Without a prescription it is illegal. The most popular versions are Deca-Durabolin(Nandrolone decanoate) and Durabolin(Nandrolone phenylpropionate). It will allow you to recover much faster from long hours of play, and it speeds up the healing process of injuries. You will gain muscle and strength even if you do not weight train, but it is more pronounced in the individuals who do. If the ATP players are using it, it is likely due to increased recovery benefits, not for the typical bodybuilding motives. The tour schedule is a grind on the body so I understand if they feel they need help to make it through the season, even if it is illegal. The medical uses are for weight gain and recovery after certain surgeries or severe trauma, along with reducing protein catabolism associated with corticosteroids. The serious side effects include male breats cancer, kidney disease, liver disease, and prostate cancer. Milder, but still undesireable include gynocomastia, acne, decreased sperm formation and a few others. Personally, the stuff is too expensive and too dangerous to risk for recreational use. Almost everyone who takes it suffers from the mild side effects, which are still hell in my book. Plus, you could wind up in jail for possession of a controlled substance. If you have any problems that would warrant hormone or steroid treatments, I suggest you visit a qualified endocrinologist. If you are thinking about that stuff they sell at GNC forget it. I have known many people who have taken that crap, including myself, and we saw nothing. Only 30 or more dollars missing from our bank statements.