As a PDE5 inhibitor, kamagra vs viagra differs from in that it specifically blocks the PDE5 enzyme, which is in charge of preventing an individual from achieving or maintaining an erection. At least an hour before having sex, take Viagra on an empty stomach. Never use Viagra with any other erectile dysfunction drug or take more than one tablet at once. The drugs increase the relaxation of your penile muscles by the action of nitric oxide, a substance produced by your body. This facilitates an erection by increasing blood flow. Headaches, nausea, indigestion, and lightheadedness are the most frequent adverse effects. Many experience only minor or no adverse effects at all.