Both Cialis and Viagra are medicines used to improve erectile dysfunction in men. The main ingredient of Cialis is tadalafil and the main ingredient of Viagra is sildenafil, both drugs belong to the same class of drugs. There is no difference between the two drugs in terms of mechanism of action, through the inhibition of phosphodiesterase, stimulate the release of nitric oxide, so that more blood flow into the penis, so that the penis erection, can be used to improve male erectile dysfunction. There is no difference between the two in terms of indications, but there will be a difference in terms of duration of action. The half-life of the drug can reach 17 hours, while Viagra or Sildenafil is a short-acting drug, the half-life of the drug is usually only a few hours, the specific patients need to use Viagra or Viagra, under the guidance of a doctor.