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AHuman RCT or meta-analysis
Hepatotoxic

Ostarine

MK-2866 · Enobosarm · GTx-024

Ostarine (MK-2866, enobosarm, GTx-024) is an investigational non-steroidal selective androgen receptor modulator (SARM) developed by GTx to treat muscle wasting/cachexia. It is not approved by the FDA or any major regulator for any use, and it is banned in sport (WADA). In short (12-week) phase II randomized trials in cancer patients and healthy older adults it produced modest, dose-dependent gains in lean body mass and physical function at 1-3 mg/day, and was generally well tolerated in those monitored settings. The main dangers for real-world (recreational, higher-dose, unmonitored) use are drug-induced liver injury (cholestatic/hepatocellular, sometimes with jaundice and prolonged recovery) reported in case reports; suppression of the hypothalamic-pituitary-testicular axis (falls in total testosterone and SHBG); an unfavorable cardiometabolic lipid shift (dose-dependent HDL-cholesterol reduction of up to ~27%); and unregulated products sold as "ostarine" that are frequently mislabeled or adulterated. There are no long-term human safety data and no data in young athletic users. This is a risk-forward reference entry, not an endorsement or a dosing guide; anyone using or considering it should be under clinician supervision with bloodwork.

Clinical readoutSARM · sarm
Hepatic strainModerate
CardiovascularModerate
HPTA suppressionHigh
Half-life
24 h
Route
Oral
Evidence
A
Active
Once-daily oral adminis…
24 h2 d3 d4 d5 d
Illustrative single-compartment washout · each mark = one half-life · t½ ≈ Not precisely established in the peer-reviewed literature; all phase I/II trials used once-daily oral dosing, consistent with a terminal half-life on the order of roughly 24 hours. Anti-doping detection windows are considerably longer than the pharmacologic effect. (Half-life value is approximate/inferred, not a directly verified figure.)
Pharmacology

Mechanism of action

Ostarine is a non-steroidal selective androgen receptor modulator: it binds the androgen receptor (AR) and acts as a tissue-selective partial agonist, producing anabolic (agonist) signaling in skeletal muscle and bone while exerting comparatively less androgenic activity in tissues such as prostate, skin (sebaceous glands/hair follicles). Unlike testosterone it is not aromatized to estradiol and is not 5-alpha-reduced to dihydrotestosterone, which underlies its tissue-selective profile. As an AR agonist it still engages androgen negative feedback and alters androgen-responsive endpoints (e.g., SHBG, total testosterone, lipids). Tissue selectivity is relative, not absolute, and does not eliminate off-target endocrine, hepatic, or cardiometabolic effects.
Kinetics

Pharmacokinetics

Half-life

Not precisely established in the peer-reviewed literature; all phase I/II trials used once-daily oral dosing, consistent with a terminal half-life on the order of roughly 24 hours. Anti-doping detection windows are considerably longer than the pharmacologic effect. (Half-life value is approximate/inferred, not a directly verified figure.)

Active duration

Once-daily oral administration was used throughout clinical development (e.g., 1-3 mg/day for 12 weeks), indicating a duration of action supporting once-daily dosing.

Route

Oral (orally bioavailable non-steroidal SARM).

Metabolism & clearance

Hepatic metabolism including CYP3A4-mediated oxidation and UGT glucuronidation to a glucuronide metabolite, with subsequent excretion. Drug-drug interaction studies showed the CYP3A4 inducer rifampin lowered exposure (Cmax ~-23%, AUC ~-43%) and the UGT inhibitor probenecid raised exposure, confirming CYP3A4/UGT-dependent clearance. Presented for monitoring/washout and interaction awareness, not for evading drug testing.

For monitoring and washout planning, not drug-test evasion.

