Zudena 100 mg tablet (udenafil) works by inhibiting the enzyme phosphodiesterase type 5 (PDE5). When a man is sexually stimulated, nitric oxide is released in the penis, leading to the production of cyclic guanosine monophosphate (cGMP). This compound relaxes the blood vessels, allowing increased blood flow to the penis. PDE5 breaks down cGMP, which can reduce blood flow. By inhibiting this enzyme, Zudena helps maintain higher levels of cGMP, facilitating sustained erections.