PDE5, or phosphodiesterase type 5, is an enzyme found in various tissues, including the penis and lungs. It plays a crucial role in regulating blood flow by breaking down a molecule called cGMP, which helps relax blood vessels. This process is essential for achieving and maintaining an erection. Inhibitors of PDE5, such as sildenafil and tadalafil, are commonly used to treat erectile dysfunction and pulmonary hypertension. By blocking the action of PDE5, these medications increase cGMP levels, leading to improved blood flow and enhanced erectile function or reduced blood pressure in the lungs.