Cialis, Viagra, and their generic equivalents (tadalafil and sildenafil, respectively) are FDA-approved ED medications belonging to a class of treatment options called PDE5 inhibitors. PDE5 (short for phosphodiesterase type 5) is an enzyme that reduces blood flow to the penis. These ED drugs help you get and stay hard by keeping the smooth muscles of the blood vessels that supply the penis relaxed, so blood can flow in more easily.