Tadalafil is a phosphodiesterase type 5 (PDE5) inhibitor, similar to sildenafil citrate (found in Viagra, Zenegra) and vardenafil (found in Levitra). It works by relaxing the blood vessels in the penis during sexual stimulation, allowing increased blood flow to the area. This enhanced blood flow helps men achieve and maintain an erection, making it easier to engage in sexual activity.