I would suggest not using it at 21 years old. Stop before you do it. It isnt exactly the healthiest thing to do. You really have no idea what you're doing. You havent researched anything, your diet is completely unknown to you; 20mg alone will get you some lean body mass, but if you dont know what you're doing, why are you doing it? Do you have side supports? Liver care?
Anyway, hope this helps.