Ideally, treatments, operations and diagnostic procedures should be thoroughly tested before they come into routine use. But that is not always the case. Drugs and medical devices have to be approved by the Food and Drug Administration, but once they are on the market, doctors can prescribe them in almost any way they see fit, a practice called off-label use.

http://www.nytimes.com/2007/12/26/health/26double.html?partner=rssnyt&emc=rssv