3 g/mol (PubChem). It has no steroid ring structure. “Enobosarm” is its official US generic (USAN) name (Dalton et al., J Cachexia Sarcopenia Muscle 2011).
Like testosterone, ostarine binds and activates the androgen receptor. The difference is in how the receptor behaves afterward. A 2023 review describes how a steroid such as DHT pushes the receptor into a full-agonist shape in the prostate, while a SARM like enobosarm produces a partial-agonist shape (Bhasin et al., J Gerontol A 2023). Because it is not a steroid, ostarine is not converted to DHT by 5α-reductase or to estradiol by aromatase.
The case for “tissue selectivity” comes mainly from animal work. In rats, ostarine increased muscle mass and bone density with limited effect on the prostate and seminal vesicles. At doses that increased muscle in intact male rats, prostate size actually fell (Dalton et al., 2011). These are animal results; they do not show that the same separation holds in people at any dose.
How it works in the body
Human pharmacokinetic data are thin. A 2024 study of ostarine metabolism noted that its pharmacokinetics “remain partially understood in humans” (Taoussi et al., Int J Mol Sci 2024). We could not find a peer-reviewed human half-life in an accessible primary source, so we do not give one. Trials used once-daily dosing.
What is documented:
| Finding | Detail | Source |
|---|---|---|
| Main metabolite in humans | Ostarine glucuronide | Coss et al., 2016 |
| Other human metabolic routes | Glucuronidation, ether cleavage, hydroxylation and sulfation (human liver cells and athletes’ urine) | Taoussi et al., 2024 |
| Absorption (rats) | Rapid and complete, with high oral bioavailability | Kim et al., Xenobiotica 2013 |
| Half-life (rats) | 0.6 h in males vs 16.4 h in females | Kim et al., 2013 |
| Excretion (rats) | About 70% in feces, mostly unchanged; 21–25% in urine within 48 h | Kim et al., 2013 |
The large male-female difference in rats is a reminder that animal half-lives cannot simply be carried over to people.
Drug interactions
A series of phase I interaction studies tested ostarine with several drugs (Coss et al., Invest New Drugs 2016):
| Co-administered drug | What it tests | Effect |
|---|---|---|
| Itraconazole | CYP3A4 inhibition | No effect on ostarine levels |
| Rifampin | CYP3A4 induction | Ostarine peak level −23%, total exposure (AUC) −43% |
| Probenecid | UGT (glucuronidation) inhibition | Ostarine AUC +50%; ostarine glucuronide AUC +112% |
| Celecoxib | CYP2C9 substrate | No significant change in celecoxib |
| Rosuvastatin | BCRP transporter substrate | No significant change in rosuvastatin |
Regulatory status
- FDA. No SARM is FDA-approved. FDA states that SARMs cannot currently be legally marketed in the US as either a dietary supplement or a drug. It warns that products are often labeled as being for research use only and are sold without warning labels (FDA consumer update).
- NIH LiverTox. Several SARMs were tested for cancer cachexia, osteoporosis and age-related muscle loss, “but none were approved for human use” (LiverTox: SARMs).
- Sport. The 2026 WADA Prohibited List names enobosarm (ostarine) as an example under S1.2, other anabolic agents. All of class S1 is banned in and out of competition, and its substances are “non-specified” (WADA 2026 List, official Austrian publication). A 2024 paper described ostarine as the most reported S1.2 substance in doping tests (Taoussi et al., 2024).
What the research shows
All controlled human data come from company-sponsored trials at 0.1–18 mg/day. No trial has tested ostarine in young, healthy, resistance-trained people, or at the doses and combinations used outside medicine.
Healthy older adults (phase 2)
The first efficacy trial randomized 120 healthy, non-obese adults (60 men over 60 and 60 postmenopausal women) to placebo or ostarine 0.1, 0.3, 1 or 3 mg/day for 86 days. Participants were told not to change diet or exercise (Dalton et al., 2011):
| Mean change from baseline (p vs placebo) | Placebo | 1 mg/day | 3 mg/day |
|---|---|---|---|
| Lean body mass (DXA) | −0.07 kg | +0.59 kg (p = 0.055) | +1.25 kg (p under 0.001) |
| Fat mass | +0.30 kg | −0.26 kg (p = 0.085) | −0.32 kg (p = 0.049) |
| Body weight | −0.1 kg | +0.3 kg (p = 0.55) | +0.9 kg (p = 0.18) |
| Stair climb power | +20.4 W | +10.7 W (p = 0.52) | +49.7 W (p = 0.049) |
| Insulin resistance (HOMA-IR) | +2.6% | −9.3% | −27.5% (p = 0.013) |
Relative to placebo, the 3 mg dose added about 1.3 kg of lean mass, which is modest. Total weight did not change, so the shift came partly from fat loss. The trial was run by GTx employees and lasted only 12 weeks.
