john w replied: "The history of Cialis cannot be discussed without mentioning Viagra or Pfizer. The FDA's approval on March 27, 1998, lead both of these pre ion drugs, Viagra and Pfizer, to a ground breaking success in just the first year of introduction as they sold drugs worth over a billion dollars. However, things changed considerably for the two giants of erectile dysfunction drugs when the FDA also approved Levitra in August 19, 2003, and Cialis in November 21, 2003. In 1993 the drug company Icos began studying IC351, which is a PDE5 enzyme inhibiter, and this is basically the process through which the erectile dysfunction drugs work. In 1994, Pfizer scientists discovered that Sildenafil Citrate, which is a white crystalline powder that temporarily normalizes erectile function of the penis by blocking an enzyme known to inhibit the production of a chemical that causes erections, caused the heart patients that were participating in a clinical study of a heart medicine to have erections. Although the scientists were not testing the chemical compound IC351 for erectile dysfunction, the compound seem to have a side effect which could potentially worth million, if not billions. Soon Icos received its very first patent in 1994 on IC351, and the clinical trials of phase I took place in 1995. In 1997, phase II clinical studies began and Icos performed its first study on patients with erectile dysfunction. Phase II lasted about two years, and after that phase III began. In 1998, ICOS Corporation, and Eli Lilly and Company, commercialized the drug for erectile dysfunction, and two year later, they filed a new drug application with the U.S. Food and Drug Admisnistration for IC351, only difference was that this time they decided to call the drug, Cialis. In May of 2002, Icos and Eli Lilly and Company reported to the American Urological Association that the phase III tests show that Cialis works for up to 36 hours, and one year later Icos and Eli Lilly and Company received the U.S. FDA's approval for Cialis. One advantage that Cialis has over Viagra is that Cialis works for up to 36 hours as compared to 4 hours for Viagra. Viagra, Levitra and Cialis work in much the same way. Chemically known as phosphodiesterase inhibitors, these drugs enhance the effects of nitric oxide, a chemical messenger that relaxes smooth muscles in the penis. This increases the amount of blood and allows a natural sequence to occur an erection in response to sexual stimulation. These medications don't automatically produce an erection. Instead they allow an erection to occur after physical and psychological stimulation. Many men experience improvement in erectile function after taking these medications regardless of the cause of their impotence. These medications share many similarities, but they have differences as well. They vary in dosage, duration of effectiveness and possible side effects. Other distinctions for example, which drug is best for certain types of men aren't yet known. No study has directly compared these three medications."