Tadalafil inhibits the enzyme PDE5 (phosphodiesterase type 5), which is found in the penis and regulates the amount of blood that flows into it. When a man is sexually stimulated, nitric oxide is released in the penis. Nitric oxide activates an enzyme that helps in the production of cyclic guanosine monophosphate (cGMP), which relaxes the blood vessels in the penis, allowing increased blood flow. PDE5 breaks down cGMP, but tadalafil blocks PDE5, allowing cGMP to remain active longer and, therefore, enhancing the erection.