What it is
Ostarine (enobosarm, developmental codes GTx-024, MK-2866, and S-22) is a non-steroidal selective androgen receptor modulator (SARM) with an arylpropionamide scaffold (C19H14F3N3O3, MW 389.33, CAS 841205-47-8, PubChem CID 11326715). It binds the androgen receptor with a reported Ki of ~3.8 nM. Unlike endogenous androgens it cannot be converted to DHT by 5α-reductase or to estrogens by aromatase, which is the structural basis for its tissue-selective anabolic profile: preferential activity in skeletal muscle and bone, with lower activity in prostate and other reproductive tissues. It was developed by GTx, Inc. (Memphis, TN) — originally under an early-2000s collaboration with Merck — and was the first SARM to advance into Phase 3 trials. GTx-024 assets are now held by Veru Inc.
In plain English
Ostarine is a SARM — a Selective Androgen Receptor Modulator. In plain terms: it attaches to the same cell-level "lock" that testosterone uses (called the androgen receptor), but it's designed to mostly affect muscle and bone while mostly leaving the prostate and other sex organs alone. It's not a steroid. Because of its shape, the body can't turn it into DHT (the strong masculinizing form of testosterone) or into estrogen, which is how most anabolic steroids cause their worst side effects. It was developed by a company called GTx in Memphis, Tennessee, and was the first SARM to ever make it to big Phase 3 trials. The rights are now owned by Veru Inc.
How it works
- 01
Tissue-selective androgen receptor binding
Ostarine is a non-steroidal AR ligand with a reported dissociation constant Ki ~3.8 nM (Narayanan 2018, Sex Med Rev). On binding, it induces an AR conformation distinct from the one induced by testosterone or DHT, producing a different coregulator-recruitment profile. The result in preclinical systems is dissociation of anabolic from androgenic activity: ostarine restores levator ani muscle mass in orchiectomized rats to near intact levels while causing only partial prostate-weight restoration — the classic dissociation signature used to triage SARM candidates.
In plain English
It uses testosterone's cell-level lock — but selectively
Ostarine attaches to the same cell-level "lock" that testosterone uses — called the androgen receptor. But it fits the lock in a slightly different shape, which brings in a different set of helper proteins inside the cell. The result in animal studies: muscles grow back almost normally in castrated rats, but the prostate only partially grows back. That "muscle yes, prostate less" split is the whole point of a SARM.
- 02
Muscle: Akt/mTOR and ubiquitin-proteasome modulation
In skeletal muscle AR activation by ostarine drives protein synthesis via Akt/mTOR signaling and suppresses protein degradation through the ubiquitin-proteasome system. Roch 2020 (Frontiers in Endocrinology) in ovariectomized rats showed that ostarine restored soleus and gastrocnemius fiber cross-sectional area and improved histomorphometric markers of muscle quality; the Roch 2024 follow-up (Int J Mol Sci) showed advantage over raloxifene and additive effects with raloxifene in the estrogen-deficient model.
In plain English
It turns up muscle building and turns down muscle breakdown
Muscle size is a balance between how fast your body builds muscle protein and how fast it tears it down. Ostarine turns up the building side and turns down the breakdown side. In rats with removed ovaries, ostarine restored individual muscle fibers to a more normal thickness. A 2024 follow-up showed it worked better than raloxifene — a drug used for osteoporosis — and that the two combined worked even better.
- 03
Bone: osteoblast activation and callus mineralization
Komrakova 2020 (Calcified Tissue International) reported that enobosarm improved fracture callus mineralization, trabecular bone volume, and mechanical strength in an aged-male osteoporotic rat model. Mechanistically AR activation in osteoblasts promotes differentiation and matrix mineralization while AR in osteoclasts attenuates resorption. The bone signal is consistent across models but has not been tested as a primary endpoint in a dedicated human trial.
In plain English
It helps bones build new material
In old male rats with osteoporosis, ostarine improved fracture healing, inner-bone structure, and mechanical strength. The reason: bone-building cells (osteoblasts) get more active, while bone-eating cells (osteoclasts) get less active. Results like this show up across several animal studies. No human trial has tested bone as the main outcome yet.
- 04
Metabolic effects on adipocytes and glucose handling
Macías-Villalobos 2020 (J Physiol Biochem) characterized ostarine effects on Wistar-rat adipocyte metabolism, showing modulation of lipogenic gene expression. Dalton 2011 Phase 2 observed placebo-separable reductions in fasting glucose, fasting insulin, and HOMA-IR at 3 mg/day — consistent with muscle-mediated improvement in whole-body glucose disposal. The metabolic effect has never been replicated as a pre-specified endpoint in a dedicated trial.
In plain English
It may improve blood sugar handling
In rat fat cells, ostarine changes which fat-related genes are active. In the 2011 Phase 2 human trial, the 3 mg/day group had meaningfully lower fasting blood sugar and insulin than placebo — probably because bigger muscles soak up more glucose. This has never been re-tested as the main outcome of a dedicated metabolic trial.
- 05
Hepatotoxicity mechanism — not fully characterized
Despite the non-steroidal scaffold that was supposed to reduce hepatotoxicity relative to 17α-alkylated anabolic steroids, cholestatic DILI has been reported repeatedly in retail-SARM users (Bedi 2021, ACG Case Reports Journal). The mechanism is not fully worked out in the peer-reviewed literature: AR-mediated effects on hepatic transporters, off-target effects of metabolites characterized by Sobolevsky 2024 (Drug Test Anal), and formulation-quality issues in the unregulated supplement market all plausibly contribute. The clinical pattern is indistinguishable from anabolic-steroid cholestasis at the bedside.
In plain English
Why it damages the liver in some users
Ostarine was supposed to be gentler on the liver than old-school oral steroids because of its non-steroid structure. But published case reports keep showing it causes a specific kind of liver damage (where bile can't flow out of the liver properly) in people who bought SARM products from retailers. The exact reason isn't fully understood yet — it could be off-target effects of its breakdown products, or contamination in the unregulated supplement supply, or something else. In the clinic, it looks the same as old-fashioned steroid liver damage.
- 06
What is NOT known about the mechanism
The coregulator-recruitment map that would explain tissue selectivity at the molecular level is incomplete. Long-term effects on the HPG axis in humans are not well-characterized — transient testosterone and SHBG suppression have been observed on-cycle in the Phase 2/3 program, with rebound after washout, but there is no long-exposure cohort to characterize recovery in recreational users who stack or cycle. Drug-drug interactions have been mapped for itraconazole, rifampin, probenecid, celecoxib, and rosuvastatin (Nanda 2016), but co-exposure patterns in real-world SARM users (often stacked with other anabolics) are not studied.
In plain English
What we still don't know about how it works
The exact reason ostarine favors muscle over prostate at the molecular level isn't fully worked out. Long-term effects on the body's natural testosterone system aren't well known — during-cycle suppression has been seen in trials, and it comes back after stopping, but no one has followed the multi-cycle gym-user pattern. Drug interactions with a few specific drugs have been mapped, but the real-world pattern of stacking ostarine with other anabolic drugs has never been studied.