Zenegra 100 mg is a generic version of the popular brand name Viagra. It contains the same active ingredient, sildenafil citrate, which helps men suffering from erectile dysfunction achieve and maintain an erection. The drug works by increasing blood flow to the penis, allowing it to become engorged with blood during sexual stimulation. Zenegra 100 mg is manufactured in India and is available at many online pharmacies for a much lower price than Viagra.