Proviron is 5-alpha reduced and not capable of forming estrogen, and also has a very high affinity for binding to the aromatase enzyme (the enzyme responsible for converting all that good testosterone in your body into all that nasty estrogen). That means if you choose to take proviron with testosterone (and I know you wouldn´t even be doing a cycle without including some form of testosterone) and/or any aromatizable steroid, it should actually serve to prevent estrogen build up by the aforementioned binding to the aromatase enzyme, which prevents aromatase from doing it´s dirty work and making a bunch of estrogen out of the other steroids you are taking. It should also be noted that Proviron also binds very well to SHBG (Sex Hormone Binding Globulin& a hormone responsible for reducing the amount of circulating free testosterone in your body)(1). As a matter of fact, in the last study I read, it bound to SHBG better than any other drug studied. Also, I´d like to note that Proviron bound to the Anabolic Receptor better than any oral anabolic (except for the insanely toxic MethylTrienolone), having an ability to bind to the AR better then testosterone, but not as well as Nandrolone (1). Unfortunately, as we know, DHT also has a high affinity for binding to receptors in the scalp and prostate, causing some possible nasty side effects, like male pattern baldness and prostate enlargement. It´s important to remember that DHT and DHT derived compounds are used quite successfully to treat gynocomastia, and in this area, Proviron is no different.
The above is from Steroid.com on proviron
In my experience with it, it has helped keep the aromitization of test down. As well as increase sex drive, and give a more hardened defined look to the muscles.