.....you increase the dose due to the bodies various action/reaction factors
after only 21 days on any gear your body starts counter measures to try to achieve homeostatis (cortisol/shbg etc etc) or in simple terms the body naturally shifts you from anabolism back to catabolism at the four week point you
will need to up the dose to continue the speed of growth achieved in the first twenty one days.....hence the reason most pros(the smart ones anyway)
run short (30 days or less cycles)combined with other drugs (hgh,slin,t3 igf-1)
to grow almost continually.........long cycles are in my opinion counter productive to long term gains.....and hopefully on the way out....the information is out there people lets start using it........