Fildena works by inhibiting the enzyme phosphodiesterase type 5 (PDE5), which breaks down a substance called cyclic guanosine monophosphate (cGMP). cGMP is responsible for relaxing the smooth muscles in the penis, allowing blood vessels to dilate and increase blood flow. By blocking PDE5, Fildena helps maintain higher levels of cGMP, enabling an erection to be achieved and sustained when sexually stimulated.