Before talking about the Cialis's case history it's necessary to remember the famous Pfizer's medication Viagra. After on the 27th of March 1998 The U. S. Food and Drug Administration approved Viagra, this was a success, and just for the first year of marketing Pfizer sold medicines, total cost of which overreached one billion dollars. Unfortunately for Pfizer the situation was altered significantly after the FDA authorized Levitra on the 19th of August, 2003 and Cialis on the 21st of November, 2003 too. In 1993 the pharmaceutical company Icos started to examine a PDE5 enzyme inhibitor called IC351 as it was the process through which the erectile dysfunction medicines affected. In 1994 during clinical researches of some cardiac medication Pfizer's researches descried the sildenafil citrate, a white crystal powder, which was a part of the medicine, caused the , taking part in the researches, to have erections as it blocked the enzyme that was for the production of the chemical substation causing erection, normalizing thus erectile function of the penis for some time.

Despite the fact that the investigators were not attempting the chemical mix IC351 for erectile dysfunction therapy, the mixture appeared to have an accompanying effect that could possibly cost millions or even billions of dollars. In 1994 at the nearest opportunity Icos got the exclusive right to IC351, and during 1995 the medical researches of the first phase were taking place. In 1997 the second phase of the medical studies was started and for the first time Icos tested its medicine on patients with erectile dysfunction. The second phase continued for about two years and when it was over the third phase was commenced.

In 1998 ICOS Corporation and Eli Lilly & Company decided exploiting the preparation for erectile dysfunction for financial profit and in two years they registered a new medicine with the U.S. FDA for IC351; they made a decision to name the medicine Cialis this time though. In May of 2002, Icos and Eli Lilly & Company informed the American Urological Association that the researches carried out in the third phase showed that Cialis's effect lasted for about 36 hours. In a year U.S. FDA approved Cialis. In comparison with Viagra Cialis has one essential advantage. The matter is that the half life of Tadalafil (Cialis), is 17.5 hours so this medicine works up to 36 hours, even if there is still about a quarter of the absorbed dose in the body by that time. As to Sildenafil (Viagra), its half life is just 4 hours.
In 2006 Eli Lilly bought ICOS Corporation for $2.1 billion dollars.