Generic Viagra® contains sildenafil, the same active ingredient as brand-name Viagra®, which means that it is just as effective for erectile dysfunction as the original ED medication. Generic Viagra® is a PDE5 (phosphodiesterase type 5) inhibitor, which works by blocking the PDE5 enzyme, which in turn temporarily relaxes smooth muscle and improves blood flow to the penis during sexual stimulation. Other similar PDE5 inhibitors are Cialis® (tadalafil), Levitra® (vardenafil) and Stendra® (avanafil).