Short answer: they are different kinds of drug that happen to share a code prefix. Ostarine (MK-2866, enobosarm) is a SARM: it acts on the androgen receptor, builds a little lean mass, and lowers testosterone and HDL cholesterol. MK-677 (ibutamoren) is not a SARM: it acts on the ghrelin receptor, raises growth hormone and IGF-1, and mostly brings hunger, water retention and higher blood sugar. In trials in older adults, both added roughly 1 kg of lean or fat-free mass.
Ostarine vs MK-677 at a glance
| Ostarine | MK-677 | |
|---|---|---|
| What it is | Selective androgen receptor modulator (SARM), nonsteroidal | GH secretagogue (ghrelin receptor agonist) |
| Approval | Not approved; furthest stage: Phase 3 | Not approved; furthest stage: Phase 3 |
| Human studies (counted in our records) | 8 | 17 |
| Half-life | ~14–24 h | Not established in humans |
| Testosterone suppression score | 2/3 | 1/3 |
| Liver risk score | 2/3 | 1/3 |
| Evidence score | 3/3 | 3/3 |
| WADA | Prohibited at all times (S1.2 Other anabolic agents) | Prohibited at all times (S2.2.4 Growth hormone releasing factors) |
| Target | Androgen receptor | Ghrelin (GHS) receptor; raises GH and IGF-1 |
| Lean mass in trials | +1.3 kg vs placebo in 86 days (3 mg/day, older adults) | +1.1 kg vs −0.5 kg placebo over 12 months (25 mg/day, older adults) |
| SwissChems product | Ostarine (MK-2866), (10mg/capsule), 60 Capsules | MK-677 (Ibutamoren), (10mg/capsule) 60 Capsules |
Scores run from 0 (none) to 3 (high); see our methodology. Data from the SARMs database.
Source: Sarmguide compound database; Ostarine: GTx, Inc. Provides Corporate Update And Reports 2013 Financial Results (press release, 4 March 2014) (2014). Source: Sarmguide compound database; Ostarine: 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 (2011).
How they work
Ostarine binds the androgen receptor and was designed to act more on muscle and bone than on the prostate. Because it is an androgen-receptor drug, the brain sees “enough androgen” and lowers natural testosterone. MK-677 copies the hunger hormone ghrelin: the pituitary releases more growth hormone, which raises IGF-1; in older adults, 12 months raised GH 1.8-fold and IGF-1 1.5-fold. It does not act on the androgen receptor, so it is not expected to shut down testosterone the way a SARM does.
What the trials show
| Outcome | Ostarine | MK-677 |
|---|---|---|
| Lean mass | +1.3 kg vs placebo (86 days); +1.0–1.5 kg in cancer patients | +1.1 kg vs −0.5 kg (12 months) |
| Fat | About 0.6 kg less than placebo | Total and belly fat unchanged; limb fat rose; weight +2.7 kg |
| Strength and function | Improved on 3 mg in Phase 2; Phase 3 lung cancer trials missed their goals | Strength and function unchanged |
| Testosterone | Total testosterone −6 to −7 nmol/L vs placebo; SHBG down | Small drop in total testosterone in one 8-week trial; LH and FSH unchanged |
| Cholesterol | HDL −17% to −27% | LDL down slightly |
| Blood sugar | Insulin resistance improved (86 days) | Fasting glucose up about 5 mg/dL; insulin sensitivity down |
| Newest research | With Wegovy: 71% less lean-mass loss (company topline, 2025) | Phase 3 in children with GH deficiency (as LUM-201) |
Side effects
| Ostarine | MK-677 |
|---|---|
| Lower testosterone, SHBG and HDL | More appetite (67% vs 36% placebo) |
| Dose-dependent ALT rises; case reports of cholestatic liver injury | Water retention and leg swelling; muscle and joint pain |
| Liver-enzyme rises at high trial doses (9–18 mg/day) in women with breast cancer | Higher blood sugar; heart-failure signal in frail older adults after hip fracture |
Half-life
Ostarine’s half-life is about 14–24 hours in Phase 1 studies. No human plasma half-life has been published for MK-677; the FDA found no detailed human pharmacokinetic data, and its GH effect is described as lasting about 24 hours.
