The reason you only run clen for two weeks at a time and many alternate with e/c/a is that Clen seems to work incredibly for periods of about 2-3 weeks, but then, the fat burning effects just stops.
Unlike ephedrine, clen is a "specific" beta 2 agonist, fitting almost perfectly into the receptor site. It stimulates the receptors so well, the body will burn fat very fast. However, being that our bodie's are masters of homeostasis, the body will quickly downregulate the receptors and desensitize them to clen.
Another problem is that clen will slow the rate of T4 to T3 conversion...not good. So, in order for clen to work for a longer amount of time, you must find a way to keep T3 levels optimal.
T3 increases the number of beta receptors on fat and muscle cells, which in turn increases the effect of clen.
This is what leads to the ever popular clen/T3 stack.
Ephedrine has been shown to increase levels of T3 after several weeks of use. It also does not downregulate the beta receptor site like clen does.
So you can now see why people use for 2 weeks on then 2 weeks off if not using T3, although clen/T3 is hard to beat...but then T3 is a whole different subject.
Wrongun!