Viagra drug. Computer artwork of a molecule of the anti-impotence drug viagra. The atoms, represented as spheres, are colour coded: carbon (pink), oxygen (red), nitrogen (blue) and hydrogen (white). Viagra is the trade name for the drug sildenafil citrate. It is used to treat erectile dysfunction. Viagra enhances the smooth muscle relaxant effects of nitric oxide, a chemical that is released in response to sexual stimulation. Relaxation of the smooth muscle allows blood to enter the penis, resulting in an erection.