Viagra is the common name of a class of drugs for the treatment of male erectile dysfunction, it is not a drug, is a class of drugs, belongs to the type 5 phosphodiesterase inhibitors, is currently the most widely used class of drugs for the clinical treatment of erectile dysfunction. These drugs mainly include short-acting class and long-acting class, the short-acting class has sildenafil and vardenafil, long-acting class of drugs have tadalafil, of which sildenafil and there are domestic drugs and imported drugs, the use of flexible choice. Sildenafil and vardenafil are generally taken more on demand, that is, before sex, and tadalafil is generally taken more orally according to the course of treatment, that is, according to a certain interval of time to take drugs.