Cancer-related muscle wasting (phase 2)
A double-blind trial randomized 159 patients with cancer and at least 2% recent weight loss to placebo, 1 mg or 3 mg daily for up to 113 days (Dobs et al., Lancet Oncol 2013). Only 100 patients had usable before-and-after scans:
| Measure | Placebo (n = 34) | 1 mg (n = 32) | 3 mg (n = 34) |
|---|---|---|---|
| Median change in lean body mass | +0.02 kg | +1.5 kg | +1.0 kg |
| p vs own baseline | 0.88 | 0.0012 | 0.046 |
| p vs placebo | — | 0.066 | 0.041 |
| Mean change in stair climb power | +2.2 W | +14.3 W | +16.8 W |
Only the 3 mg dose beat placebo on lean mass, and only narrowly. Serious adverse events were similar across groups, and none were judged drug-related.
Phase 3 POWER trials: a failed program
Two identical phase 3 trials (POWER 1 and 2) gave 3 mg/day or placebo for 147 days to patients starting chemotherapy for non-small-cell lung cancer. Success needed two co-primary “responder” endpoints at day 84 (Crawford et al., Curr Oncol Rep 2016):
- stair climb power improved by at least 10%;
- no loss of lean body mass.
Results posted to ClinicalTrials.gov (NCT01355484; NCT01355497):
| Trial | n analyzed | Physical function responders, ostarine vs placebo | Lean mass responders, ostarine vs placebo |
|---|---|---|---|
| POWER 1 (taxane chemo) | 321 | 29.4% vs 24.2% | 41.9% vs 30.4% |
| POWER 2 (non-taxane chemo) | 320 | 19.5% vs 24.8% | 46.5% vs 37.9% |
More patients on ostarine kept their lean mass, but physical function did not improve consistently. In POWER 2, fewer patients on ostarine than on placebo met the physical-function target. The registry postings do not report p values. A 2023 review summarized that in phase 3, enobosarm “failed to show consistent improvements in physical function” (Bhasin et al., 2023). Ostarine was never approved for muscle wasting.
Breast cancer
Activating the androgen receptor can slow some estrogen-receptor-positive breast cancers. In an open-label phase 2 trial, 136 postmenopausal women with pretreated ER-positive, HER2-negative advanced breast cancer took 9 mg or 18 mg daily (Palmieri et al., Lancet Oncol 2024):
| Outcome | 9 mg/day | 18 mg/day |
|---|---|---|
| Clinical benefit at 24 weeks (AR-positive patients) | 32% (95% CI 20–47) | 29% (95% CI 17–43) |
| Grade 3–4 drug-related adverse events | 8% | 16% |
| Grade 3–4 raised liver enzymes | 4% | 3% |
| Grade 3–4 high blood calcium | 3% | 3% |
The higher dose was no more effective and caused more severe side effects. There was no placebo or standard-therapy comparison.
Other uses tested
- Stress urinary incontinence. In a phase 2 trial of 491 postmenopausal women, a 50% or greater drop in leak episodes at 12 weeks occurred in 94 of 163 (1 mg), 96 of 163 (3 mg) and 87 of 165 (placebo) women. The registry posting gives no statistical comparison (NCT03241342).
- Muscle preservation during GLP-1 weight loss. Veru Inc. completed a phase 2 trial of 3 or 6 mg/day with semaglutide (NCT06282458). As of this writing, no results are posted on the registry and we found no peer-reviewed publication (ClinicalTrials.gov).
Side effects and risks
Liver injury
In the healthy older-adult trial, raised ALT (a liver enzyme) was reported as an adverse event in 5 of 24 people (20.8%) on 3 mg versus none on placebo. One participant stopped because ALT reached 4.2 times the upper limit of normal; it returned to normal after stopping (Dalton et al., 2011). Across 18 SARM trials in healthy adults, raised ALT was reported in a mean of 7.1% of SARM-exposed participants (Vignali et al., J Xenobiot 2023).
