Viagra and Cialis are both PDE-5 inhibitors. They relax the smooth muscle of the blood vessels that supply the penis. This relaxation increases blood flow and promotes an erection. However, Viagra and Cialis have different active ingredients. In Viagra, the PDE-5 inhibitor is sildenafil, while in Cialis, it is tadalafil. Some men may respond better to one active ingredient than the other. However, sildenafil and tadalafil work by the same mechanism; therefore, they should never be taken together. It is also not recommended to take more than one Viagra or Cialis tablet at a time.