From what I can gather listening to Jewett, James Hollingshead, and other large pro's who like Mast, it's best used along with HIGH test dosages (that's right, exactly opposite to what Todd Lee says - go figure) to 1) ameliorate E2 sides, and 2) run up the total MG amount for growth (this is assuming that one can tolerate mast). As you get bigger and need more mg's to grow, the game becomes more about running high doses of what you can tolerate. Most pro's level out around 1250 test weekly (seems to be a dose response curve that flattens out around that level), and the most tolerable compounds to add to that which also help with E2 are EQ, primo, and Mast. Aside from price and availability of genuine product (quoting Jewett here), the best thing to do is run what you tolerate as high as you can. That said, for the really big guys who need 2500+ to grow, that often means using at least 2 of these 3 secondary anabolics.