Outside trials, LiverTox reports that SARMs from supplements have been implicated in “multiple cases of severe cholestatic jaundice” resembling anabolic-steroid jaundice (LiverTox). Cholestasis means bile flow is blocked, causing jaundice and itching. Published ostarine cases include:
- a man in his early 40s who used ostarine for 2 months, developed jaundice, and had biopsy-confirmed cholestatic injury that improved over several months after stopping (Bedi et al., ACG Case Rep J 2021);
- a 31-year-old man who developed itching and dark urine 2 weeks after starting an ostarine-only product, with ALT of 346 U/L (Weinblatt and Roy, J Med Cases 2022).
The same safety review also collected case reports of Achilles tendon rupture and rhabdomyolysis (muscle breakdown) in SARM users (Vignali et al., 2023). Case reports cannot establish how often these events occur.
Cholesterol
Ostarine lowered HDL (“good” cholesterol) in a dose-dependent way. At 3 mg/day, HDL fell by 14.7 mg/dL (about 27%) over 12 weeks, versus no change on placebo. LDL did not change significantly, total cholesterol fell, and triglycerides trended lower (p = 0.06) (Dalton et al., 2011).
Hormones
“SARMs don’t affect hormones” is not what the trial data show. In older men on 3 mg/day for 12 weeks (Dalton et al., 2011):
- total testosterone fell by 7.4 nmol/L versus placebo (p under 0.001);
- SHBG (sex hormone-binding globulin, the protein that carries testosterone in blood) fell by about 61%;
- free testosterone, LH, FSH and estradiol did not change significantly.
In postmenopausal women, 3 mg lowered LH, FSH and SHBG. Hemoglobin and hematocrit (the share of blood made up of red cells) rose slightly but significantly at 3 mg. These findings cover only 3 mg or less for 12 weeks. No published trial measures hormonal suppression at the higher doses used outside medicine.
About “no suppression” claims. Online claims that ostarine does not suppress hormones usually rely on the free-testosterone and LH results above. They do not mention the fall in total testosterone and SHBG, the 3 mg ceiling, or the 12-week duration.
FDA’s list of possible SARM risks also includes heart attack or stroke, psychosis, sexual dysfunction, infertility and testicular shrinkage (FDA consumer update).
Quality and contamination
In a 2017 JAMA study, researchers bought 44 products sold online as SARMs and tested them (Van Wagoner et al., JAMA 2017):
| Finding | Products |
|---|---|
| Contained any SARM (ostarine, LGD-4033 or andarine) | 23 (52%) |
| Contained other unapproved drugs (e.g. ibutamoren, GW501516, SR9009) | 17 (39%) |
| Contained substances not on the label | 11 (25%) |
| Contained no active compound | 4 (9%) |
| Amount matched the label | 18 (41%) |
Contamination also matters for athletes. In a controlled study, a single oral dose of just 1 μg of ostarine could be detected in urine for up to 9 days. This supports the concern that trace contamination of ordinary supplements can cause a positive test (Walpurgis et al., Drug Test Anal 2020).
Related library pages: LGD-4033, RAD-140, Andarine (S4), Cardarine (GW501516).
Key takeaways
- An unapproved investigational drug. Ostarine (enobosarm) is a nonsteroidal SARM that never gained approval. FDA says SARMs cannot legally be sold as dietary supplements.
- Small lean-mass gains in trials. At 3 mg/day for about 12–16 weeks, lean mass rose by roughly 1–1.5 kg in older adults and cancer patients.
- Phase 3 did not deliver. In two lung cancer trials with about 640 patients, more patients kept their lean mass, but physical function did not improve consistently.
- Not free of side effects. Even at 3 mg/day, HDL fell by about 27% and total testosterone and SHBG fell. Liver enzyme rises occurred in trials, and cholestatic liver injury has been reported in users.
- Products are unreliable. In one analysis only about half of “SARM” products contained a SARM, and only 41% matched their label.
- Banned in sport. WADA prohibits ostarine at all times, and trace amounts can be detected in urine for days.
Sources
- Dalton JT, Barnette KG, Bohl CE, et al. The selective androgen receptor modulator GTx-024 (enobosarm) improves lean body mass and physical function in healthy elderly men and postmenopausal women: results of a double-blind, placebo-controlled phase II trial. J Cachexia Sarcopenia Muscle. 2011;2(3):153–161. PubMed; PMC.
- Dobs AS, Boccia RV, Croot CC, et al. Effects of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised controlled phase 2 trial. Lancet Oncol. 2013;14(4):335–345. PubMed.