Stacking ostarine and MK-677
They are often stacked because they work through different routes, but no study has tested the combination. The closest human data come from a case report of a different SARM, LGD-4033, with MK-677: over 5 weeks, testosterone fell 62%, HDL fell 36% and LDL rose 40%. Our stack checker shows the overlapping risks; see also ostarine stacks.
Legal status and sport
Neither is approved. In December 2025, FDA warning letters to an online seller named products sold as “Ostarine MK-2866” and “MK-677 Ibutamoren” as unapproved new drugs. In Australia both are prescription-only, and possession without authority is illegal. WADA bans ostarine under S1.2 (SARMs) and MK-677 under S2.2.4 (growth hormone secretagogues), at all times.
Verdict
Ostarine is the better-studied muscle compound of the two in shorter trials and has some evidence for keeping muscle during GLP-1 weight loss, at the cost of lower testosterone and HDL and rare serious liver injury. MK-677 adds weight partly as water, brings hunger and higher blood sugar, and has a heart-failure signal in frail patients, but it barely touches testosterone. Choose what to read next: ostarine before and after · MK-677 before and after · cardarine vs ostarine
Frequently asked questions
Is MK-677 the same as ostarine?
No, unrelated drugsEvidence: Human data
Ostarine (MK-2866) is a SARM that acts on the androgen receptor; MK-677 (ibutamoren) is a growth hormone secretagogue that acts on the ghrelin receptor. The similar codes do not mean they are related.
Is MK-677 a SARM?
NoEvidence: Human data
It is a growth hormone secretagogue that copies ghrelin and does not act on the androgen receptor, though it is often sold alongside SARMs.
Which builds more muscle, ostarine or MK-677?
About the same in trialsEvidence: Trial
In older adults, both added about 1–1.3 kg of lean or fat-free mass: ostarine over 86 days, MK-677 over 12 months. MK-677 did not improve strength.
Does MK-677 shut down testosterone like ostarine?
Much lessEvidence: Trial
Ostarine lowered total testosterone by 6–7 nmol/L versus placebo. MK-677 caused a small drop in one 8-week trial, without changing LH or FSH.
Can you stack ostarine and MK-677?
Never studiedEvidence: Human data
No study has tested the combination. In a case report of LGD-4033 plus MK-677, testosterone fell 62% and HDL 36% over 5 weeks.
Which has fewer side effects?
They differEvidence: Trial
Ostarine lowers testosterone and HDL and has liver injury case reports. MK-677 increases appetite, water retention and blood sugar, with a heart-failure signal in frail older adults.
Are ostarine and MK-677 legal?
Neither is approvedEvidence: Law
FDA warning letters in December 2025 named products sold as both as unapproved new drugs. WADA bans both at all times.
Sources
- 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
- Effects of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised controlled phase 2 trial
- EMERALD study protocol (ISRCTN12213700, University of Liverpool, version 6.0, 2021): a window of opportunity study of enobosarm in ER+/AR+ early breast cancer
- Veru Announces Positive Topline Data from Phase 2b QUALITY Clinical Study: Enobosarm Preserved Lean Mass in Patients Receiving WEGOVY (Semaglutide) for Weight Reduction (press release, 27 January 2025)
- 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
- Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial
- Discrepancy between serum leptin values and total body fat in response to the oral growth hormone secretagogue MK-677
- FDA Briefing Document, Pharmacy Compounding Advisory Committee, 29 October 2024: Ibutamoren Mesylate
- LGD-4033 and MK-677 use impacts body composition, circulating biomarkers, and skeletal muscle androgenic hormone and receptor content: A case report
- FDA Warning Letter to an online seller of products marketed as SARMs (MARCS-CMS 719257), 12 December 2025
- Therapeutic Goods (Poisons Standard—June 2026) Instrument 2026 (F2026L00633)
- World Anti-Doping Code International Standard: Prohibited List 2026
