These two compounds get lumped together constantly in online forums, but MK-677 and RAD140 could not be more different pharmacologically. One raises growth hormone. The other hits the androgen receptor. Choosing between them - or deciding to run both - comes down to what you actually want out of your physique and how much risk you’re willing to accept.
Quick Comparison
| Feature | MK-677 (Ibutamoren) | RAD140 (Testolone) |
|---|---|---|
| Primary Mechanism | Ghrelin receptor agonist - raises GH and IGF-1 [1] | Selective androgen receptor modulator (SARM) [2] |
| Typical Dosing | 10-25 mg/day orally [3] | 10-20 mg/day orally |
| FDA Status | Not approved - research compound | Not approved - pre-clinical [2] |
| Best For | Recovery, sleep quality, joint health, long-term body composition | Lean muscle gain, strength, recomposition |
| Approximate Cost | $40-60/month | $50-80/month |
| Common Side Effects | Water retention, increased appetite, elevated blood glucose [4] | Testosterone suppression, potential liver stress, hair shedding |
| Half-Life | ~24 hours (active metabolites); parent compound 4.7 hours [3] | ~60 hours |
| PCT Required | No | Yes, in most cases |
What Is MK-677?
MK-677, also called Ibutamoren, is a growth hormone secretagogue that mimics the hunger hormone ghrelin [5]. It binds to the ghrelin receptor in the hypothalamus and pituitary gland, triggering your body to produce and release more of its own growth hormone. This is a critical distinction - MK-677 does not introduce exogenous GH. It amplifies your natural pulsatile secretion pattern [5].
Developed in 1995 and later studied by Merck for GH deficiency, the compound stands out because it’s orally bioavailable [5]. Most GH-releasing peptides require subcutaneous injection, which limits their appeal for anyone who isn’t comfortable pinning daily. MK-677 is a capsule or liquid you take once a day. Clinical trials have used it in elderly populations to increase bone mineral density, lean body mass, and IGF-1 levels [6].
Despite being grouped with SARMs in online vendor catalogs, MK-677 is pharmacologically distinct from every SARM on the market [1]. It does not interact with the androgen receptor at all. It won’t suppress your natural testosterone production, it won’t cause androgenic side effects like hair loss or acne, and it doesn’t require post-cycle therapy. What it will do is make you hungry - sometimes ravenously so - and hold water like a sponge for the first few weeks [7].
What Is RAD140?
RAD140 (Testolone) is a genuine selective androgen receptor modulator developed by Radius Health Inc. [2]. It was designed to deliver anabolic effects in muscle and bone tissue while minimizing androgenic activity in the prostate and other sensitive tissues [2]. The original research direction targeted non-cancer muscle wasting, age-related sarcopenia, and potentially neurodegenerative diseases [8].
On paper, RAD140 is impressive. It demonstrates tissue-selective anabolic activity that rivals testosterone’s muscle-building capacity while substantially reducing androgenic impact on the prostate [9]. Users consistently report rapid increases in strength, muscle hardness, and vascularity within the first two to three weeks of a cycle. It’s one of the most potent SARMs available, and some researchers have described its anabolic profile as closer to traditional anabolic steroids than to milder SARMs like Ostarine [9].
The catch: RAD140 is still in pre-clinical stages and has not completed human clinical trials [2]. Everything the fitness community knows about its effects in humans comes from anecdotal reports and self-experimentation. It suppresses endogenous testosterone production - sometimes significantly - and there are documented cases of elevated liver enzymes during use. This is not a compound you take casually.
Key Differences Between MK-677 and RAD140
Mechanism of Action
This is the fundamental split. MK-677 works through the GH/IGF-1 axis by activating the ghrelin receptor [5]. RAD140 works through the androgen receptor, mimicking testosterone’s effects in select tissues [2]. They don’t overlap pharmacologically at all.
The practical implication: MK-677 gives you growth hormone benefits - improved recovery, better sleep quality, enhanced collagen synthesis, and gradual improvements in body composition over months. RAD140 gives you androgen-mediated benefits - rapid muscle protein synthesis, increased strength, and a harder, drier look. The timelines are different too. RAD140 users notice strength gains within two weeks. MK-677’s effects on body composition take six to eight weeks to become apparent, though sleep and recovery improvements often kick in within days.
Side Effect Profile
MK-677’s main nuisances are water retention and appetite stimulation [4]. New users commonly gain 10-20 pounds of water weight in the first month before it stabilizes [7]. Blood glucose and cortisol can also spike, particularly with poor timing - dosing too close to bedtime may disrupt sleep through elevated cortisol, which is ironic given that many people take it specifically for sleep benefits [4]. The hunger can be genuinely difficult to manage. For someone trying to cut body fat, the ghrelin activation can feel like it’s working directly against your goals.
RAD140 carries heavier risks. Testosterone suppression is virtually guaranteed at effective doses, and the degree of suppression varies unpredictably between individuals. Some users bounce back within weeks of discontinuation; others need a proper PCT protocol with SERMs like Nolvadex or Enclomiphene to restore hormonal function. Liver toxicity, while not universal, has been reported in case studies. Hair shedding is common in those genetically predisposed, and unlike finasteride-responsive hair loss from traditional androgens, SARM-induced shedding doesn’t respond to 5-alpha reductase inhibitors because it isn’t mediated by DHT.
