YK11, also known as Myostine, is considered one of the strongest SARMs on the market. In fact, it’s so strong and has comparable anabolic activity to steroids that it may actually be a synthetic steroid that was mislabeled a SARM in the first place.
YK11 was first studied by Japanese researcher Yuichiro Kanno in 2011. It was found to be a partial agonist of the Androgen Receptor (AR) with gene-selective properties, leading researchers to suggest that the compound is a SARM. 
To date, limited studies have been undertaken, although preliminary findings in vitro on muscle cells are very promising. Initial studies suggest that YK11 may have greater muscle building effects than those of DHT and a propensity to act as a myostatin inhibitor.
How it works
As a relatively new compound, exactly how YK11 works to improve lean muscle mass in humans can only be discussed based on intial in vitro studies.
However, what we do know is that YK11 may be a Myostatin blocker – essentially allowing users to reap continuous gains without stalling and requiring a break in a cycle.
Myostatin is a protein that is released to limit muscle growth to ensure they do not get ‘too large’. It is found predominantly in skeletal muscle, and studies show that individuals with a gene mutation that limits Myostatin production are both stronger and more muscular than those with normal amounts.
YK11 may help to inhibit the production of Myostatin in muscles by attaching itself to the Androgen Receptor. From there, it may induce muscles to create more Follistatin, which in turn limits the levels of Myostatin, allowing increased muscle growth beyond genetic capability.
The confusion over whether Myostatin is a SARM or a steroid lies in the naming convetions (or lack thereof) in relation to newer compounds. Kanno, the original Japanese researcher, discovered that YK11 works via the androgen receptor, which is why he initially claimed it to be a SARM.
Newer research suggests that the backbone of YK11 is identical to those of steroids. In reality, we won’t know for sure until more studies are conducted.
There are 3 primary benefits that have been observed in both initial studies and user-reported reviews.
- YK11 may block Myostatin, allowing continues lean body mass gains
The primary benefit of YK11 and the reason it shows huge promise in the bodybuilding world is its potential to activate more Follistatin in muscles, subsequently blocking the production of Myostatin. By blocking Myostatin, a user could potentially unleash huge gains with continuous muscle growth. 
- YK11 may be comparable to testosterone in its ability to build strength in muscles
It’s a commonly held belief that while SARMs produce less negative side effects than testosterone, they’re also less effective at creating an anabolic effect in muscle growth.
Initial studies  show that muscle cells produce more anabolic factors when exposed to YK11 than they do when exposed to DHT (testosterone). This suggests that YK11 is potentially more powerful than testosterone at producing impressive gains with less side effects than steroids.
- YK11 shows potential for fast, large gains
Many users report seeing results from YK11 in as little as a single week of use, with the amazing results after an eight-week cycle.
YK11 has only been tested in vitro – meaning that it has been tested on cells in a petri dish rather than on live animals or humans – where it has shown great promise in building lean muscle mass. However, this also means that until animal or human trials are conducted, we will not know what the potential side effects may be.
Anecdotal evidence from individuals who have cycled YK11 suggests that for those prone to hair loss, shedding may occur. Other users have noted zero hair loss, so it may be a case-by-case situation based on each individual.
Other anecdotal evidence suggests that mild acne may be a side effect of YK11, but this also disappears after a few weeks or once the cycle is complete.
It has also been suggested that YK11 may cause mild, reversible liver damage. This means that any damage caused is repaired by the body once the cycle is complete.
User reviews suggest that the ideal human dosage for YK11 is 2-5mg twice per day for eight weeks (a total of 5-10mg per day for men).
Anecdotally, some users suggest increasing the dosage to 20mg per day in total over the course of a cycle, although it’s best to start at a lower dosage and find your sweet spot.
5-10mg total per day split into two doses.
0.5-2mg total per day split into two doses.
There have been quite a few anecdotal reports from users who have stacked YK11 with RAD140 with incredible results.
Users report increased strength and an increase in lean muscle mass similar to testosterone results but without the water bloat. It works great for users looking to bulk with lean, dry gains.
- RAD140 (Testolone) – 15-30mg
- YK11 (Myostine) – 5-10mg