Among athletes Proviron is primarily used as an antiestrogen. It is believed to act as an antiaromatase in the body, preventing or slowing the conversion of steroids into estrogen. The result is somewhat comparable to Arimidex (though less profound), the drug acting to prevent the buildup of estrogen in the body. This effect of Proviron is in contrast to Nolvadex, which only blocks the ability of estrogen to bind and activate receptors in certain tissues. The anti-aromatization effect is preferred, as it is a more direct and efficient means of dealing with the problem of estrogenic side effects. A related disadvantage to Nolvadex is that if discontinued too early, a rebound effect may occur as high serum estrogen levels are again free to take action. This of course could mean a rapid onset of side effects such as gynecomastia and water retention. Most athletes actually prefer to use both Mesterolone and Nolvadex, especially during strongly estrogenic cycles. Proviron and Nolvadex attack estrogen at a different angle, side effects are often greatly minimized.