Zudena 100 mg contains udenafil, a PDE5 inhibitor used to treat erectile dysfunction (ED) in adult men. It works by increasing blood flow to the penis during sexual stimulation, helping an erection develop and remain firm. It does not increase sexual desire and does not cause an erection without sexual stimulation. Udenafil generally begins working within about 30–60 minutes, although the timing can vary. The duration of action may be relatively long compared with some other ED medicines.