Fildena 100 mg contains sildenafil citrate, a phosphodiesterase type 5 (PDE5) inhibitor. Sildenafil works by blocking the PDE5 enzyme, leading to increased levels of cyclic guanosine monophosphate (cGMP). This results in the relaxation of blood vessels in the penis, enhancing blood flow and helping to achieve and maintain an erection in response to sexual stimulation.