it'll come out on the black market. a round of people will use them. they'll either spend a shitload of money on something that doesn't work or they'll spend a shitload of money and die gigantic and in 5 years. there's no "reversing" myostatin blocking. you grow until you can't grow anymore...making you basically rip your own body to shreds. the massive muscles will be so strong you'll shatter your own bones, your lungs will collapse under the pressure and weight, your heart will shut down, etc. it seems like it would be similar to dying of tetanus (a VERY painful death). if it does work...people will use it until that first batch starts dying. people are dumb and willing to risk everything for the easy way out. mark my words...people will die if they get this to work in humans.
im not saying it doesn't have medical uses, i haven't done any research into muscular dystrophy or other muscle wasting diseases, but from personal research i'd have to say there are other ways to better combat these diseases...but that's a whole other discussion