VIAGRA contains the active substance Sildenafil which belongs to a group of medicines called phosphodiesterase type 5 (PDE 5) inhibitors. It works by helping to relax the blood vessels in your penis, allowing blood to flow into your penis when you get sexually excited. It is the most popular treatment for ED although there are now many alternative treatments available.   Following sexual stimulation Viagra works by helping the blood vessels in your penis to relax, allowing the flow of blood into your penis. The result of this is improved erectile function. It starts working within 30-60 minutes.