- Crawford J, Prado CM, Johnston MA, et al. Study design and rationale for the phase 3 clinical development program of enobosarm, a selective androgen receptor modulator, for the prevention and treatment of muscle wasting in cancer patients (POWER trials). Curr Oncol Rep. 2016;18(6):37. PubMed.
- GTx, Inc. Phase III studies of the effect of GTx-024 on muscle wasting in patients with non-small cell lung cancer (POWER 1 and POWER 2). ClinicalTrials.gov NCT01355484 and NCT01355497; results posted. ClinicalTrials.gov; ClinicalTrials.gov.
- Bhasin S, Krishnan V, Storer TW, et al. Androgens and selective androgen receptor modulators to treat functional limitations associated with aging and chronic disease. J Gerontol A Biol Sci Med Sci. 2023;78(Suppl 1):25–31. PubMed.
- Palmieri C, Linden H, Birrell SN, et al. Activity and safety of enobosarm, a novel, oral, selective androgen receptor modulator, in androgen receptor-positive, oestrogen receptor-positive, and HER2-negative advanced breast cancer (Study G200802): a randomised, open-label, multicentre, multinational, parallel design, phase 2 trial. Lancet Oncol. 2024;25(3):317–325. PubMed.
- GTx, Inc. Study to assess enobosarm (GTx-024) in postmenopausal women with stress urinary incontinence. ClinicalTrials.gov NCT03241342; results posted. ClinicalTrials.gov.
- Veru Inc. A phase 2 dose-finding and proof-of-concept study to evaluate the effect on body composition and safety of enobosarm in patients treated with GLP-1 receptor agonists for chronic weight management. ClinicalTrials.gov NCT06282458. ClinicalTrials.gov.
- Coss CC, Jones A, Dalton JT. Pharmacokinetic drug interactions of the selective androgen receptor modulator GTx-024(Enobosarm) with itraconazole, rifampin, probenecid, celecoxib and rosuvastatin. Invest New Drugs. 2016;34(4):458–467. PubMed.
- Kim J, Wang R, Veverka KA, Dalton JT. Absorption, distribution, metabolism and excretion of the novel SARM GTx-024 [(S)-N-(4-cyano-3-(trifluoromethyl)phenyl)-3-(4-cyanophenoxy)-2-hydroxy-2-methylpropanamide] in rats. Xenobiotica. 2013;43(11):993–1009. PubMed.
- Taoussi O, Bambagiotti G, Gameli PS, et al. In vitro and in vivo human metabolism of ostarine, a selective androgen receptor modulator and doping agent. Int J Mol Sci. 2024;25(14):7807. PubMed.
- National Center for Biotechnology Information. PubChem Compound Summary for CID 11326715, Enobosarm. PubChem.
- Hoofnagle JH. Selective androgen receptor modulators. In: LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. National Institute of Diabetes and Digestive and Kidney Diseases; updated September 20, 2025. PubMed.
- Bedi H, Hammond C, Sanders D, Yang HM, Yoshida EM. Drug-induced liver injury from enobosarm (ostarine), a selective androgen receptor modulator. ACG Case Rep J. 2021;8(1):e00518. PubMed.
- Weinblatt D, Roy S. Drug-induced liver injury secondary to enobosarm: a selective androgen receptor modulator. J Med Cases. 2022;13(5):244–248. PubMed.
- Vignali JD, Pak KC, Beverley HR, et al. Systematic review of safety of selective androgen receptor modulators in healthy adults: implications for recreational users. J Xenobiot. 2023;13(2):218–236. PubMed.
- Van Wagoner RM, Eichner A, Bhasin S, Deuster PA, Eichner D. Chemical composition and labeling of substances marketed as selective androgen receptor modulators and sold via the internet. JAMA. 2017;318(20):2004–2010. PubMed.
- Walpurgis K, Rubio A, Wagener F, et al. Elimination profiles of microdosed ostarine mimicking contaminated products ingestion. Drug Test Anal. 2020;12(11-12):1570–1580. PubMed.
- Food and Drug Administration. FDA warns of use of selective androgen receptor modulators (SARMs) among teens, young adults. Consumer update; content current as of April 26, 2023. FDA.
- World Anti-Doping Agency. Prohibited List 2026 (in force January 1, 2026), official German translation published in the Austrian Federal Law Gazette, BGBl. III Nr. 219/2025, December 30, 2025. RIS.