As discussed in the following paragraphs, perhaps some of the best candidates are the phosphodiesterase 5 (PDE5A)-specific inhibitors, which include sildenafil, tadalafil and vardenafil (marketed under the trade names Viagra, Cialis and Levitra, respectively). These pharmacological agents enhance the vasodilatory action of NO by inhibiting the breakdown of cGMP, the second messenger for NO, thus causing sustained relaxation of vascular smooth muscle (Michel, 2006).