Viagra. Computer artwork of a molecule of the drug Viagra (sildenafil citrate (chemical formula C27.H35.N6.O11.S)). The drug is used to treat male impotence. The atoms represented as cylinders are colour-coded: carbon (green), hydrogen (white), nitrogen (blue), oxygen (red), sulphur (yellow). Unlike treatments that directly cause penile erections, Viagra affects the response to sexual stimulation. Viagra enhances the smooth muscle relaxant effects of nitric oxide, which is released in response to sexual stimulation. Smooth muscle relaxation allows blood to enter the penis leading to an erection.