http://www.bodybuilding.com/fun/catarimidex.htm
"Anastrozole operates by blocking the aromatase enzyme, the primary enzyme for the conversion of testosterone to estrogen."
"ften times during a cycle most will want to allow for some estrogen, since it heavily promotes strength and gains as well (increases GH, upgrades the androgen receptor, improves glucose utilization). These people will generally opt for an estrogen receptor antagonist such as Nolvadex (tamoxifen) or Clomid (Clomiphene). These products do not stop the formation of estrogen, but stop the estrogen from exerting its effects by competitively taking up the receptors for this hormone. This allows them to stop any problems dead in their tracks, acting very fast, but upon discontinuation allowing for immediate influx of estrogen again as well."