Sildenafil is a medication primarily used to treat erectile dysfunction (ED), a condition that affects a significant percentage of men, especially as they age. The drug works by inhibiting an enzyme called phosphodiesterase type 5 (PDE5). This enzyme is responsible for breaking down cyclic guanosine monophosphate (cGMP), a molecule that plays a crucial role in relaxing the smooth muscles in the penis. When PDE5 is inhibited, cGMP levels rise, leading to increased blood flow into the penile tissues, ultimately resulting in an erection.