If pacquiao was on steroids, why couldnt he just stop taking them for the floyd fight

anyone have a good answer??? Idk alot about roids, but i know as long as you keep working out you will retain the muscles that you have. My only explanation is Lets say pac used steroids for the cotto fight, the last time he would of roided up was around 11/10/09, so the roids might not be out of his system yet and thats why he might not want the drug test, but i heard the first test wont be until the first week of january. anyone know how long roids stay in your system???