Sildenafil, the active ingredient in Silditop 100 mg, is a PDE5 inhibitor. It works by blocking the enzyme phosphodiesterase type 5 (PDE5), which is responsible for regulating blood flow in the penis. By inhibiting PDE5, sildenafil helps to increase the levels of cyclic GMP (cGMP), a molecule that causes the blood vessels in the penis to relax and expand. This leads to increased blood flow to the penis, making it easier to achieve an erection during sexual arousal.