Hormonal Impact
MK-677 does not suppress the hypothalamic-pituitary-gonadal axis. Your testosterone, LH, and FSH remain untouched [1]. You can run it for extended periods - some users go six months or longer - without worrying about hormonal recovery afterward.
RAD140 directly suppresses LH and FSH through negative feedback on the hypothalamus, just like exogenous testosterone does. Cycles are typically limited to 8-12 weeks, followed by 4-6 weeks of PCT. Running it longer or at higher doses increases the depth and duration of suppression.
Legality and Testing
Both compounds are prohibited by WADA and banned in drug-tested competition [10]. Neither is FDA-approved for any indication. They’re sold as “research chemicals” in most jurisdictions, which exists in a legal gray area. The key difference for tested athletes: MK-677 has a longer detection window due to its effects on GH markers, while RAD140 can be detected directly in urine.
MK-677 vs RAD140: Which Should You Choose?
Choose MK-677 if your priorities are recovery, longevity, and gradual recomposition. You’re not looking for dramatic eight-week transformations. You want better sleep, faster healing from training, healthier joints, and a slow, steady improvement in body composition without suppressing your natural hormones. MK-677 also makes sense if you’re in a gaining phase and don’t mind the appetite increase - it essentially turns eating into the easy part. It’s a particularly strong choice for natural lifters over 35 who are noticing recovery declining but don’t want to commit to TRT or SARM cycles.
Choose RAD140 if you want measurable muscle and strength gains on a defined timeline and you accept the hormonal trade-offs. You’re running an 8-12 week cycle with a specific physique goal, you have PCT drugs on hand, and you’ve had baseline bloodwork done. RAD140 delivers more dramatic results in a shorter window, but those results come with suppression, potential liver stress, and mandatory recovery time afterward. It’s better suited for experienced lifters who understand cycle management.
If you’re a woman, MK-677 carries significant water retention issues that make it generally unappealing [7]. RAD140 at very low doses (5 mg/day) has been used by female athletes, but virilization risk exists with any androgen receptor agonist. Neither compound is well-studied in women.
If budget is the primary concern, MK-677 wins on value because it can be run continuously without PCT costs, bloodwork intervals are less critical (though still recommended), and a single purchase covers months of use rather than a discrete cycle.
Can You Stack MK-677 and RAD140?
This is actually one of the more logical SARM-adjacent stacks because the two compounds work through entirely independent pathways. MK-677 handles the GH/IGF-1 side - recovery, sleep, collagen synthesis, nutrient partitioning - while RAD140 drives androgen-mediated muscle growth [1][2]. There’s no pharmacological redundancy.
A common protocol runs RAD140 at 10-15 mg/day for 8-10 weeks while MK-677 continues at 12.5-25 mg/day through the cycle and into PCT. The rationale: MK-677 doesn’t suppress testosterone, so it can bridge the gap between the end of your RAD140 cycle and full hormonal recovery. Users report that the GH support helps preserve muscle during the suppressed PCT period when anabolic signaling from androgens is at its lowest.
The downsides of stacking are additive side effects. Water retention from MK-677 masks the dry, hard look RAD140 produces. Appetite stimulation from MK-677 works against you if you’re trying to use RAD140 for a cut. And running two research chemicals simultaneously means two sets of unknowns interacting in your body. If bloodwork comes back abnormal, isolating which compound caused the issue becomes harder.
If you do stack them, get bloodwork at minimum before starting, at the four-week mark, and four weeks after discontinuing everything. Monitor fasting glucose (MK-677), liver enzymes (RAD140), and a full hormonal panel including LH, FSH, total testosterone, and IGF-1.
Read the Full Guide(s)
References
- elite_athlete - MK-677 is Not a SARM (https://www.youtube.com/watch?v=cBgkPynoMBA)
- mpmd - Introduction to RAD140 (Testolone) (https://www.youtube.com/watch?v=kTBAaXWWVmY)
- mpmd - MK-677 Dosage and Administration (https://www.youtube.com/watch?v=Nng8T-bPdaU)
- mpmd - MK-677 Pharmacology and Misconceptions (https://www.youtube.com/watch?v=VVPZRucd-bQ)
- ryan_russo - Mechanism of Action of MK-677 (Ibutamoren) (https://www.youtube.com/watch?v=sXGHc22fPOM)
- mpmd - Clinical Applications of MK-677 (https://www.youtube.com/watch?v=Nng8T-bPdaU)
- dr_gabrielle_lyon - MK-677 for Muscle Mass (https://www.youtube.com/watch?v=073WMTTRzO8)
- elite_athlete - Research Origins and Potential Medical Applications (https://www.youtube.com/watch?v=LiR2AbfoPJk)
- elite_athlete - RAD140 Potency and Anabolic Comparison to Testosterone (https://www.youtube.com/watch?v=LiR2AbfoPJk)
- greg-doucette - What Is MK-677 and How Does It Work? (https://www.youtube.com/watch?v=6Dkva6GHvNM)