Viagra, Levitra and Cialis are used for the treatment of erectile dysfunction when you are unable to achieve adequate erection for sexual activity. These drugs can be very good for most men with only few side effects. The erectile improvement is same for all three medications can tadalafil (Cialis) can effect erection longer. If you are looking for real, effective and complete options to treat your ED problem when you get generic Viagra online, you should consider taking a look at Kamagra, Caverta, Silagra and other ED top-quality drugs, such as Zenegra and Edegra. There are three oral medications approved for the treatment of erectile dysfunction: Sildenafil (Viagra), vardenafil (Levitra), and tadalafil (Cialis). All three belong to a class of drugs called selective enzyme inhibitors. Sildenafil (Viagra), vardenafil (Levitra), and tadalafil (Cialis) block the enzyme phosphodiesterase-5 (PDE5). Blocking this enzyme helps maintain levels of cyclic guanosine monophosphate (GMP), a chemical produced in the penis during sexual arousal. Balanced levels of GMP causes the smooth muscles of the penis to relax and increases blood flow.