Reported effects

Physiological & performance effects

  • Dose-dependent increase in total lean body mass (e.g., ~1.3 kg vs placebo at 3 mg/day over 12 weeks in healthy older adults; significant increases at 1 mg and 3 mg in cancer patients)
  • Modest improvement in physical function (stair-climb power) at 3 mg/day
  • Reduction in total fat mass and improved insulin resistance (lower fasting glucose, HOMA-IR) in short-term trials
  • Small increases in hemoglobin/hematocrit at 3 mg/day
  • Marked reduction in sex hormone-binding globulin (SHBG) and total testosterone (androgen-pathway pharmacodynamics)
  • Effects studied only over 12 weeks at 1-3 mg/day in clinical populations; efficacy/safety of higher recreational doses is not established
Safety

Adverse effects by system

Cardiovascular

Dose-dependent, unfavorable lipid shift: HDL-cholesterol fell ~17% at 1 mg and ~27% at 3 mg vs placebo over 12 weeks; total cholesterol and triglycerides also fell, LDL largely unchanged (Dalton 2011). No RCT-level data on hard cardiovascular outcomes; class reviews of SARMs flag potential cardiovascular risk. Long-term cardiovascular effects are unknown.

Hepatic

Transient ALT elevations occurred in RCTs (8/120 subjects in Dalton 2011; one discontinued at ALT 4.2x ULN, which normalized off drug). More severe drug-induced liver injury (cholestatic/hepatocellular, with jaundice, pruritus, dark urine) has been reported in case reports of enobosarm-containing supplements and across the SARM class; injury generally resolved weeks-to-months after discontinuation.

Endocrine / HPTA

Androgen-receptor agonism suppresses androgen-pathway markers: significant reductions in total testosterone and marked reductions in SHBG in men; reductions in LH/FSH seen in postmenopausal women (not clinically significant there). In the low-dose 12-week trial, free testosterone and LH in men were not significantly changed, but suppression of endogenous testosterone/gonadotropins is mechanistically expected and reported anecdotally at higher recreational doses. No long-term recovery data.

Reproductive

Via HPTA/androgen suppression, reduced endogenous testosterone could impair spermatogenesis/fertility and libido; this is mechanistically expected but not directly quantified for ostarine in humans. No adequate human fertility data. Contraindicated in pregnancy (androgenic; theoretical fetal virilization risk).

Neuropsychiatric

No psychiatric adverse effects were characterized in the phase II RCTs. SARM-class reviews list possible psychiatric/mood disturbance, but there are no adequate ostarine-specific human data; considered not established.

Renal

No signal of nephrotoxicity in short-term RCTs (subjects required creatinine <=1.5 mg/dL at entry); no adequate human data on renal effects with prolonged or high-dose recreational use.

Hematologic

Small but statistically significant increases in hemoglobin and hematocrit at 3 mg/day over 12 weeks; magnitude was modest in trials but erythrocytosis is a known androgen class effect and could be greater at higher/longer exposure.

Dermatologic

Ostarine did not significantly increase sebum production or hirsutism in women in the 12-week trial; dry skin was reported as an occasional grade 3 event in an enobosarm-plus-pembrolizumab oncology study (confounded by combination therapy). Androgenic skin effects (acne) are theoretically possible but not well documented for ostarine specifically.

Recovery

HPTA suppression & recovery

Suppression: Present; dose-dependent and expected for an AR agonist. In low-dose (1-3 mg) 12-week trials, total testosterone and SHBG fell significantly in men while free testosterone and LH were not significantly suppressed; at higher recreational doses/longer durations, meaningful suppression of endogenous testosterone and gonadotropins (LH/FSH) is anticipated but not well quantified in humans.

No robust human data define the time course of axis recovery after stopping ostarine. Because suppression is a class effect, do not assume rapid or complete spontaneous recovery. Any concern about suppressed testosterone, gonadotropins, symptomatic hypogonadism, or fertility should be evaluated and managed by an endocrinologist with serial LH/FSH/total and free testosterone testing. This entry does not describe or endorse any self-directed hormonal recovery ('PCT') or SERM protocol; management is individualized and clinician-led.

