A majority of drugs taken have a action/reaction period. Times are different depending on the compound taken, but steroids and other performance enhancers tend to go problem free for about 2-4 weeks before other factors in your body start to react to the newly acquired levels of hormones or what not. Your body will attempt to stay in a state of homeostasis so if your testosterone levels are jacked after awhile your body will try and compensate for those high levels by also increasing your estrogen and cortisol levels. Both of these levels if not kept in check will dramatically reduce the amount of gains that can be made. Beyond at, if these levels are also not kept in check towards the end of your cycle and post-cycle you are even worse yet bound to lose alot of the gains you made during your cycle, because after the exogenous testosterone is discontinued your body's normal production is quite low to even non-existent, and estrogen and cortisol levels will remain high if not even bounce higher for awhile.