If you are a man of a certain age (or are anyone of any age who listens to AM radio in any major market) you have heard about Viagara, Cialis and Levitra. These are the big brand names of the most popular erectile dysfunction (ED) drugs (sildenafil, tadalafil and vardenafil, respectively), and ED drugs are doing gangbusters business. But have you ever wondered whether these drugs are differentiated by anything other than their brand names?