Bloodwork & vitals

Monitoring

Recommended labs & checks
Liver function panel (ALT, AST, ALP, total/direct bilirubin, GGT) at baseline and periodicallyFasting lipid panel (especially HDL-C) at baseline and on treatmentTotal and free testosterone, SHBG, LH, FSH (HPTA assessment)Complete blood count / hematocrit and hemoglobin (erythrocytosis)Fasting glucose (metabolic monitoring)

Cadence: Baseline before any use; re-check liver panel and lipids within a few weeks of starting and periodically thereafter; promptly if symptoms arise. Endocrine (HPTA) labs at baseline and after discontinuation to assess recovery. All monitoring should be arranged with a clinician.

Warning signs — seek care
  • Jaundice (yellow eyes/skin), dark urine, pale stools, pruritus (itching)
  • Right-upper-quadrant abdominal pain, persistent nausea, marked fatigue
  • Chest pain, shortness of breath, or other cardiovascular symptoms
  • Symptoms of low testosterone (low libido, mood change, erectile dysfunction) or testicular changes
  • Headaches, visual changes, or symptoms suggestive of elevated hematocrit
Do not use if

Contraindications

  • Not FDA-approved; use is off-label/unapproved for any indication
  • Pre-existing or active liver disease, or unexplained transaminase elevation
  • Pregnancy and breastfeeding (androgenic; theoretical fetal harm/virilization)
  • History of, or high risk for, dyslipidemia/cardiovascular disease (given HDL suppression)
  • Polycythemia/erythrocytosis or conditions worsened by increased hematocrit
  • Hormone-sensitive conditions or undiagnosed prostate issues in men, without specialist oversight
  • Concurrent hepatotoxic drugs, heavy alcohol use, or other agents that suppress the HPTA
  • Competitive athletes subject to anti-doping rules (WADA-prohibited)
Combinations

Interaction profile

  • ModerateWith a thermogenic stimulant: Additive cardiovascular strain
  • ModerateWith an anabolic steroid: Hormonal
  • ContraindicatedWith DNP: Additive cardiovascular strain

Check a specific combination in the interaction checker.

Harm reduction

Reducing harm & when to stop

  • Ostarine is not FDA-approved and no long-term human safety data exist; there is no verified 'safe' dose for recreational use - trial data cover only 1-3 mg/day for 12 weeks in supervised clinical populations.
  • Stop immediately and seek medical care for any sign of liver injury: jaundice, dark urine, pale stools, itching, right-upper-quadrant pain, persistent nausea, or profound fatigue.
  • Get baseline bloodwork before use and monitor on-treatment: liver panel, fasting lipids (HDL), CBC/hematocrit, and a full HPTA hormone panel - do this with a clinician.
  • Products sold as 'ostarine/MK-2866' are unregulated and frequently mislabeled, underdosed, overdosed, or adulterated with other SARMs or anabolic steroids, which increases and obscures risk.
  • Avoid combining with alcohol, other hepatotoxic drugs/supplements, or other hormonal agents; these compound liver and endocrine risk.
  • Expect an unfavorable HDL-cholesterol drop; those with cardiovascular or lipid risk should be especially cautious and involve a clinician.
  • For any suppression of testosterone or fertility concerns, consult an endocrinologist rather than self-managing hormonal 'recovery'; recovery time is not well characterized.
  • Banned by WADA and most sporting bodies - detectable in anti-doping testing for an extended window.
  • This entry is informational and risk-forward; it is not medical advice and not an endorsement of use.
Evidence

Citations (11)

Every clinical claim above is tied to a primary source. Overall evidence grade A — graded to the best available evidence for its core claims.

  1. 01

    Ostarine/enobosarm is a non-steroidal SARM that produced dose-dependent, statistically significant increases in lean body mass and improved physical function (stair-climb power) at 3 mg/day over 12 weeks in healthy elderly men and postmenopausal women; adverse-event incidence was similar to placebo.

