viagra has an active ingredient by the name of Sildenafil, which is a phosphodiesterase type 5 inhibitor. This inhibits the enzyme phosphodiesterase type 5 from breaking down cyclic GMP. Cyclic GMP dilates blood vessels for a longer amount of time. Thus when phosphodiesterase type 5 concentration is inhibited cyclic GMP concentration is higher resulting in higher oxygen flow to capillary beds/muscles and more efficient aerobic metabolism