Test breaks down into either estrogen or DHT, Dihydrotestosterone. DHT can cause some unwanted side effects like enlarged prostate and hair loss for those that are prone. Propecia is a DHT blocker, meaning that it will reduce the conversion of test to DHT and specifically reduce the chance of hair loss. Reducing DHT is not always a good thing though. DHT hs it's benefits. It is a powerful androgen and takes part in normal sexual function. Threre have been many reports of guys have ED while taking propecia. If you are not prone to baldness, you might want to think twice about taking this "just in case". Also be aware that it does work for balding from test, but will not stop balding from other sterods