You're kidding, right. Your age is showing my man. And I mean that in all respect. Listen to George. Kickstart it or better yet, don't do it. At 21 you may not have any business even thinking about it...and based on your lack of research, taking just one would be a mistake. Do yourself a favor and spend the next 4 to 5 years training hard, dieting correctly, and research your entire hormonal balances before you buy anything. Lewd but true. Just don't do it and move on.