Viagra , which contains the active ingredient sildenafil, is one of the most popular medications available for treating erectile dysfunction (ED), with millions of users in the United States alone and many more worldwide 비아그라 구매.Sildenafil belongs to a class of medications called PDE5 inhibitors. It functions by blocking the effects of an enzyme called phosphodiesterase type 5, or PDE5, which regulates blood flow to your penis.