Zudena 100 mg is a medication used to treat erectile dysfunction (ED) in men. The active ingredient in Zudena is udenafil, which is a phosphodiesterase type 5 (PDE5) inhibitor. Like other PDE5 inhibitors, udenafil works by increasing blood flow to the penis, allowing men to achieve and maintain an erection when sexually stimulated. Udenafil is the active ingredient in Zudena. It is a newer PDE5 inhibitor that is used to treat ED. Udenafil, the active ingredient in Zudena, has a relatively fast onset of action. It typically starts to work within 30 minutes to 1 hour after ingestion, although individual responses may vary.