Drugs may only be prescribed for an approved set of diseases. Using a drug for a new disease indication without a study to back it up falls"experimental treatment". In other words, using a drug for an unapproved indication is doing scientific experiments on humans without approval which of course is unethical (and illegal). That said it's easier to get permission to use a new drug for a new disease indication if the drug has had previous clinical history of treating other diseases. At least in my country such permission is handed out on a case by case basis after the responsible physician has applied to the proper regulatory authorities.