    RCTThe 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 trialPMID 22031847 ↗

  2. 02

    In the same 12-week RCT, HDL-cholesterol was reduced dose-dependently (~17% at 1 mg, ~27% at 3 mg vs placebo), total cholesterol and triglycerides fell, and LDL was largely unchanged.

    RCTThe 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 trialPMID 22031847 ↗

  3. 03

    In the same RCT, total testosterone and SHBG were significantly reduced in men while free testosterone and LH were not significantly changed at 1-3 mg; small significant increases in hemoglobin/hematocrit occurred at 3 mg; transient ALT elevations occurred in 8/120 subjects with one discontinued at ALT 4.2x ULN that normalized off drug; insulin resistance and fasting glucose improved.

    RCTThe 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 trialPMID 22031847 ↗

  4. 04

    In a double-blind randomized phase 2 trial in cancer patients, enobosarm 1 mg and 3 mg significantly increased total lean body mass vs placebo without the toxic effects associated with androgens/progestational agents.

    RCTEffects of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised controlled phase 2 trial.PMID 23499390 ↗

  5. 05

    Enobosarm advanced to identically designed phase 3 muscle-wasting trials (POWER 1/2) in NSCLC at 3 mg/day once daily; clinical development targeted prevention/treatment of cancer-associated muscle wasting.

    RCTStudy Design and Rationale for the Phase 3 Clinical Development Program of Enobosarm... (POWER Trials).PMID 27138015 ↗

  6. 06

    Enobosarm is orally administered and cleared partly via CYP3A4 metabolism and UGT glucuronidation to a glucuronide metabolite; the CYP3A4 inducer rifampin reduced enobosarm Cmax by ~23% and AUC by ~43%, and a UGT inhibitor (probenecid) increased exposure.

    RCTPharmacokinetic drug interactions of the selective androgen receptor modulator GTx-024 (Enobosarm) with itraconazole, rifampin, probenecid, celecoxib and rosuvastatin.PMID 27105861 ↗

  7. 07

    Drug-induced liver injury (hepatocellular pattern with jaundice/dark urine) has been reported after use of an enobosarm-containing muscle-building supplement, improving after discontinuation.

    Case reportDrug-Induced Liver Injury Secondary to Enobosarm: A Selective Androgen Receptor Modulator.PMID 35655632 ↗

  8. 08

    SARMs as a class are associated with drug-induced liver injury; case reports document cholestatic/hepatocellular injury with jaundice that resolves over months after stopping, and products are widely available online despite lacking FDA approval.

    Case reportSelective Androgen Receptor Modulators (SARMs)-Induced Liver Injury: A Case Report and Review of LiteraturePMID 36945289 ↗

  9. 09

    SARM use has caused clinically significant hepatotoxicity (jaundice, elevated liver enzymes) that resolved on discontinuation, prompting FDA warnings.

    Case reportSelective Androgen Receptor Modulator Induced Hepatotoxicity.PMID 35340496 ↗

  10. 10

    Off-label SARM use in healthy adults is associated with hepatotoxicity, cardiovascular complications, endocrine disturbance, and psychiatric symptoms, driven partly by the misconception that SARMs are safer than anabolic steroids.

    Case reportLGD-4033 and a Case of Drug-Induced Liver Injury: Exploring the Clinical Implications of Off-Label Selective Androgen Receptor Modulator Use in Healthy Adults.PMID 39421081 ↗

  11. 11

    In an oncology combination study, enobosarm 18 mg/day with pembrolizumab was generally well tolerated with few grade 3 events (dry skin, diarrhea, musculoskeletal ache), though findings are confounded by combination therapy and small numbers.

    RCTA Phase II Clinical Trial of Pembrolizumab and Enobosarm in Patients with Androgen Receptor-Positive Metastatic Triple-Negative Breast Cancer.PMID 33141975 ↗

Last reviewed 2026-07-06 · Verified against PubMed · Educational, not medical advice