The three top anti ED drugs in the market today are Viagra, Levitra and Cialis. The generic versions of these drugs are also extremely popular since they are cheaper and just as effective. Generic Viagra, Cialis and Levitra have similar effects but have a few dissimilarities. The first difference between the three is that Generic Viagra is sildenafil citrate, Generic Cialis is tadalafil and generic Levitra is vardenafil. These three chemicals react in different ways to the body though the ultimate effect is the same. Generic Viagra, Cialis and Levitra are also different in the time the take to react and the length of their effects. Sildenafil citrate takes about half an hour to start working and lasts for up to four hors. On the other hand, tadalafil and vardenafil have a longer span. Generic Cialis� effects last up to thirty six hours and generic Levitra�s effects up to twenty four hours. In spite of these small differences, all three have been proved effective as a treatment for impotency and can be used interchangeably. To choose between Generic Viagra, Cialis and Levitra, you can try all three and decide yourself which works best for you.