The active ingredient, sildenafil citrate, is essentially a selective inhibitor of the cyclic guanosine monophosphate (cGMP)-specific PDE-5 enzyme. The PDE-5 enzyme degrades cGMP, which is required for proper blood flow in the penile muscles. As a result, obtaining and maintaining an erection becomes difficult. PDE-5 inhibitors inhibit the activity of this enzyme, preventing the breakdown of cGMP. However, Kamagra only works when sexual stimulation is present. Because the stimulation causes the muscles lining the inner wall of the penis to release nitric oxide (NO), which activates the cGMP molecules. This causes penile muscle vasodilation and increases blood flow.