Fildena Professional contains sildenafil citrate as its active ingredient, a potent inhibitor of phosphodiesterase type 5 (PDE5). PDE5 is an enzyme that breaks down cyclic guanosine monophosphate (cGMP), a molecule responsible for regulating blood flow in the penis. By inhibiting PDE5, sildenafil citrate prolongs the effects of cGMP, leading to sustained vasodilation and improved blood flow to the penis, thereby facilitating the attainment and maintenance of erections.