Fildena 100 mg works by inhibiting the enzyme phosphodiesterase type 5 (PDE5). This enzyme breaks down cyclic guanosine monophosphate (cGMP), a substance that promotes relaxation and dilation of blood vessels in the penis. By inhibiting PDE5, Sildenafil Citrate increases cGMP levels, leading to improved blood flow to the penis and facilitating an erection in response to sexual stimulation.