Viagra. Computer model of a molecule of the drug sildenafil. It is sold under the brand names Viagra and Revatio. It is a phosphodiesterase inhibitor, and is an impotency drug, used to treat erectile dysfunction. This drug was developed by Pfizer. The molecule's chemical formula is C22.H30.N6.O4.S. Atoms (spheres) are colour-coded: carbon (grey), hydrogen (light blue), nitrogen (dark blue), oxygen (orange), sulphur (yellow).