As a result,it affects the flow of blood. The release of nitric oxide and adequate inflow of blood are the two most important processes for achieving erections. Never use two doses of Viagra on the same day. Tadalafil which is the active medical ingredient in this drug belongs to the group of drugs which are known as phosphodiesterase type 5 inhibitor and it works by inhibiting this enzyme and prevents the degradation of cGMP.